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PROFESSOR: All right, so we're
going to try to finish up

9
00:00:24,280 --> 00:00:26,180
oxidation reduction today.

10
00:00:26,180 --> 00:00:29,770
The next unit we're heading
into is transition metals.

11
00:00:29,770 --> 00:00:35,535
So, in the last class we were
talking about delta e knots of

12
00:00:35,535 --> 00:00:39,940
cells, so the standard potential
of a particular

13
00:00:39,940 --> 00:00:41,610
electric chemical cell.

14
00:00:41,610 --> 00:00:44,610
And as we ended class last
time, you had a clicker

15
00:00:44,610 --> 00:00:48,340
question, which you told me what
the reaction was at the

16
00:00:48,340 --> 00:00:51,530
anode and the reaction at the
cathode for this particular

17
00:00:51,530 --> 00:00:54,500
electric chemical cell.

18
00:00:54,500 --> 00:00:56,630
So, now we're going to consider,
we're going to

19
00:00:56,630 --> 00:01:05,850
calculate what this delta e
nought is for this cell.

20
00:01:05,850 --> 00:01:09,450
So we have an equation that we
can use to calculate delta e

21
00:01:09,450 --> 00:01:13,750
nought of a cell, and that is
that the delta e or the

22
00:01:13,750 --> 00:01:17,080
potential of the cell, is
equal to the standard

23
00:01:17,080 --> 00:01:20,520
reduction potential for the
reaction that's occurring at

24
00:01:20,520 --> 00:01:24,320
the cathode, minus the standard
reduction potential

25
00:01:24,320 --> 00:01:27,470
for the reaction that's
occurring at the anode.

26
00:01:27,470 --> 00:01:30,770
So you told me which reaction
was occurring at the anode and

27
00:01:30,770 --> 00:01:32,950
which at the cathode
last time.

28
00:01:32,950 --> 00:01:37,080
So we can just go ahead and use
that information in here.

29
00:01:37,080 --> 00:01:40,200
So the reaction that's occurring
at the cathode is

30
00:01:40,200 --> 00:01:44,090
the copper reaction, and the
reaction and the anode is the

31
00:01:44,090 --> 00:01:47,030
zinc reaction, and note here,
I have these written as

32
00:01:47,030 --> 00:01:50,030
reduction potentials, because
we're going to be plugging in

33
00:01:50,030 --> 00:01:57,720
the standard reduction potential
into this equation.

34
00:01:57,720 --> 00:02:00,750
So, where do we get our standard
reduction potentials?

35
00:02:00,750 --> 00:02:05,070
Well, your textbook has tables
of many standard reduction

36
00:02:05,070 --> 00:02:05,850
potentials.

37
00:02:05,850 --> 00:02:08,660
And, of course, on an exam
you would be given that

38
00:02:08,660 --> 00:02:09,750
information.

39
00:02:09,750 --> 00:02:12,900
And the standard reduction
potentials were measured

40
00:02:12,900 --> 00:02:17,270
against the standard
hydrogen electrode.

41
00:02:17,270 --> 00:02:20,330
So, now we talked last time
about what some of these

42
00:02:20,330 --> 00:02:22,020
abbreviations mean.

43
00:02:22,020 --> 00:02:24,410
So we can look up the values
and they're all going to be

44
00:02:24,410 --> 00:02:28,630
listed as reduction reactions,
because they are standard

45
00:02:28,630 --> 00:02:30,430
reduction potentials.

46
00:02:30,430 --> 00:02:32,720
So all the reactions, if
you're looking for ones

47
00:02:32,720 --> 00:02:35,600
written as oxidation, you'll be
out of luck, they'll all be

48
00:02:35,600 --> 00:02:37,140
written as reductions.

49
00:02:37,140 --> 00:02:39,940
So here, written as a reduction,
zinc plus 2, two

50
00:02:39,940 --> 00:02:43,400
electrons to zinc solid,
copper plus 2 plus two

51
00:02:43,400 --> 00:02:46,750
electrons to copper solid, and
you can look up those standard

52
00:02:46,750 --> 00:02:48,490
reduction potentials.

53
00:02:48,490 --> 00:02:51,570
And then you can plug them
in to your equation.

54
00:02:51,570 --> 00:02:54,680
Again, the equation is the
standard reduction potential

55
00:02:54,680 --> 00:02:58,310
for the couple at the cathode
minus the standard reduction

56
00:02:58,310 --> 00:03:01,160
potential for the reaction
at the anode.

57
00:03:01,160 --> 00:03:05,570
So the reaction at the cathode
is the copper reaction, and so

58
00:03:05,570 --> 00:03:07,750
we have -- we put in here 0 .

59
00:03:07,750 --> 00:03:14,330
3 4 0 2, and then minus, and the
zinc reduction potential's

60
00:03:14,330 --> 00:03:16,550
negative, so it's minus
a minus 0 .

61
00:03:16,550 --> 00:03:20,430
7 6 2 8, and then if we add
that together we get a

62
00:03:20,430 --> 00:03:23,170
positive value of 1 .

63
00:03:23,170 --> 00:03:25,350
1 0 3.

64
00:03:25,350 --> 00:03:30,030
And so in doing these, this is a
pretty easy thing to do, but

65
00:03:30,030 --> 00:03:32,360
I'm sort of emphasizing it
because a lot of people

66
00:03:32,360 --> 00:03:34,170
out-clever themselves
in doing this.

67
00:03:34,170 --> 00:03:39,360
They say, well, one thing's
being oxidized in this

68
00:03:39,360 --> 00:03:42,090
reaction, one thing's being
reduced, so I'm looking up a

69
00:03:42,090 --> 00:03:44,480
standard reduction potential,
I'm going to change the sign,

70
00:03:44,480 --> 00:03:46,630
and then plug it in and
change the sign again.

71
00:03:46,630 --> 00:03:49,210
And so they change the sign so
many times and they come up

72
00:03:49,210 --> 00:03:50,380
with the wrong answer.

73
00:03:50,380 --> 00:03:53,970
So just a hint in doing these
problems, if you always think

74
00:03:53,970 --> 00:03:57,100
that the equation is asking you
for the standard reduction

75
00:03:57,100 --> 00:03:59,830
potential, look that value
up and put it in.

76
00:03:59,830 --> 00:04:03,680
Don't do anything fancy with
signs, just use this equation

77
00:04:03,680 --> 00:04:07,510
as written, plugging in the
standard reduction potentials,

78
00:04:07,510 --> 00:04:10,640
and you'll be all set and you
won't have any problems

79
00:04:10,640 --> 00:04:11,540
getting it wrong.

80
00:04:11,540 --> 00:04:14,720
So try not to out-clever
yourself and flip signs around

81
00:04:14,720 --> 00:04:17,260
many times in doing
these problems.

82
00:04:17,260 --> 00:04:22,120
All right, so we have a value,
a positive value then for our

83
00:04:22,120 --> 00:04:24,360
potential for the cell.

84
00:04:24,360 --> 00:04:28,160
So we can ask the question, in
this type of cell, would the

85
00:04:28,160 --> 00:04:31,290
flow of electrons
be spontaneous?

86
00:04:31,290 --> 00:04:35,690
What tells us if something
is spontaneous?

87
00:04:35,690 --> 00:04:39,460
Delta g tells us if something
is going to be spontaneous.

88
00:04:39,460 --> 00:04:42,480
And we mentioned last time
a connection between

89
00:04:42,480 --> 00:04:46,410
delta g and delta e.

90
00:04:46,410 --> 00:04:50,490
So, delta g tells us if
something will be spontaneous,

91
00:04:50,490 --> 00:04:54,660
and we talked last time of this
equation that delta g

92
00:04:54,660 --> 00:04:59,100
nought for the cell is equal to
minus n, n here being moles

93
00:04:59,100 --> 00:05:04,100
of electrons times Faraday's
constant, times the delta e

94
00:05:04,100 --> 00:05:05,480
nought for the cell.

95
00:05:05,480 --> 00:05:09,550
So here again, we're relating
back to thermodynamics, back

96
00:05:09,550 --> 00:05:12,610
to delta g, and we're thinking
about whether things are going

97
00:05:12,610 --> 00:05:14,890
to be spontaneous or not.

98
00:05:14,890 --> 00:05:18,925
So, if we have a positive value
for delta e nought of

99
00:05:18,925 --> 00:05:23,690
the cell, what's going to be
true then for delta g?

100
00:05:23,690 --> 00:05:27,180
It'll be negative, and so
will it be spontaneous?

101
00:05:27,180 --> 00:05:29,120
Yes.

102
00:05:29,120 --> 00:05:33,160
So, if delta e nought is
positive, delta g will be

103
00:05:33,160 --> 00:05:38,940
negative, and if delta g is
negative, the reaction will,

104
00:05:38,940 --> 00:05:41,210
in fact, be spontaneous.

105
00:05:41,210 --> 00:05:47,080
So the answer, then, is yes.

106
00:05:47,080 --> 00:05:51,950
So now let me introduce some
more terms to you.

107
00:05:51,950 --> 00:05:55,780
Galvanic cell is an electric
chemical cell in which a

108
00:05:55,780 --> 00:06:00,290
spontaneous chemical reaction
is used to generate an

109
00:06:00,290 --> 00:06:02,050
electric current.

110
00:06:02,050 --> 00:06:04,965
So on a problem, which you may
have on your problem-set and

111
00:06:04,965 --> 00:06:07,860
have already looked at this,
it'll say something about for

112
00:06:07,860 --> 00:06:09,310
a galvanic cell.

113
00:06:09,310 --> 00:06:12,050
Well, that wasn't just kind of
random information they're

114
00:06:12,050 --> 00:06:14,940
throwing out, they're telling
you that the reaction's going

115
00:06:14,940 --> 00:06:16,970
to be spontaneous
in that cell.

116
00:06:16,970 --> 00:06:20,050
So that will often tell you
what reaction had to be

117
00:06:20,050 --> 00:06:22,470
happening at the anode, and
what reaction had to be

118
00:06:22,470 --> 00:06:24,220
happening at the cathode.

119
00:06:24,220 --> 00:06:27,500
Because you need to have it be
spontaneous, you need a value

120
00:06:27,500 --> 00:06:30,170
for delta e nought, then
it's positive.

121
00:06:30,170 --> 00:06:33,580
So, the information that it's
a galvanic cell tells you a

122
00:06:33,580 --> 00:06:36,590
lot about the problem.

123
00:06:36,590 --> 00:06:39,690
In contrast, we have
electrolitic cell, and in this

124
00:06:39,690 --> 00:06:46,790
case, we can put in energy to
provide, to be able to drive a

125
00:06:46,790 --> 00:06:48,820
non-spontaneous reaction.

126
00:06:48,820 --> 00:06:52,580
So you can generate a current
to then force and

127
00:06:52,580 --> 00:06:54,970
non-spontaneous reaction
to go.

128
00:06:54,970 --> 00:07:02,400
So these are 2 different
kinds of cells.

129
00:07:02,400 --> 00:07:05,850
So again, whether something's
spontaneous or not comes back

130
00:07:05,850 --> 00:07:08,050
to our friend delta g.

131
00:07:08,050 --> 00:07:12,230
So if a cell is operating
spontaneously, that means

132
00:07:12,230 --> 00:07:14,620
you're going to have a delta
e nought of the cell that's

133
00:07:14,620 --> 00:07:16,820
positive, which means that
the delta g of the

134
00:07:16,820 --> 00:07:18,160
cell will be negative.

135
00:07:18,160 --> 00:07:21,720
And we can calculate these delta
e noughts of the cell

136
00:07:21,720 --> 00:07:24,320
from the standard reduction
potentials, which some nice

137
00:07:24,320 --> 00:07:26,010
person measured for
us against the

138
00:07:26,010 --> 00:07:27,190
standard hydrogen electrode.

139
00:07:27,190 --> 00:07:31,910
And so we can look up those
values and then we can

140
00:07:31,910 --> 00:07:36,270
calculate delta e nought of a
cell, we'll know something

141
00:07:36,270 --> 00:07:39,240
about whether it will be a
spontaneous reaction or not.

142
00:07:39,240 --> 00:07:48,700
So now let's think about the
size and the sign of standard

143
00:07:48,700 --> 00:07:52,390
reduction potentials and what
they tell us about a

144
00:07:52,390 --> 00:07:54,180
particular reaction.

145
00:07:54,180 --> 00:07:57,290
So the meaning of the standard
reduction potential that you

146
00:07:57,290 --> 00:07:59,150
can look up in your book.

147
00:07:59,150 --> 00:08:01,760
So first let's think about what
happens or what would be

148
00:08:01,760 --> 00:08:05,720
true if we had a large positive
delta e nought.

149
00:08:05,720 --> 00:08:10,590
And that's going to mean that
the element is easy to reduce.

150
00:08:10,590 --> 00:08:12,700
So let's look at an example.

151
00:08:12,700 --> 00:08:15,590
At the top of your table,
you're going to see this

152
00:08:15,590 --> 00:08:18,140
particular reduction with this

153
00:08:18,140 --> 00:08:20,010
particular reduction potential.

154
00:08:20,010 --> 00:08:24,920
So we have f two gas plus 2
electrons going to 2 f minus.

155
00:08:24,920 --> 00:08:27,970
And the standard reduction
potential for this is measured

156
00:08:27,970 --> 00:08:29,840
at plus 2 .

157
00:08:29,840 --> 00:08:32,450
8 7 volts.

158
00:08:32,450 --> 00:08:35,960
So, as written, that's the
value of the standard

159
00:08:35,960 --> 00:08:37,650
reduction potential.

160
00:08:37,650 --> 00:08:40,550
That's a large positive number,
so that's going to

161
00:08:40,550 --> 00:08:45,460
mean that it's easy to
add electrons to f 2.

162
00:08:45,460 --> 00:08:49,470
The delta g nought would
be favorable for that.

163
00:08:49,470 --> 00:08:57,020
So then, you can tell me, does
that make f 2 a good oxidizing

164
00:08:57,020 --> 00:09:26,300
agent or not and why?

165
00:09:26,300 --> 00:09:42,390
OK, let's do 10 seconds.

166
00:09:42,390 --> 00:09:44,630
Good.

167
00:09:44,630 --> 00:09:46,520
Yes, it is easy to reduce.

168
00:09:46,520 --> 00:09:49,420
So it's easy to add electrons,
which makes it a good

169
00:09:49,420 --> 00:09:50,950
oxidizing agent.

170
00:09:50,950 --> 00:09:54,940
So let's go back
to the slides.

171
00:09:54,940 --> 00:10:00,010
So, a good oxidizing agent is
something that oxidizes other

172
00:10:00,010 --> 00:10:01,950
elements and gets
reduced itself.

173
00:10:01,950 --> 00:10:04,140
So it goes around oxidizing
things,

174
00:10:04,140 --> 00:10:06,520
it's an agent of oxidation.

175
00:10:06,520 --> 00:10:10,480
So something that easy to reduce
is going to be a good

176
00:10:10,480 --> 00:10:12,010
oxidizing agent.

177
00:10:12,010 --> 00:10:15,070
And something that's easy to
reduce is going to have a

178
00:10:15,070 --> 00:10:20,700
large positive standard
reduction potential.

179
00:10:20,700 --> 00:10:24,540
So, one way to sort of remember
this is for a

180
00:10:24,540 --> 00:10:26,280
particular couple.

181
00:10:26,280 --> 00:10:30,250
If something has a large
positive delta e nought, the

182
00:10:30,250 --> 00:10:35,270
oxidized species, the oxidized
species here is the f 2, it's,

183
00:10:35,270 --> 00:10:38,890
of these two things, it's
the one that's oxidized.

184
00:10:38,890 --> 00:10:41,330
So the oxidized species
is very oxidizing.

185
00:10:41,330 --> 00:10:45,480
So that's one way
to remember it.

186
00:10:45,480 --> 00:10:48,270
Large positive delta
e nought, oxidized

187
00:10:48,270 --> 00:10:52,810
species is very oxidizing.

188
00:10:52,810 --> 00:10:57,180
So, here is a list that is
similar to what you would see

189
00:10:57,180 --> 00:11:00,110
in the back of your book, and
we just talked about this

190
00:11:00,110 --> 00:11:05,320
couple between fluoride gas and
a fluoride minus up here

191
00:11:05,320 --> 00:11:09,490
with this large positive
standard reduction potential,

192
00:11:09,490 --> 00:11:10,930
and as it says up
here, oxidized

193
00:11:10,930 --> 00:11:13,680
form is strongly oxidizing.

194
00:11:13,680 --> 00:11:16,820
Now, here we have positive
numbers, and now we start to

195
00:11:16,820 --> 00:11:19,540
go to small negative numbers,
and by the time we get down

196
00:11:19,540 --> 00:11:22,830
here, we have a large negative
number for the standard

197
00:11:22,830 --> 00:11:25,620
reduction potential, and this
is between lithium plus and

198
00:11:25,620 --> 00:11:27,380
lithium solid.

199
00:11:27,380 --> 00:11:31,140
So, let's consider what would be
true down on the other end

200
00:11:31,140 --> 00:11:33,540
of the table.

201
00:11:33,540 --> 00:11:37,780
So a large negative delta e
nought means that the element

202
00:11:37,780 --> 00:11:40,580
is hard to reduce.

203
00:11:40,580 --> 00:11:43,460
So let's look at this reaction
with lithium.

204
00:11:43,460 --> 00:11:47,480
So we have lithium plus
with lithium plus 1.

205
00:11:47,480 --> 00:11:50,100
When you add one electron to
it, you get lithium solid.

206
00:11:50,100 --> 00:11:53,480
And the standard reduction
potential for doing that

207
00:11:53,480 --> 00:11:55,590
reaction is minus 3 .

208
00:11:55,590 --> 00:11:59,840
0 4 5 volts.

209
00:11:59,840 --> 00:12:03,950
So, that's hard to add electrons
to lithium plus 1 --

210
00:12:03,950 --> 00:12:08,300
lithium plus 1 is very happy
being lithium plus 1, it

211
00:12:08,300 --> 00:12:12,870
doesn't want that electron back,
and so that would be a

212
00:12:12,870 --> 00:12:17,460
non-spontaneous, unfavorable
reaction.

213
00:12:17,460 --> 00:12:25,040
So, is lithium plus 1 a
good oxidizing agent?

214
00:12:25,040 --> 00:12:29,780
No, it's not a good oxidizing
agent, but something around

215
00:12:29,780 --> 00:12:32,310
here is going to be
a good agent.

216
00:12:32,310 --> 00:12:36,650
Lithium solid is a good
reducing agent.

217
00:12:36,650 --> 00:12:42,880
So, lithium solid likes to
reduce other things, lithium

218
00:12:42,880 --> 00:12:45,940
solid likes to be oxidized,
lithium prefers to be

219
00:12:45,940 --> 00:12:47,660
lithium plus 1.

220
00:12:47,660 --> 00:12:52,200
So it's very happy to be lithium
plus 1, so the solid

221
00:12:52,200 --> 00:12:57,090
form is a good reducing agent.

222
00:12:57,090 --> 00:13:00,500
So, if we think about what's
true at the bottom of the

223
00:13:00,500 --> 00:13:05,590
table, then if we have a couple
of plus 1 to the solid,

224
00:13:05,590 --> 00:13:08,660
then if you have a large
negative standard reduction

225
00:13:08,660 --> 00:13:12,480
potential, the reduced species
is very reducing.

226
00:13:12,480 --> 00:13:16,770
So the reduced species here is
the lithium solid, it is a

227
00:13:16,770 --> 00:13:18,930
good reducing agent.

228
00:13:18,930 --> 00:13:26,690
So, large negative reduced
species is reducing.

229
00:13:26,690 --> 00:13:29,870
And if we go back to work table
then, up here we have

230
00:13:29,870 --> 00:13:31,700
the big positive numbers.

231
00:13:31,700 --> 00:13:34,950
The oxidized form of this
first couple is very

232
00:13:34,950 --> 00:13:38,530
oxidizing, at the bottom you
have the large negative

233
00:13:38,530 --> 00:13:41,140
numbers, and the reduced
form of that

234
00:13:41,140 --> 00:13:43,530
couple is very reducing.

235
00:13:43,530 --> 00:13:46,290
And you're going to be asked
questions, given different

236
00:13:46,290 --> 00:13:50,400
elements in problem-sets or on
exams and saying which of

237
00:13:50,400 --> 00:13:53,110
these is the better oxidizing
agent, which of these is the

238
00:13:53,110 --> 00:13:57,550
better reducing agent, and be
able to compare, and you need

239
00:13:57,550 --> 00:14:01,260
to think about where it is,
which are the bigger positive

240
00:14:01,260 --> 00:14:04,090
numbers, bigger negative
numbers, and you can make

241
00:14:04,090 --> 00:14:07,590
those comparisons, and if you
remember on the top, big

242
00:14:07,590 --> 00:14:10,990
positive, oxidized form very
oxidizing, big negative,

243
00:14:10,990 --> 00:14:13,880
reduced form very reducing,
you'll be all set to answer

244
00:14:13,880 --> 00:14:17,930
those questions.

245
00:14:17,930 --> 00:14:20,820
And some of this should be
sort of intuitive of the

246
00:14:20,820 --> 00:14:23,450
things that we've talked about
already in this course.

247
00:14:23,450 --> 00:14:27,400
So we think about the periodic
table for a minute, and here

248
00:14:27,400 --> 00:14:31,800
are some of the potentials,
lithium is easy to oxidize,

249
00:14:31,800 --> 00:14:33,430
it's a good reducing agent.

250
00:14:33,430 --> 00:14:37,130
If it's lithium plus 1, then
it has a noble gas

251
00:14:37,130 --> 00:14:39,330
configuration, it's
very happy there.

252
00:14:39,330 --> 00:14:42,540
Whereas fluoride can get its
noble gas configuration if

253
00:14:42,540 --> 00:14:46,470
it's f minus, so it's easy to
reduce and that makes it a

254
00:14:46,470 --> 00:14:48,020
good oxidizing agent.

255
00:14:48,020 --> 00:14:50,880
So you can think about this
in terms of it's sort of

256
00:14:50,880 --> 00:14:53,500
intuitive if you think about
what you know about those

257
00:14:53,500 --> 00:14:59,230
elements going into this unit.

258
00:14:59,230 --> 00:15:04,080
So now, let's think about
calculating a standard

259
00:15:04,080 --> 00:15:06,190
reduction potential for
this particular

260
00:15:06,190 --> 00:15:07,750
electric chemical cell.

261
00:15:07,750 --> 00:15:11,960
So we're given equations and
we want to calculate a

262
00:15:11,960 --> 00:15:16,050
standard potential for this.

263
00:15:16,050 --> 00:15:21,410
So, in doing this then, we're
going to use our standard

264
00:15:21,410 --> 00:15:24,830
reduction potentials on this
table, it's a little hard to

265
00:15:24,830 --> 00:15:26,960
see, so I'll go back
over there.

266
00:15:26,960 --> 00:15:30,140
So now we have to figure out
if we see this equation,

267
00:15:30,140 --> 00:15:35,720
what's the reaction that's
going on at the cathode?

268
00:15:35,720 --> 00:15:38,410
First tell me what's happening
at the cathode -- is it an

269
00:15:38,410 --> 00:15:41,480
oxidation or reduction happening
at the cathode?

270
00:15:41,480 --> 00:15:46,590
The reduction is happening
at the cathode.

271
00:15:46,590 --> 00:15:53,510
Now look at that equation up
there and tell me what is

272
00:15:53,510 --> 00:15:55,390
being reduced -- which
element is being

273
00:15:55,390 --> 00:15:59,270
reduced in that equation?

274
00:15:59,270 --> 00:16:01,120
The iron is being reduced.

275
00:16:01,120 --> 00:16:05,510
So we're going to write the 1/2
reaction, that's balance,

276
00:16:05,510 --> 00:16:09,100
so there were two iron 3's.

277
00:16:09,100 --> 00:16:16,100
And on the other side there's
two iron plus 2's, and how

278
00:16:16,100 --> 00:16:22,880
many electrons am I talking
about here? two electrons.

279
00:16:22,880 --> 00:16:25,860
So we have the reduction
reaction.

280
00:16:25,860 --> 00:16:32,390
Then at the anode, the anode has
an oxidation going on, and

281
00:16:32,390 --> 00:16:36,690
we only have one choice left
of what's being oxidized.

282
00:16:36,690 --> 00:16:41,580
So, we have -- and
again balanced.

283
00:16:41,580 --> 00:16:47,910
We have 2 i minus, and this is
an aqueous solution going to i

284
00:16:47,910 --> 00:16:54,770
2 solid plus two electrons.

285
00:16:54,770 --> 00:17:00,210
So we have now our two 1/2
reactions written out, and we

286
00:17:00,210 --> 00:17:04,000
can look at the potentials for
the standard reduction

287
00:17:04,000 --> 00:17:08,040
potentials, which we'll
need to calculate

288
00:17:08,040 --> 00:17:13,600
the e for the cell.

289
00:17:13,600 --> 00:17:17,280
So now we want to calculate
e nought for the cell, and

290
00:17:17,280 --> 00:17:20,360
that's going to be
equal to the e

291
00:17:20,360 --> 00:17:25,420
for the cathode reaction.

292
00:17:25,420 --> 00:17:30,010
And that particular couple at
the cathode, we're looking

293
00:17:30,010 --> 00:17:35,880
then at iron 3 plus going to
iron 2 plus -- that's the

294
00:17:35,880 --> 00:17:40,650
reduction potential that we're
going to be looking up.

295
00:17:40,650 --> 00:17:46,400
And then we also need another
standard reduction potential,

296
00:17:46,400 --> 00:17:50,870
the one for the couple at the
anode, and the couple at the

297
00:17:50,870 --> 00:17:55,380
anode that we're looking
up is i 2 to i minus.

298
00:17:55,380 --> 00:18:00,550
So, if you can see that up
there, it's also in your

299
00:18:00,550 --> 00:18:05,810
handout, then we can plug
in those values.

300
00:18:05,810 --> 00:18:10,480
So, for iron, we have plus 0 .

301
00:18:10,480 --> 00:18:19,160
7 7 0 volts minus plus 0 .

302
00:18:19,160 --> 00:18:24,190
5 3 5 volts.

303
00:18:24,190 --> 00:18:28,480
And that's going to equal 0 .

304
00:18:28,480 --> 00:18:34,990
2 3 5 volts, which is
a positive number.

305
00:18:34,990 --> 00:18:38,020
So what would be true about this
reaction in terms of it

306
00:18:38,020 --> 00:18:42,030
being spontaneous or
non-spontaneous?

307
00:18:42,030 --> 00:18:44,360
Right, so it would be
spontaneous, because you have

308
00:18:44,360 --> 00:18:48,200
a positive value for delta e, so
we'd have a negative value

309
00:18:48,200 --> 00:18:49,360
for delta g.

310
00:18:49,360 --> 00:18:55,970
All right, so now, let's think
about what the good oxidizing

311
00:18:55,970 --> 00:19:00,260
and reducing agents are here,
and let's have a clicker

312
00:19:00,260 --> 00:19:06,300
question for that.

313
00:19:06,300 --> 00:19:11,830
So, which is the better
oxidizing agent, iron plus 3

314
00:19:11,830 --> 00:19:17,100
or i 2, and which is the better
reducing agent, i minus

315
00:19:17,100 --> 00:19:19,320
or iron plus 2?

316
00:19:19,320 --> 00:20:01,240
So think about that one.

317
00:20:01,240 --> 00:20:18,810
All right, let's take
10 seconds.

318
00:20:18,810 --> 00:20:21,220
Very good.

319
00:20:21,220 --> 00:20:24,990
So the trick here is to, again,
think about these two

320
00:20:24,990 --> 00:20:26,600
different potentials.

321
00:20:26,600 --> 00:20:30,470
Now, unlike the example we had
before, we had a big positive

322
00:20:30,470 --> 00:20:32,630
and a big negative
value, these are

323
00:20:32,630 --> 00:20:34,150
both positive values.

324
00:20:34,150 --> 00:20:40,460
But one of them is bigger than
the other, and so the better

325
00:20:40,460 --> 00:20:47,180
oxidizing one is going to be
the one of these two, these

326
00:20:47,180 --> 00:20:49,620
are both the oxidized form.

327
00:20:49,620 --> 00:20:52,730
So the one with the bigger
positive number, the oxidized

328
00:20:52,730 --> 00:20:56,730
species will be better at
oxidizing, and so that would

329
00:20:56,730 --> 00:21:01,310
be the iron plus 3, that's the
bigger positive number.

330
00:21:01,310 --> 00:21:03,950
Whereas here, when we're
considering which is the

331
00:21:03,950 --> 00:21:06,670
better reducing agent, we're
looking at the two reduced

332
00:21:06,670 --> 00:21:10,320
species here, and here the
thing with the smaller a

333
00:21:10,320 --> 00:21:14,280
positive number, the reduced
form will be a better reducing

334
00:21:14,280 --> 00:21:16,260
agent, more reducing.

335
00:21:16,260 --> 00:21:19,670
So again, you can look at where
those are in the table

336
00:21:19,670 --> 00:21:22,810
and the size difference
between them to

337
00:21:22,810 --> 00:21:24,250
get the right answer.

338
00:21:24,250 --> 00:21:27,570
So, very good.

339
00:21:27,570 --> 00:21:35,710
OK, so now, I want to consider
a biological example for a

340
00:21:35,710 --> 00:21:38,690
minute, and I'm going to ask a
question that we we'll then

341
00:21:38,690 --> 00:21:41,360
answer at the end of class.

342
00:21:41,360 --> 00:21:45,430
So in cells, things have
reduction potentials.

343
00:21:45,430 --> 00:21:48,240
Vitamin B12 has a reduction
potential.

344
00:21:48,240 --> 00:21:52,930
In fact, it has one of the
largest negative reduction

345
00:21:52,930 --> 00:21:56,970
potentials of any biologically
occurring molecule.

346
00:21:56,970 --> 00:22:00,920
And so it has to be reduced
to be active in the body.

347
00:22:00,920 --> 00:22:04,940
So how can something with a very
large negative reduction

348
00:22:04,940 --> 00:22:08,380
potential be reduced?

349
00:22:08,380 --> 00:22:10,200
That's the question.

350
00:22:10,200 --> 00:22:11,740
Why should you care?

351
00:22:11,740 --> 00:22:14,660
Well, because vitamin
B12 needs to be

352
00:22:14,660 --> 00:22:16,540
reduced to be active.

353
00:22:16,540 --> 00:22:20,000
And the proper functioning of
enzymes, there's only actually

354
00:22:20,000 --> 00:22:24,180
two that you have in humans that
require B12 -- one that

355
00:22:24,180 --> 00:22:28,680
requires B12 and folic acid is
thought to be important in

356
00:22:28,680 --> 00:22:33,010
preventing heart disease, birth
defects, and B12 hs

357
00:22:33,010 --> 00:22:36,250
recently been linked
to mental health.

358
00:22:36,250 --> 00:22:40,300
In particular, a lack of B12 has
been linked to dementia.

359
00:22:40,300 --> 00:22:44,590
So, these are all pretty
significant things.

360
00:22:44,590 --> 00:22:49,070
So, one thing that I think is
kind of interesting here when

361
00:22:49,070 --> 00:22:54,970
they talk about heart disease,
how many people have heard of

362
00:22:54,970 --> 00:22:57,550
cholesterol?

363
00:22:57,550 --> 00:23:02,940
How many people have heard
of homocysteine?

364
00:23:02,940 --> 00:23:05,980
Would you be surprised to know
that homocysteine is a better

365
00:23:05,980 --> 00:23:08,630
indicator of whether you'll
have heart trouble than

366
00:23:08,630 --> 00:23:10,800
cholesterol?

367
00:23:10,800 --> 00:23:11,210
Yes.

368
00:23:11,210 --> 00:23:13,410
You would think that you would
have heard of the one that was

369
00:23:13,410 --> 00:23:14,260
a better link.

370
00:23:14,260 --> 00:23:17,840
Well, homocysteine is actually
a better link to indicate

371
00:23:17,840 --> 00:23:20,930
whether you might have heart
problems, but the thing is

372
00:23:20,930 --> 00:23:23,780
that there's not a whole lot
of money to be made there,

373
00:23:23,780 --> 00:23:26,460
whereas there's a lot of money
to be made in drugs that lower

374
00:23:26,460 --> 00:23:27,480
cholesterol.

375
00:23:27,480 --> 00:23:30,320
So you may realize, if you think
about where you get your

376
00:23:30,320 --> 00:23:33,020
medical information, it's often
from commercials where

377
00:23:33,020 --> 00:23:34,910
someone's trying to sell
you something.

378
00:23:34,910 --> 00:23:38,110
And so if there isn't much money
to be made, say, the

379
00:23:38,110 --> 00:23:40,780
treatment for a condition might
be taking vitamins,

380
00:23:40,780 --> 00:23:43,730
which there's not a whole lot of
money to be made there, you

381
00:23:43,730 --> 00:23:45,310
don't hear as much about it.

382
00:23:45,310 --> 00:23:47,780
So, it's important to consider
the source of information

383
00:23:47,780 --> 00:23:51,270
about your health, and as
scientists, you can all

384
00:23:51,270 --> 00:23:54,560
evaluate information about
your health now.

385
00:23:54,560 --> 00:23:57,800
All right, so vitamin B12 is
very important, you really

386
00:23:57,800 --> 00:24:00,620
need vitamin B12 to be
a healthy person.

387
00:24:00,620 --> 00:24:04,640
And most people at MIT are maybe
at this stage are not

388
00:24:04,640 --> 00:24:08,080
that worried of heart disease.

389
00:24:08,080 --> 00:24:10,820
Many of you are probably not
worried yet about having

390
00:24:10,820 --> 00:24:13,510
children with birth defects,
but maybe some of you are

391
00:24:13,510 --> 00:24:18,340
worried about mental recognition
of particular

392
00:24:18,340 --> 00:24:22,300
facts for exams coming up, and
so you may be concerned about

393
00:24:22,300 --> 00:24:25,610
B12 to keep sharp mentally.

394
00:24:25,610 --> 00:24:29,980
So, where do you get vitamin
B12 and folic acid in your

395
00:24:29,980 --> 00:24:32,570
diet and how is it reduced?

396
00:24:32,570 --> 00:24:36,210
So let's first consider where
you get it in your diet.

397
00:24:36,210 --> 00:24:42,970
So, does anyone know where you
get vitamin B12 in your diet?

398
00:24:42,970 --> 00:24:47,900
I heard vegetables.

399
00:24:47,900 --> 00:24:52,110
Anyone agree with vegetables?

400
00:24:52,110 --> 00:24:55,740
I didn't say if anyone liked
vegetables, does anyone agree

401
00:24:55,740 --> 00:24:58,070
that vitamin B12 come
from vegetables.

402
00:24:58,070 --> 00:25:00,000
It doesn't.

403
00:25:00,000 --> 00:25:14,240
Where does vitamin
B12 come from?

404
00:25:14,240 --> 00:25:19,030
So, red meat is a good source of
vitamin B12, all meat is a

405
00:25:19,030 --> 00:25:21,360
good source the vitamin B12.

406
00:25:21,360 --> 00:25:25,700
Plants do not use vitamin
B12 at all.

407
00:25:25,700 --> 00:25:31,150
So, people who are vegan are
in trouble in terms of how

408
00:25:31,150 --> 00:25:34,170
much vitamin B12 they get, but
luckily there are vitamin

409
00:25:34,170 --> 00:25:36,780
tablets that can help
take care of this.

410
00:25:36,780 --> 00:25:41,260
So meat is really the best
source of vitamin B12.

411
00:25:41,260 --> 00:25:47,110
So, I'm always looking for good
references to vitamin

412
00:25:47,110 --> 00:25:49,300
B12, and I saw one recently.

413
00:25:49,300 --> 00:25:54,740
Has anyone seen the HBO
series, True Blood.

414
00:25:54,740 --> 00:25:55,650
One person.

415
00:25:55,650 --> 00:25:58,900
OK, if you watch that, you will
notice that if you date a

416
00:25:58,900 --> 00:26:02,990
vampire, you have to make sure
you get extra vitamin B12, and

417
00:26:02,990 --> 00:26:07,000
that Sookie, of True Blood is
taking vitamin B12 tablets

418
00:26:07,000 --> 00:26:09,810
just to be on the safe side.

419
00:26:09,810 --> 00:26:13,380
So, vitamin B12.

420
00:26:13,380 --> 00:26:17,470
What about folic acid?

421
00:26:17,470 --> 00:26:24,410
Anyone know where you
get folic acid?

422
00:26:24,410 --> 00:26:27,450
In the fall, what happens,
people say let's go outside

423
00:26:27,450 --> 00:26:32,210
and look at the trees or look
at the pretty foliage.

424
00:26:32,210 --> 00:26:38,190
Any suggestion of where
folic acid comes from?

425
00:26:38,190 --> 00:26:43,150
Oh, wait a minute.

426
00:26:43,150 --> 00:26:46,180
I was at a meeting once where
there was a whole lecture

427
00:26:46,180 --> 00:26:50,160
about how Norwegian beer was the
best source of folic acid,

428
00:26:50,160 --> 00:26:53,410
and it was, perhaps, not
coincidentally reported by

429
00:26:53,410 --> 00:26:56,300
Norwegian scientists.

430
00:26:56,300 --> 00:26:59,190
They would not vouch for any
other kinds, but some -- but

431
00:26:59,190 --> 00:27:02,210
it does come from barleys,
vegetables,

432
00:27:02,210 --> 00:27:03,440
this kind of thing.

433
00:27:03,440 --> 00:27:08,870
So, here is a secret
for healthy diet.

434
00:27:08,870 --> 00:27:11,720
Have some of you seen the orange
juice commercial that

435
00:27:11,720 --> 00:27:16,960
says "Drink orange juice, it's
good for your heart." That's

436
00:27:16,960 --> 00:27:20,510
because of this, so that
actually is potentially true.

437
00:27:20,510 --> 00:27:24,120
So you get a lot of folic acid
in orange juice, and folic

438
00:27:24,120 --> 00:27:27,050
acid is, in fact, good
for your heart.

439
00:27:27,050 --> 00:27:31,320
So, who would have guessed,
some of the claims are

440
00:27:31,320 --> 00:27:33,780
actually true.

441
00:27:33,780 --> 00:27:36,480
All right, but we still have a
problem, we still need to know

442
00:27:36,480 --> 00:27:38,770
how its reduced in the body.

443
00:27:38,770 --> 00:27:40,880
And so we're going to come back
to that at the end of the

444
00:27:40,880 --> 00:27:46,110
lecture and answer that
if we get that far.

445
00:27:46,110 --> 00:27:48,630
All right, so we need to do a
couple more things first so

446
00:27:48,630 --> 00:27:52,180
everyone can finish their
problem-sets, and the things

447
00:27:52,180 --> 00:27:54,040
we're going to do is we're
going to look adding and

448
00:27:54,040 --> 00:27:56,940
subtracting 1/2 cell reactions,
and then get to one

449
00:27:56,940 --> 00:28:02,610
of my favorite things, which
is the Nernst equation.

450
00:28:02,610 --> 00:28:03,290
All right.

451
00:28:03,290 --> 00:28:05,750
So, some of you may have
encountered this problem on

452
00:28:05,750 --> 00:28:07,500
the problem-set already.

453
00:28:07,500 --> 00:28:10,810
Suppose you need you know a
standard reduction potential

454
00:28:10,810 --> 00:28:14,470
and it's not given to you in
the back of the book, but

455
00:28:14,470 --> 00:28:17,600
other things are given to you
in the back of the book.

456
00:28:17,600 --> 00:28:19,290
Can you calculate the
thing you need

457
00:28:19,290 --> 00:28:21,000
from related equations?

458
00:28:21,000 --> 00:28:26,050
So suppose you really want to
know the reduction couple for

459
00:28:26,050 --> 00:28:28,480
copper 2 going to copper 1.

460
00:28:28,480 --> 00:28:29,990
But that's not given.

461
00:28:29,990 --> 00:28:34,030
You find copper 2 with two
electrons going to copper 0,

462
00:28:34,030 --> 00:28:37,830
and you find copper 0 going
to copper plus 1 with one

463
00:28:37,830 --> 00:28:42,180
electron, and you'll realize
that if you combine these

464
00:28:42,180 --> 00:28:44,710
equations, you'll get
the desired one.

465
00:28:44,710 --> 00:28:48,980
So if you add these together,
you have copper 2 with one

466
00:28:48,980 --> 00:28:51,630
electron, one electron
cancels out here.

467
00:28:51,630 --> 00:28:54,240
The copper solids cancel
out and you're left

468
00:28:54,240 --> 00:28:56,030
with copper plus 1.

469
00:28:56,030 --> 00:28:59,240
So you add these together,
and you know the

470
00:28:59,240 --> 00:29:00,590
potentials for these.

471
00:29:00,590 --> 00:29:03,340
How do you get the standard
potential for the thing that

472
00:29:03,340 --> 00:29:06,730
you've come up with, for
the sum of those?

473
00:29:06,730 --> 00:29:10,520
So, as someone actually asked
me before class, you have to

474
00:29:10,520 --> 00:29:15,540
go back to free energy, and you
do, but I'm going to drive

475
00:29:15,540 --> 00:29:18,400
an equation so that you don't
have to go all the way back,

476
00:29:18,400 --> 00:29:20,700
but this is, in fact, how you
do the problem, you think

477
00:29:20,700 --> 00:29:24,140
about the different
free energies.

478
00:29:24,140 --> 00:29:29,860
So, the new reaction, the new
delta g for that new reaction

479
00:29:29,860 --> 00:29:32,810
will be equal to the delta g
knot for the reduction minus

480
00:29:32,810 --> 00:29:34,730
the oxidation reaction here.

481
00:29:34,730 --> 00:29:37,910
But we can substitute for
delta g this minus n,

482
00:29:37,910 --> 00:29:42,000
Faraday's constant times our
reduction potential, and put

483
00:29:42,000 --> 00:29:44,580
it in to this equation here.

484
00:29:44,580 --> 00:29:47,290
And so we have it for the new
reaction, and then the 2

485
00:29:47,290 --> 00:29:51,040
reactions that we're
adding together.

486
00:29:51,040 --> 00:29:55,320
So we have Faraday's constant in
common, so that's going to

487
00:29:55,320 --> 00:29:56,410
cancel out.

488
00:29:56,410 --> 00:30:00,270
And we can also move n 3 to
the other side, because we

489
00:30:00,270 --> 00:30:04,670
want to solve for this new e
value, this new standard

490
00:30:04,670 --> 00:30:05,980
reduction potential.

491
00:30:05,980 --> 00:30:08,740
And so that's going to be equal
to the number of moles

492
00:30:08,740 --> 00:30:12,790
that are involved in the
reaction that's the reduction

493
00:30:12,790 --> 00:30:16,150
times its reduction potential
minus the number of moles

494
00:30:16,150 --> 00:30:19,380
involved in the oxidation
reaction with its reduction

495
00:30:19,380 --> 00:30:23,255
potential, and then the number
of moles for the reaction that

496
00:30:23,255 --> 00:30:25,590
-- this new reaction
in question.

497
00:30:25,590 --> 00:30:29,800
So we can use, then, this
equation to look for a 1/2

498
00:30:29,800 --> 00:30:35,270
cell reaction that is the sum
of two other reactions.

499
00:30:35,270 --> 00:30:37,900
So let's go and use that
equation, then.

500
00:30:37,900 --> 00:30:41,960
So here we have the known values
-- we know the couple

501
00:30:41,960 --> 00:30:45,560
for copper 2 to copper 0, we
know the couple for copper 1

502
00:30:45,560 --> 00:30:53,040
to copper solid, and this
is what we want to know.

503
00:30:53,040 --> 00:30:56,090
So we can use this equation.

504
00:30:56,090 --> 00:30:59,850
Which of these reactions is the
reduction, what value am I

505
00:30:59,850 --> 00:31:06,220
going to put in here?

506
00:31:06,220 --> 00:31:17,400
Which of these goes into
the reduction?

507
00:31:17,400 --> 00:31:23,310
Which of these is a reduction
reaction?

508
00:31:23,310 --> 00:31:24,090
Yeah.

509
00:31:24,090 --> 00:31:29,340
And how many moles of electrons
are involved?

510
00:31:29,340 --> 00:31:31,500
So, we put in 2 times 0 .

511
00:31:31,500 --> 00:31:38,470
3 4 0 volts, and then over here
in the oxidation, there's

512
00:31:38,470 --> 00:31:42,680
one electron involved, and we
put in our other potential.

513
00:31:42,680 --> 00:31:44,160
So 1 times 0 .

514
00:31:44,160 --> 00:31:46,320
5 2 2.

515
00:31:46,320 --> 00:31:49,440
And what are the number of moles
of electrons for our

516
00:31:49,440 --> 00:31:52,410
desired, final reaction?

517
00:31:52,410 --> 00:31:54,140
1.

518
00:31:54,140 --> 00:31:56,950
And so, then we can do the
math and come up with an

519
00:31:56,950 --> 00:31:58,740
answer of 0 .

520
00:31:58,740 --> 00:32:00,930
1 5 8 volts.

521
00:32:00,930 --> 00:32:03,940
And so now we've just come
up with a new reduction

522
00:32:03,940 --> 00:32:07,180
potential, we've calculated a
new reduction potential for

523
00:32:07,180 --> 00:32:11,220
this 1/2 cell reaction
right here.

524
00:32:11,220 --> 00:32:14,460
So, this equation will be given
to you on an exam and

525
00:32:14,460 --> 00:32:19,600
all you need to do is
know how to use it.

526
00:32:19,600 --> 00:32:23,030
So we've calculated now the
standard reduction potential

527
00:32:23,030 --> 00:32:27,920
for copper 2 to copper plus 1.

528
00:32:27,920 --> 00:32:31,260
So, just a little note about
when you're going to use this

529
00:32:31,260 --> 00:32:34,290
and when you're going to use
the other equation that I

530
00:32:34,290 --> 00:32:35,280
showed you.

531
00:32:35,280 --> 00:32:39,460
So, if we're talking about a
whole electric chemical cell,

532
00:32:39,460 --> 00:32:41,950
the number of moles that are
released at the anode are

533
00:32:41,950 --> 00:32:44,950
going to equal the number of
moles taken up at the cathode

534
00:32:44,950 --> 00:32:47,220
and be the number of
moles involved

535
00:32:47,220 --> 00:32:49,190
in the overall equation.

536
00:32:49,190 --> 00:32:54,140
So this equation is not
necessary, and for a full

537
00:32:54,140 --> 00:32:56,510
electric chemical cell, we're
going to use the equation that

538
00:32:56,510 --> 00:32:59,940
I gave you before for this delta
e nought for the cell,

539
00:32:59,940 --> 00:33:01,430
so we just use this.

540
00:33:01,430 --> 00:33:04,240
But if it's not a whole electric
chemical cell you're

541
00:33:04,240 --> 00:33:06,550
talking about, if you're talking
about calculating a

542
00:33:06,550 --> 00:33:09,910
1/2 cell potential, then you
need to use this equation.

543
00:33:09,910 --> 00:33:13,010
Both of these equations will
be given to you on

544
00:33:13,010 --> 00:33:14,590
sheets for the exam.

545
00:33:14,590 --> 00:33:17,280
This one's for an electric
chemical cell, this one's for

546
00:33:17,280 --> 00:33:20,030
calculating a 1/2
cell potential.

547
00:33:20,030 --> 00:33:23,690
So again, if it's 1/2 cell
potential, you want to use

548
00:33:23,690 --> 00:33:27,790
this equation.

549
00:33:27,790 --> 00:33:31,290
OK, so now the Nernst
equation.

550
00:33:31,290 --> 00:33:36,570
So, some of you may have
encountered in your life where

551
00:33:36,570 --> 00:33:41,410
you go to turn something on
and you discover that the

552
00:33:41,410 --> 00:33:44,020
battery is dead.

553
00:33:44,020 --> 00:33:49,280
So, an exhausted battery is
a sign that your chemical

554
00:33:49,280 --> 00:33:58,500
reaction has reached
equilibrium.

555
00:33:58,500 --> 00:33:59,845
At equilibrium, you're going
to have a zero difference

556
00:33:59,845 --> 00:34:00,020
potential across your
electrodes, and the battery

557
00:34:00,020 --> 00:34:03,200
will be not useful to
you at that point.

558
00:34:03,200 --> 00:34:07,060
So, if you -- instead of getting
annoyed next time you

559
00:34:07,060 --> 00:34:09,660
have a dead battery, you can
think about all, ah, it's

560
00:34:09,660 --> 00:34:14,150
finally reached equilibrium.

561
00:34:14,150 --> 00:34:18,150
So, we need to think about,
then, to think about how these

562
00:34:18,150 --> 00:34:22,010
cell reactions are happening,
how the potential is going to

563
00:34:22,010 --> 00:34:26,230
change with the composition of
the ingredients in those

564
00:34:26,230 --> 00:34:29,560
electrochemical cells.

565
00:34:29,560 --> 00:34:34,080
So, we know something about
equilibrium and components of

566
00:34:34,080 --> 00:34:36,240
reactions again.

567
00:34:36,240 --> 00:34:38,790
So this is a nice lecture to
give coming up with a week

568
00:34:38,790 --> 00:34:41,880
before the exam, because now
we're going back and reviewing

569
00:34:41,880 --> 00:34:46,810
the first material on the exam
from chemical equilibrium.

570
00:34:46,810 --> 00:34:50,710
So, we know that delta g is
going to change as the

571
00:34:50,710 --> 00:34:54,540
components change until
equilibrium is reached.

572
00:34:54,540 --> 00:34:57,680
And when equilibrium is reached
our delta g is going

573
00:34:57,680 --> 00:35:00,700
to be equal to zero.

574
00:35:00,700 --> 00:35:06,010
Before equilibrium is reached,
delta g will depend on the

575
00:35:06,010 --> 00:35:10,460
delta g nought for the equation,
r t and the natural

576
00:35:10,460 --> 00:35:14,970
log of q, where q is what?

577
00:35:14,970 --> 00:35:17,350
What's q?

578
00:35:17,350 --> 00:35:19,570
The reaction quotient.

579
00:35:19,570 --> 00:35:24,390
So we saw this before and we're
back to using it again.

580
00:35:24,390 --> 00:35:27,800
So what do we know about the
relationship between delta g

581
00:35:27,800 --> 00:35:30,870
nought and delta e?

582
00:35:30,870 --> 00:35:34,860
We know that delta g nought is
equal to minus n, moles of

583
00:35:34,860 --> 00:35:39,790
electrons times Faraday's
constant times delta e nought.

584
00:35:39,790 --> 00:35:44,100
So, we can combine some
things around.

585
00:35:44,100 --> 00:35:47,110
So we can take our equation
that we saw in chemical

586
00:35:47,110 --> 00:35:51,820
equilibrium and substitute in
values that are related to

587
00:35:51,820 --> 00:35:53,870
standard reduction potentials.

588
00:35:53,870 --> 00:35:57,810
So we can put in, for this delta
g, minus n Faraday's

589
00:35:57,810 --> 00:36:00,230
constant times the delta e.

590
00:36:00,230 --> 00:36:03,340
And for delta g nought, we put
in the same thing but now we

591
00:36:03,340 --> 00:36:05,170
have delta e nought.

592
00:36:05,170 --> 00:36:08,810
And we have r t, gas constant
times temperature times the

593
00:36:08,810 --> 00:36:12,020
natural log of q, our
reaction quotient.

594
00:36:12,020 --> 00:36:16,850
Now we can divide both sides
by n and Faraday's constant

595
00:36:16,850 --> 00:36:19,650
and we come up with the
Nernst equation.

596
00:36:19,650 --> 00:36:24,040
So the Nernst equation tells us
the potential for a cell at

597
00:36:24,040 --> 00:36:28,100
any given time, at any given
component of ingredients in

598
00:36:28,100 --> 00:36:34,320
the cell, any amounts of, say,
your zinc plus 2, compared to

599
00:36:34,320 --> 00:36:37,690
the standard potential for that
cell, which you're going

600
00:36:37,690 --> 00:36:40,520
to calculate from your standard
reduction potentials

601
00:36:40,520 --> 00:36:44,370
in the table, and then you have
this term, gas constant

602
00:36:44,370 --> 00:36:47,510
times temperature, number of
moles of electrons involved,

603
00:36:47,510 --> 00:36:51,000
Faraday's constant, and you need
to know q, the particular

604
00:36:51,000 --> 00:36:56,020
composition of the cell
at that given time.

605
00:36:56,020 --> 00:36:59,160
So, just a hint in doing Nernst
equation problems, if

606
00:36:59,160 --> 00:37:02,990
you're given a problem and it's
giving you concentrations

607
00:37:02,990 --> 00:37:07,120
of things at a particular time,
that should be a clue

608
00:37:07,120 --> 00:37:10,430
that the Nernst equation is
going to be what you're going

609
00:37:10,430 --> 00:37:13,090
to want to use here.

610
00:37:13,090 --> 00:37:15,580
So, let let's look
at an example.

611
00:37:15,580 --> 00:37:19,140
Say we have a -- we want to
calculate the cell potential

612
00:37:19,140 --> 00:37:23,460
at a given time at 25 degrees,
and we know our zinc plus 2

613
00:37:23,460 --> 00:37:24,690
ions or 0 .

614
00:37:24,690 --> 00:37:28,270
1 0 molar and copper
2 ions are 0 .

615
00:37:28,270 --> 00:37:34,700
0 0 1 0 molar, and this is the
equation for that cell.

616
00:37:34,700 --> 00:37:44,940
So we can look up, again, our
values in the table, and the

617
00:37:44,940 --> 00:37:47,540
first thing we're going to do,
if we're going to use to

618
00:37:47,540 --> 00:37:51,260
Nernst equation, is to calculate
the standard

619
00:37:51,260 --> 00:37:56,130
potential for that particular
cell, and here are the values

620
00:37:56,130 --> 00:38:00,630
from the table, and why don't
you go ahead and calculate

621
00:38:00,630 --> 00:38:48,060
that for me.

622
00:38:48,060 --> 00:39:11,240
All right, so let's just take
10 more seconds on that.

623
00:39:11,240 --> 00:39:16,770
OK, so let's see if we can get
into the 90's pretty soon,

624
00:39:16,770 --> 00:39:19,400
because we should be
able to do that for

625
00:39:19,400 --> 00:39:20,680
this particular one.

626
00:39:20,680 --> 00:39:25,560
All right, let's go back to
the presentation here.

627
00:39:25,560 --> 00:39:29,970
So it was actually, this
was pretty easy.

628
00:39:29,970 --> 00:39:33,150
I didn't actually mean for these
two things to show up,

629
00:39:33,150 --> 00:39:35,610
to help you out of what the
reaction at the cathode and

630
00:39:35,610 --> 00:39:37,220
the anode was, so that
was an easier

631
00:39:37,220 --> 00:39:39,550
question than I had intended.

632
00:39:39,550 --> 00:39:41,680
So, first you need to think
about if you're given an

633
00:39:41,680 --> 00:39:44,380
equation here, what's happening
at the cathode,

634
00:39:44,380 --> 00:39:46,540
what's happening at the anode.

635
00:39:46,540 --> 00:39:49,860
As it's written, you can look
and see so copper plus 2 is

636
00:39:49,860 --> 00:39:54,980
going to copper solid, zinc
solid's going to zinc plus 2,

637
00:39:54,980 --> 00:39:58,440
so you can think about which is
the cathode reaction, which

638
00:39:58,440 --> 00:40:01,980
is the anode, which has a
reduction, which has an

639
00:40:01,980 --> 00:40:05,610
oxidation going on, and then
once you know which couples

640
00:40:05,610 --> 00:40:09,540
you're talking about, then you
can plug in your values.

641
00:40:09,540 --> 00:40:14,330
So, at the cathode, we're going
from copper plus 2 to

642
00:40:14,330 --> 00:40:15,520
copper solid.

643
00:40:15,520 --> 00:40:18,720
We put in our standard reduction
potential, using the

644
00:40:18,720 --> 00:40:21,880
equation we have a minus, and
then the standard reduction

645
00:40:21,880 --> 00:40:25,070
potential at the anode, so we
have the oxidation from zinc

646
00:40:25,070 --> 00:40:29,960
solid to zinc plus 2, and we
put in this value and we

647
00:40:29,960 --> 00:40:33,870
calculate the number here.

648
00:40:33,870 --> 00:40:39,340
So then, we have that, we
also need to know q.

649
00:40:39,340 --> 00:41:10,330
So why don't you tell
me what q is now?

650
00:41:10,330 --> 00:41:27,450
OK, let's just do 10 more
seconds on this.

651
00:41:27,450 --> 00:41:32,070
OK.

652
00:41:32,070 --> 00:41:38,320
So here, if we go back, q is
going to be equal to products

653
00:41:38,320 --> 00:41:42,750
over reactants, except things
that are solids are not are

654
00:41:42,750 --> 00:41:45,550
not changing during
the equilibrium so

655
00:41:45,550 --> 00:41:46,770
we leave those out.

656
00:41:46,770 --> 00:41:49,960
So it's going to be
concentration of zinc plus 2

657
00:41:49,960 --> 00:41:52,700
over the concentration of
copper, which gives you a

658
00:41:52,700 --> 00:41:53,740
value of 1 .

659
00:41:53,740 --> 00:41:56,220
0 times 10 to the 2.

660
00:41:56,220 --> 00:42:01,680
So, just a review of q, if you
need help calculating q's,

661
00:42:01,680 --> 00:42:04,690
we're going to have to extra
problems for the exam coming

662
00:42:04,690 --> 00:42:06,530
up to be posted on Friday.

663
00:42:06,530 --> 00:42:12,110
So, this'll be used
in several units.

664
00:42:12,110 --> 00:42:14,620
So then you need to know n, and
then we have everything to

665
00:42:14,620 --> 00:42:16,280
go back to the Nernst
equation.

666
00:42:16,280 --> 00:42:23,550
So how many moles of electrons
are involved in this?

667
00:42:23,550 --> 00:42:26,100
2.

668
00:42:26,100 --> 00:42:29,450
Sometimes it's not so obvious,
so this can trick people, so

669
00:42:29,450 --> 00:42:31,680
make sure that you pay
attention to this.

670
00:42:31,680 --> 00:42:33,330
This one is a pretty
obvious one.

671
00:42:33,330 --> 00:42:36,280
All right, now we have
everything that we can put in.

672
00:42:36,280 --> 00:42:41,215
You calculated the standard
potential for the cell, you

673
00:42:41,215 --> 00:42:44,020
calculated q, and you told
me how many moles of

674
00:42:44,020 --> 00:42:45,140
electrons there are.

675
00:42:45,140 --> 00:42:46,930
So we can go and put this in.

676
00:42:46,930 --> 00:42:49,110
So we calculated positive 1 .

677
00:42:49,110 --> 00:42:52,590
0 3 volts minus the gas
constant times the

678
00:42:52,590 --> 00:42:57,420
temperature, it was at room
temperature, natural log of q.

679
00:42:57,420 --> 00:43:02,260
We had two moles of electrons
and Faraday's constant here.

680
00:43:02,260 --> 00:43:07,020
And now if we do the math,
this whole term

681
00:43:07,020 --> 00:43:08,110
comes out to 0 .

682
00:43:08,110 --> 00:43:13,740
0 5 9 2 volts, and we get our
answer, which is a positive

683
00:43:13,740 --> 00:43:15,030
number there.

684
00:43:15,030 --> 00:43:18,320
So, just a note about
units and constants.

685
00:43:18,320 --> 00:43:20,180
Where did volts come
from here?

686
00:43:20,180 --> 00:43:24,860
Well, the moles canceled out,
and the kelvin canceled out,

687
00:43:24,860 --> 00:43:28,600
and we are left with joules per
Coulomb, and conveniently

688
00:43:28,600 --> 00:43:31,390
for us, joules per Coulomb
is a volt.

689
00:43:31,390 --> 00:43:34,610
So, all our units add up here.

690
00:43:34,610 --> 00:43:38,430
And just a note about a
significant figures in doing

691
00:43:38,430 --> 00:43:43,310
these problems, the Nernst
equation, boy, significant

692
00:43:43,310 --> 00:43:44,120
figure fun.

693
00:43:44,120 --> 00:43:46,010
If you want to make sure
you know significant

694
00:43:46,010 --> 00:43:47,250
figures, here you go.

695
00:43:47,250 --> 00:43:51,420
Significant figure rules for
logs, we have significant

696
00:43:51,420 --> 00:43:54,760
figure rules for multiplication
and division,

697
00:43:54,760 --> 00:43:58,860
and then significant figure
rules for subtraction.

698
00:43:58,860 --> 00:44:03,730
So, one equation gives you
every type of significant

699
00:44:03,730 --> 00:44:04,890
figure rule.

700
00:44:04,890 --> 00:44:10,030
So, that can be a lot
of fun as well.

701
00:44:10,030 --> 00:44:13,060
All right, but I'm going to try
to make it easier for you

702
00:44:13,060 --> 00:44:17,080
on an exam and avoid
some math mistakes.

703
00:44:17,080 --> 00:44:19,360
All the problems I'm going
to give you are at room

704
00:44:19,360 --> 00:44:20,610
temperature.

705
00:44:20,610 --> 00:44:24,100
And so, the gas constant is a
constant, temperature for

706
00:44:24,100 --> 00:44:26,480
these problems is going to be
a constant, always room

707
00:44:26,480 --> 00:44:29,430
temperature, Faraday's constant
is a constant.

708
00:44:29,430 --> 00:44:34,530
So, all of these, I'm going to
give you the value that they

709
00:44:34,530 --> 00:44:40,140
all add up to, and if you use
log instead of natural log,

710
00:44:40,140 --> 00:44:41,830
there's this value as well.

711
00:44:41,830 --> 00:44:46,550
So, these will be given to you
on an exam, so you'll see

712
00:44:46,550 --> 00:44:51,540
these equations on the exam as
well and you can use them.

713
00:44:51,540 --> 00:44:55,070
So, this is for natural log,
this is for log, so we are not

714
00:44:55,070 --> 00:44:58,190
going to test your ability to
multiply room temperature

715
00:44:58,190 --> 00:45:01,290
times the gas constant divided
by Faraday's constant.

716
00:45:01,290 --> 00:45:04,740
So that's going to make it a
little easier in doing these

717
00:45:04,740 --> 00:45:07,890
problems.

718
00:45:07,890 --> 00:45:11,460
All right, so what about
at equilibrium.

719
00:45:11,460 --> 00:45:15,640
What does q equal
at equilibrium?

720
00:45:15,640 --> 00:45:18,650
So, q equals k at equilibrium.

721
00:45:18,650 --> 00:45:22,260
What does delta g equal
at equilibrium?

722
00:45:22,260 --> 00:45:23,380
Zero.

723
00:45:23,380 --> 00:45:28,990
And so that means that we knew
before, from this equation,

724
00:45:28,990 --> 00:45:33,010
when this is equal to zero, then
delta g knot equals minus

725
00:45:33,010 --> 00:45:38,390
r t, natural log of k, so q
equals k at equilibrium.

726
00:45:38,390 --> 00:45:41,580
And so we had this equation
that we used before.

727
00:45:41,580 --> 00:45:46,010
We now have this equation, so
here it related delta g knot

728
00:45:46,010 --> 00:45:49,930
to the equilibrium constant,
here we relate delta g nought

729
00:45:49,930 --> 00:45:52,830
to delta e nought And
so, I think you

730
00:45:52,830 --> 00:45:54,420
can see what's coming.

731
00:45:54,420 --> 00:46:00,560
We are going to relate these two
together and come up with

732
00:46:00,560 --> 00:46:04,620
an expression where are you
can calculate equilibrium

733
00:46:04,620 --> 00:46:08,550
constants from standard
reduction potentials.

734
00:46:08,550 --> 00:46:12,190
So, you may be asked
to do this as well.

735
00:46:12,190 --> 00:46:14,680
So everything in these
units are, in fact,

736
00:46:14,680 --> 00:46:16,060
related to each other.

737
00:46:16,060 --> 00:46:20,192
And the only thing we have left
today, the answer to this

738
00:46:20,192 --> 00:46:23,960
question about how B12 is
reduced in the body.

739
00:46:23,960 --> 00:46:26,640
So, you're just going to have
to wait to find out, don't

740
00:46:26,640 --> 00:46:29,050
worry you won't get heart
disease between now and

741
00:46:29,050 --> 00:46:31,320
Friday, and I'll let
you know how it all

742
00:46:31,320 --> 00:46:33,240
works out on Friday.