1
00:00:00,000 --> 00:00:00,024
The following content is
provided under a Creative

2
00:00:00,024 --> 00:00:00,033
Commons license.

3
00:00:00,033 --> 00:00:00,057
Your support will help MIT
OpenCourseWare continue to

4
00:00:00,057 --> 00:00:00,081
offer high quality educational
resources for free.

5
00:00:00,081 --> 00:00:00,108
To make a donation or view
additional materials from

6
00:00:00,108 --> 00:00:00,132
hundreds of MIT courses, visit
MIT OpenCourseWare at

7
00:00:00,132 --> 00:00:00,150
ocw.mit.edu.

8
00:00:00,150 --> 00:00:24,190
PROFESSOR: OK, so today we're
going to start talking about

9
00:00:24,190 --> 00:00:29,720
acids and bases, and this is
acid-base equilibrium, so you

10
00:00:29,720 --> 00:00:32,610
can't forget anything that
you've learned from the last

11
00:00:32,610 --> 00:00:34,930
two lectures about
equilibrium.

12
00:00:34,930 --> 00:00:38,050
And then we're going to talk
about, after this, oxidation

13
00:00:38,050 --> 00:00:40,490
reduction equilibrium, and
so there's a lot of

14
00:00:40,490 --> 00:00:42,410
equilibrium going on.

15
00:00:42,410 --> 00:00:45,010
So today we're going to give
you some definitions.

16
00:00:45,010 --> 00:00:48,780
We're going to talk about
autoionization of water.

17
00:00:48,780 --> 00:00:51,190
We're going to talk about the
p h function, which most

18
00:00:51,190 --> 00:00:52,460
people are familiar with.

19
00:00:52,460 --> 00:00:57,620
We may think about what the p
h's are of some commonly found

20
00:00:57,620 --> 00:01:00,520
ingredients around campus.

21
00:01:00,520 --> 00:01:04,420
And talk about the strengths
of those acids and bases.

22
00:01:04,420 --> 00:01:07,740
And then if we have time, we'll
start thinking about how

23
00:01:07,740 --> 00:01:15,090
to work a problem associated
with a weak acid.

24
00:01:15,090 --> 00:01:18,900
So actually, I guess, do we want
to do that other clicker

25
00:01:18,900 --> 00:01:21,610
question, maybe, before
we get started?

26
00:01:21,610 --> 00:01:24,090
At some point, we're going to
ask you a question about when

27
00:01:24,090 --> 00:01:26,450
you want forums. So, I'm not
sure if we're going to have

28
00:01:26,450 --> 00:01:30,460
that for you today or not, but
we need to -- we want to have

29
00:01:30,460 --> 00:01:34,280
these pizza forums where you can
come, and the time isn't

30
00:01:34,280 --> 00:01:36,490
always working, so we thought
we could use the clickers to

31
00:01:36,490 --> 00:01:38,930
figure out what the best
time is for people.

32
00:01:38,930 --> 00:01:41,860
So we may have that
for you later.

33
00:01:41,860 --> 00:01:46,090
So this is the narrowest
definition of

34
00:01:46,090 --> 00:01:48,150
an acid and a base.

35
00:01:48,150 --> 00:01:51,920
So the narrowest definition of
an acid and a base is that an

36
00:01:51,920 --> 00:01:55,805
acid is a substance when you
dissolve it in water, it

37
00:01:55,805 --> 00:02:01,140
increases the concentration of
hydrogen ions, or h plus.

38
00:02:01,140 --> 00:02:05,260
Whereas a base is a substance
that when dissolved in water,

39
00:02:05,260 --> 00:02:11,030
increases the concentration of
hydroxide ions, or o h minus.

40
00:02:11,030 --> 00:02:13,530
So that's pretty narrow.

41
00:02:13,530 --> 00:02:15,210
We can be broader.

42
00:02:15,210 --> 00:02:20,060
We can talk about
Bronsted-Lowry, and here, an

43
00:02:20,060 --> 00:02:23,420
acid is described as a substance
that can donate a

44
00:02:23,420 --> 00:02:24,690
hydrogen ion.

45
00:02:24,690 --> 00:02:28,560
And a base is described as a
substance that can accept a

46
00:02:28,560 --> 00:02:30,880
hydrogen ion.

47
00:02:30,880 --> 00:02:45,870
So let's look at some examples
using that definition.

48
00:02:45,870 --> 00:03:02,770
So let's look at an example, so
c h 3, c o o h plus water,

49
00:03:02,770 --> 00:03:10,100
and I guess I should put that in
aqueous, going to hydronium

50
00:03:10,100 --> 00:03:22,540
ion plus c h 3, c o
o minus aqueous.

51
00:03:22,540 --> 00:03:26,790
All right, so what
is going on here?

52
00:03:26,790 --> 00:03:29,420
So if we look at the things on
one side of the equation and

53
00:03:29,420 --> 00:03:34,270
the other, you can see that this
is an acid that has lost

54
00:03:34,270 --> 00:03:39,090
its hydrogen ion, so the
hydrogen is gone, whereas the

55
00:03:39,090 --> 00:03:43,140
water molecule has gained a
hydrogen ion, and so now it's

56
00:03:43,140 --> 00:03:46,180
h 3 o plus.

57
00:03:46,180 --> 00:03:53,670
So we have an acid here, it's
acting as an acid, and acid

58
00:03:53,670 --> 00:03:58,280
acts as a substance that gives
up a hydrogen ion, and the

59
00:03:58,280 --> 00:04:02,030
water is acting as a base,
it's accepting

60
00:04:02,030 --> 00:04:04,010
that hydrogen ion.

61
00:04:04,010 --> 00:04:07,680
And when it accepts the hydrogen
ion, it becomes an

62
00:04:07,680 --> 00:04:12,870
acid in the reverse direction,
whereas when the acid gives

63
00:04:12,870 --> 00:04:16,560
off the hydrogen ion,
it becomes a base

64
00:04:16,560 --> 00:04:18,460
in the reverse direction.

65
00:04:18,460 --> 00:04:22,870
So in the reverse direction,
hydronium ions is giving off a

66
00:04:22,870 --> 00:04:29,015
hydrogen ion to this base,
reforming the acid, and after

67
00:04:29,015 --> 00:04:32,850
a hydronium ion gives off
its hydrogen ion,

68
00:04:32,850 --> 00:04:35,540
it forms water again.

69
00:04:35,540 --> 00:04:39,990
So that would be an example of
Bronsted-Lowry talking about

70
00:04:39,990 --> 00:04:43,355
substances as acids and bases
whether they accept or donate

71
00:04:43,355 --> 00:04:44,790
a hydrogen ion.

72
00:04:44,790 --> 00:04:48,650
And this is the definition we're
going to be using mostly

73
00:04:48,650 --> 00:04:51,330
throughout this unit.

74
00:04:51,330 --> 00:04:57,720
So, let's look at a little
movie of this going on.

75
00:04:57,720 --> 00:05:01,700
So, in this movie, we have our
water molecules with red and

76
00:05:01,700 --> 00:05:08,890
the white dots here are their
hydrogen atoms, and now we're

77
00:05:08,890 --> 00:05:11,180
going to come in and have an
acid come in, there's the

78
00:05:11,180 --> 00:05:14,060
acid, it has the hydrogen
ion on it in white.

79
00:05:14,060 --> 00:05:18,120
It meets up with a water
molecule, and now you formed

80
00:05:18,120 --> 00:05:18,740
hydronium ion.

81
00:05:18,740 --> 00:05:22,960
And that forms another water
molecule and it passes it

82
00:05:22,960 --> 00:05:26,910
along, so there's a different
molecule of hydronium ion.

83
00:05:26,910 --> 00:05:35,230
So that's what's going on
in this definition.

84
00:05:35,230 --> 00:05:36,180
All right.

85
00:05:36,180 --> 00:05:40,000
So this brings us to another
term, which is conjugate acid

86
00:05:40,000 --> 00:05:44,220
base pairs, so you can talk
about something being a

87
00:05:44,220 --> 00:05:50,300
conjugate base of a particular
acid, and so a conjugate base

88
00:05:50,300 --> 00:05:53,880
of an acid is a base that's
formed after the acid has

89
00:05:53,880 --> 00:05:56,070
donated its hydrogen ion.

90
00:05:56,070 --> 00:06:00,950
A conjugate acid of a base is
the acids that its formed when

91
00:06:00,950 --> 00:06:04,050
the base accepts the
hydrogen ion.

92
00:06:04,050 --> 00:06:07,350
So you can look at those
examples here.

93
00:06:07,350 --> 00:06:11,740
So we have a pair that we've
drawn here with this red line,

94
00:06:11,740 --> 00:06:16,050
an acid base pair, and the other
pair is the water and

95
00:06:16,050 --> 00:06:21,580
the hydronium ion.

96
00:06:21,580 --> 00:06:24,330
All right, let's look at a
couple more examples to get

97
00:06:24,330 --> 00:06:27,610
the sense of this and
figure out what are

98
00:06:27,610 --> 00:06:34,130
the acid base pairs.

99
00:06:34,130 --> 00:06:39,540
So, one more example.

100
00:06:39,540 --> 00:06:50,570
So now let's look at h c o 3
minus an aqueous solution and

101
00:06:50,570 --> 00:06:58,690
water, going to, again,
hydronium ion, and c o 3 minus

102
00:06:58,690 --> 00:07:06,930
2, also an aqueous solution.

103
00:07:06,930 --> 00:07:12,600
So, what is h c o 3 minus
acting as here?

104
00:07:12,600 --> 00:07:14,990
As an acid.

105
00:07:14,990 --> 00:07:18,430
And so what does that
make water?

106
00:07:18,430 --> 00:07:20,970
A base.

107
00:07:20,970 --> 00:07:26,700
And so the conjugate acid of
that base, again, is the

108
00:07:26,700 --> 00:07:29,810
hydronium ion concentration.

109
00:07:29,810 --> 00:07:34,910
And the conjugate base
of the acid is c o 3

110
00:07:34,910 --> 00:07:38,170
minus 2, over here.

111
00:07:38,170 --> 00:07:41,210
And so in the reverse direction,
this base will be

112
00:07:41,210 --> 00:07:46,020
accepting a hydrogen ion from
the acid, forming the

113
00:07:46,020 --> 00:07:48,670
conjugates on the other side.

114
00:07:48,670 --> 00:07:51,530
OK, now you can do an example.

115
00:07:51,530 --> 00:07:56,830
Let's have a clicker question.

116
00:07:56,830 --> 00:08:41,790
So identify what the acid and
the base pairs are here.

117
00:08:41,790 --> 00:08:57,270
All right, let's take
10 seconds.

118
00:08:57,270 --> 00:08:59,720
That's quite good actually.

119
00:08:59,720 --> 00:09:04,770
So, that's correct and you can
write it in your notes that

120
00:09:04,770 --> 00:09:08,280
here we see that there's a
little bit of change, water's

121
00:09:08,280 --> 00:09:09,740
doing something different.

122
00:09:09,740 --> 00:09:12,660
So instead of the conjugate of
water being the hydronium ion,

123
00:09:12,660 --> 00:09:15,200
we see it's a hydroxide.

124
00:09:15,200 --> 00:09:18,980
So here, the water is acting
as an acid giving off a

125
00:09:18,980 --> 00:09:25,040
hydrogen ion to this
h c o 3 minus.

126
00:09:25,040 --> 00:09:28,200
And so now we have a second
hydrogen ion over here, we

127
00:09:28,200 --> 00:09:33,300
have the h 2 species, and that
the conjugate of water is o h.

128
00:09:33,300 --> 00:09:36,610
So this is acting as a base,
it's accepting a hydrogen ion,

129
00:09:36,610 --> 00:09:39,850
this is donating it, it's an
acid, this is the conjugate

130
00:09:39,850 --> 00:09:42,310
acid of that base, and
this is the conjugate

131
00:09:42,310 --> 00:09:43,770
base of that acid.

132
00:09:43,770 --> 00:09:46,830
So in the reverse direction,
this is an acid giving off a

133
00:09:46,830 --> 00:09:51,170
hydrogen ion to the hydroxide
forming water.

134
00:09:51,170 --> 00:09:53,590
So one thing that you'll notice
about these examples

135
00:09:53,590 --> 00:09:56,850
that we've written up is that
when you see water in the

136
00:09:56,850 --> 00:09:59,250
equation, you don't really know
what it's going to be

137
00:09:59,250 --> 00:10:02,270
doing until you actually look
at what the products are and

138
00:10:02,270 --> 00:10:03,640
then you can figure it out.

139
00:10:03,640 --> 00:10:07,020
So water can act as either
an acid or a

140
00:10:07,020 --> 00:10:09,290
base in these equations.

141
00:10:09,290 --> 00:10:16,280
And if we go to the lecture
notes, the term is amphoteric,

142
00:10:16,280 --> 00:10:19,910
which is a molecule that can
act as either an acid or a

143
00:10:19,910 --> 00:10:22,620
base, depending on the
reaction conditions.

144
00:10:22,620 --> 00:10:24,640
So depending on if it's mixed
with something that's a

145
00:10:24,640 --> 00:10:27,730
stronger acid or a stronger
base than it is.

146
00:10:27,730 --> 00:10:34,400
And an example, one of the most
common examples is water.

147
00:10:34,400 --> 00:10:39,960
So now let's consider a broader
example of acids and

148
00:10:39,960 --> 00:10:43,250
bases, and these are Lewis acids
and bases, and we're

149
00:10:43,250 --> 00:10:46,150
going to actually come back to
this around Thanksgiving time

150
00:10:46,150 --> 00:10:48,410
when we talk about transition
metals.

151
00:10:48,410 --> 00:10:51,780
And so here it's really broad
-- we're not actually even

152
00:10:51,780 --> 00:10:55,370
going to talk about a
hydrogen ion at all.

153
00:10:55,370 --> 00:10:59,790
So in this case, we're talking
about a Lewis base as a

154
00:10:59,790 --> 00:11:04,550
species that donates lone pair
electrons, and a Lewis acid is

155
00:11:04,550 --> 00:11:09,260
a species that accepts
such electrons.

156
00:11:09,260 --> 00:11:12,650
So here would be an example.

157
00:11:12,650 --> 00:11:16,120
So we can think about forming a
complex, and which thing is

158
00:11:16,120 --> 00:11:20,420
going to act as an
acid or a base.

159
00:11:20,420 --> 00:11:24,320
One will be donating its lone
pair electrons and the other

160
00:11:24,320 --> 00:11:26,160
will be accepting.

161
00:11:26,160 --> 00:11:29,320
So this is a very broad, a much
broader definition, and

162
00:11:29,320 --> 00:11:31,790
so when you talk about acid base
here, so always say as a

163
00:11:31,790 --> 00:11:38,060
Lewis acid or Lewis base to make
it clear what's going on.

164
00:11:38,060 --> 00:11:41,380
So again we have our base
donating its lone pair

165
00:11:41,380 --> 00:11:44,010
electrons and the
acid accepting.

166
00:11:44,010 --> 00:11:47,660
All right, so those are
our definitions

167
00:11:47,660 --> 00:11:50,680
of acids and bases.

168
00:11:50,680 --> 00:11:56,060
So now let's come back to this
issue of water and how water

169
00:11:56,060 --> 00:12:00,050
can act as an acid or a base.

170
00:12:00,050 --> 00:12:03,760
So if it can act as an acid or
a base, it seems like it can

171
00:12:03,760 --> 00:12:08,380
react with itself to do some
chemistry, and it can.

172
00:12:08,380 --> 00:12:11,700
So up here, you could have one
water molecule acting as an

173
00:12:11,700 --> 00:12:15,310
acid, giving up its hydrogen ion
to another water acting as

174
00:12:15,310 --> 00:12:18,460
a base, forming hydronium
ion and also

175
00:12:18,460 --> 00:12:21,580
forming hydroxide ion.

176
00:12:21,580 --> 00:12:26,040
So then you can ask the
question, well, how much h 2 o

177
00:12:26,040 --> 00:12:28,630
is in a typical glass
of water.

178
00:12:28,630 --> 00:12:31,810
How much, so you don't like
the idea that I'm drinking

179
00:12:31,810 --> 00:12:34,150
hydroxide ions, how much
hydronium ion and how much

180
00:12:34,150 --> 00:12:40,230
hydroxide ion are in this glass
of water, how much h 2 o

181
00:12:40,230 --> 00:12:44,510
is in a glass of water?

182
00:12:44,510 --> 00:12:47,400
So that's the question.

183
00:12:47,400 --> 00:12:52,440
So here's the equation again,
and we can think about how to

184
00:12:52,440 --> 00:12:53,490
calculate it.

185
00:12:53,490 --> 00:12:58,330
What do we really want to
know in this question?

186
00:12:58,330 --> 00:13:00,060
What are we really asking?

187
00:13:00,060 --> 00:13:05,730
How much, at an equilibrium
situation, how much are

188
00:13:05,730 --> 00:13:09,170
products and how much reactants
do you have?

189
00:13:09,170 --> 00:13:12,530
What can you tell me about
ratios of products and

190
00:13:12,530 --> 00:13:15,910
reactants at equilibrium?

191
00:13:15,910 --> 00:13:20,030
K. And how are some ways
you can calculate k's?

192
00:13:20,030 --> 00:13:29,690
Different terms, but, right,
it's what about -- what is k?

193
00:13:29,690 --> 00:13:32,690
So we have the equilibrium
constant, and there a couple

194
00:13:32,690 --> 00:13:35,330
different ways one might
calculate k -- you might be

195
00:13:35,330 --> 00:13:40,850
given concentrations at
equilibrium, or you might be

196
00:13:40,850 --> 00:13:45,030
given information
about delta g.

197
00:13:45,030 --> 00:13:51,060
So you can calculate k's from
delta g nought, and this is an

198
00:13:51,060 --> 00:13:53,690
equation that you'll use pretty
often -- delta g nought

199
00:13:53,690 --> 00:13:57,320
equals minus r t natural
log of k.

200
00:13:57,320 --> 00:14:00,560
So if we want to know k, we
need to find out what

201
00:14:00,560 --> 00:14:02,580
delta g nought is.

202
00:14:02,580 --> 00:14:06,530
What are ways to calculate
delta g?

203
00:14:06,530 --> 00:14:12,010
So you've seen some of those
-- oh, you're probably

204
00:14:12,010 --> 00:14:13,100
recognizing these already,

205
00:14:13,100 --> 00:14:15,350
temperature and our gas constant.

206
00:14:15,350 --> 00:14:19,480
So we can solve for k
in terms of delta g.

207
00:14:19,480 --> 00:14:23,350
And how do we calculate
that delta g.

208
00:14:23,350 --> 00:14:25,930
Well, there are a
couple of ways.

209
00:14:25,930 --> 00:14:32,990
So you can think about the delta
g's of formations, and

210
00:14:32,990 --> 00:14:37,980
we can think about one of my
personal favorites, which is

211
00:14:37,980 --> 00:14:41,770
relationship between delta
h and t delta s.

212
00:14:41,770 --> 00:14:45,220
So you can think about your
enthalpies and your entropies

213
00:14:45,220 --> 00:14:48,450
at a certain temperature, and
you can calculate delta g, and

214
00:14:48,450 --> 00:14:50,920
from that you can get the
equilibrium constant.

215
00:14:50,920 --> 00:14:54,790
So this is just a little review
showing the relevance

216
00:14:54,790 --> 00:14:57,775
of material you've learned
before to the material we're

217
00:14:57,775 --> 00:14:59,800
covering now.

218
00:14:59,800 --> 00:15:04,610
So we're going to pick one and
just calculate the delta g.

219
00:15:04,610 --> 00:15:09,220
So we can look up these values
of formation for our products

220
00:15:09,220 --> 00:15:13,510
and our reactants and plug them
in, and we get a value

221
00:15:13,510 --> 00:15:16,890
for delta g nought
of plus 79 .

222
00:15:16,890 --> 00:15:20,310
89 kilojoules per mole.

223
00:15:20,310 --> 00:15:23,830
So we have a positive
value here.

224
00:15:23,830 --> 00:15:26,770
So without doing any more math,
which we'll do it a

225
00:15:26,770 --> 00:15:32,530
minute, do you expect a large
or small value for k for the

226
00:15:32,530 --> 00:15:36,630
equilibrium constant if delta
g nought is positive 79 .

227
00:15:36,630 --> 00:15:37,580
89 kilojoules per mole.

228
00:15:37,580 --> 00:15:39,280
STUDENT: Small.

229
00:15:39,280 --> 00:15:42,130
PROFESSOR: We would expect
it to be small.

230
00:15:42,130 --> 00:15:46,140
And you probably already knew
that that there's a lot of h 2

231
00:15:46,140 --> 00:15:47,540
o in a glass of water.

232
00:15:47,540 --> 00:15:50,660
So again, from that perspective,
we'd also expect

233
00:15:50,660 --> 00:15:51,970
it to be small.

234
00:15:51,970 --> 00:15:56,190
So now we can plug those values
in, we calculated this

235
00:15:56,190 --> 00:15:59,950
delta g, we know the gas
constant, we're at room

236
00:15:59,950 --> 00:16:04,820
temperature, and so we get
a value of k as 1 .

237
00:16:04,820 --> 00:16:11,610
0 times 10 to the minus 14 at
room temperature, and that's a

238
00:16:11,610 --> 00:16:14,040
small number.

239
00:16:14,040 --> 00:16:19,030
So, the very small value
indicates that only a small

240
00:16:19,030 --> 00:16:25,280
percentage all your h 2 0 has
ionized, and that mostly,

241
00:16:25,280 --> 00:16:29,740
there's h 2 o in a glass of
water, not so many ions.

242
00:16:29,740 --> 00:16:33,100
Not many of the molecules have
ionized, because k is a small

243
00:16:33,100 --> 00:16:37,010
number, not a lot of products
at this equilibrium.

244
00:16:37,010 --> 00:16:41,700
So there's a lot of h 2
o in a glass of water.

245
00:16:41,700 --> 00:16:45,080
So, this particular k has
a special name, and

246
00:16:45,080 --> 00:16:48,530
it's k w, w for water.

247
00:16:48,530 --> 00:16:52,780
And this term and this number,
if you haven't memorized it in

248
00:16:52,780 --> 00:16:55,240
high school, you probably will
by the time you're done with

249
00:16:55,240 --> 00:16:56,470
problem-sets.

250
00:16:56,470 --> 00:16:59,270
This is a very valuable number,
you'll be using it a

251
00:16:59,270 --> 00:17:03,890
lot in calculating acid base
problems, and you will end up

252
00:17:03,890 --> 00:17:08,820
memorizing it whether
you want to or not.

253
00:17:08,820 --> 00:17:16,710
So, then k w equals your
hydronium ion concentration

254
00:17:16,710 --> 00:17:21,070
times your hydroxide
ion concentration.

255
00:17:21,070 --> 00:17:25,210
Now for a minute, let's consider
why that's true.

256
00:17:25,210 --> 00:17:30,270
So our reaction, it's products
over reactants for an

257
00:17:30,270 --> 00:17:33,540
equilibrium constant, but now,
all of a sudden, I don't have

258
00:17:33,540 --> 00:17:36,140
my reactants going on in here.

259
00:17:36,140 --> 00:17:39,700
So the k w is expressed in terms
of the concentration a

260
00:17:39,700 --> 00:17:43,020
hydronium ions times the
concentration of hydroxide,

261
00:17:43,020 --> 00:17:46,140
and it doesn't have this water
term at the bottom.

262
00:17:46,140 --> 00:17:50,070
And that'll be true for
any problem in which

263
00:17:50,070 --> 00:17:52,210
the water is a solvent.

264
00:17:52,210 --> 00:17:56,510
And so, it's really not going
to be changing very much.

265
00:17:56,510 --> 00:18:00,100
A solvent is nearly pure, and
when you have in a nearly pure

266
00:18:00,100 --> 00:18:04,830
solvent or solid, it's not
included in the equilibrium

267
00:18:04,830 --> 00:18:05,780
expression.

268
00:18:05,780 --> 00:18:09,790
So we'll see other examples
of this as we go along.

269
00:18:09,790 --> 00:18:13,420
So you always want to ask
yourself is this the solvent.

270
00:18:13,420 --> 00:18:19,000
If so, we're talking about very
dilute things going on in

271
00:18:19,000 --> 00:18:21,140
solvent, the solvent
concentration isn't changing

272
00:18:21,140 --> 00:18:26,170
very much, so it drops
out of our term.

273
00:18:26,170 --> 00:18:30,070
So, because k w is an
equilibrium constant, the

274
00:18:30,070 --> 00:18:33,690
products are always going to be
equal to the same thing at

275
00:18:33,690 --> 00:18:35,060
the same temperature.

276
00:18:35,060 --> 00:18:39,780
So, at room temperature, or 298
kelvin, it's always going

277
00:18:39,780 --> 00:18:40,970
to be equal to 1 .

278
00:18:40,970 --> 00:18:43,650
0 times 10 to the minus 14,
and that's why it's such a

279
00:18:43,650 --> 00:18:45,910
valuable number.

280
00:18:45,910 --> 00:18:49,310
And when we're talking about
acid base problems, you're

281
00:18:49,310 --> 00:18:53,310
almost always going to be at
room temperature, just to not

282
00:18:53,310 --> 00:18:55,930
make life more complicated
for you.

283
00:18:55,930 --> 00:18:59,040
So, you can pretty much assume,
it should be in big

284
00:18:59,040 --> 00:19:01,200
bold letters if the temperature
is not room

285
00:19:01,200 --> 00:19:06,810
temperature, so that you
can use these values.

286
00:19:06,810 --> 00:19:14,290
All right, so let's look at
the p h function now.

287
00:19:14,290 --> 00:19:18,000
So what does p h equal?

288
00:19:18,000 --> 00:19:24,200
So p h is equal to the minus the
log of the hydronium ion

289
00:19:24,200 --> 00:19:30,550
concentration, and let's also
talk about p o h, and that's

290
00:19:30,550 --> 00:19:39,190
equal to minus log of the
hydroxide ion concentration.

291
00:19:39,190 --> 00:19:45,000
I just told you that k w is
equal to the concentration of

292
00:19:45,000 --> 00:19:48,140
hydronium ions times
the concentration

293
00:19:48,140 --> 00:19:51,160
of hydroxide ions.

294
00:19:51,160 --> 00:19:54,160
And now we can express
this in another very

295
00:19:54,160 --> 00:19:56,800
useful way for you.

296
00:19:56,800 --> 00:20:02,280
If we take the log of all sides,
actually let's take the

297
00:20:02,280 --> 00:20:13,260
minus log of everything, so
minus log of k w equals the

298
00:20:13,260 --> 00:20:23,180
minus log of hydronium ion
concentration, minus the log

299
00:20:23,180 --> 00:20:28,890
of hydroxide ion concentration,
and we end up

300
00:20:28,890 --> 00:20:35,760
with terms of p k w being equal
to p h, because minus

301
00:20:35,760 --> 00:20:40,710
log of the hydronium ion
concentration is p h, and

302
00:20:40,710 --> 00:20:45,350
minus the log of the hydroxide
concentration is p o h.

303
00:20:45,350 --> 00:20:47,990
So plus p o h.

304
00:20:47,990 --> 00:20:51,960
And we know that this term at
room temperature is 1 .

305
00:20:51,960 --> 00:20:56,670
0 times 10 to the minus 14.

306
00:20:56,670 --> 00:21:00,730
So this term at room
temperature is 14 .

307
00:21:00,730 --> 00:21:09,940
0 0, again at 25 degrees
c or 298 kelvin.

308
00:21:09,940 --> 00:21:12,920
So this is also a useful
expression.

309
00:21:12,920 --> 00:21:19,190
If you know the p h, you can
calculate the p o h if you're

310
00:21:19,190 --> 00:21:24,080
at room temperature, remembering
this number of 14.

311
00:21:24,080 --> 00:21:26,440
So these are things that you
will be doing a lot in the

312
00:21:26,440 --> 00:21:28,980
problems and you will start
remembering all of these

313
00:21:28,980 --> 00:21:31,910
numbers really well.

314
00:21:31,910 --> 00:21:36,040
So p h and p o h, what
does p h do for you?

315
00:21:36,040 --> 00:21:39,850
Well, the p h tells you about
the strength of the acid.

316
00:21:39,850 --> 00:21:48,060
So the p h of pure water should
be neutral, which is 7.

317
00:21:48,060 --> 00:21:51,420
And now, tell me what the
p h of an acid is and

318
00:21:51,420 --> 00:22:09,530
the p h of a base.

319
00:22:09,530 --> 00:22:11,910
Let's just do 10 seconds
on this, this is pretty

320
00:22:11,910 --> 00:22:12,240
straightforward.

321
00:22:12,240 --> 00:22:28,140
So this tests previous knowledge
on this topic, and

322
00:22:28,140 --> 00:22:30,250
it is very good.

323
00:22:30,250 --> 00:22:34,390
People know about what
the p h's are.

324
00:22:34,390 --> 00:22:36,550
So that's right.

325
00:22:36,550 --> 00:22:40,810
So the p h of an acid solution
is less than 7, and the p h of

326
00:22:40,810 --> 00:22:42,950
a base solution is
greater than 7.

327
00:22:42,950 --> 00:22:48,170
And the EPA defines corrosive as
something where the p h is

328
00:22:48,170 --> 00:22:52,150
lower than 3 or greater
than 12 .

329
00:22:52,150 --> 00:22:54,440
5.

330
00:22:54,440 --> 00:23:01,340
So, if here is our scale of p
h, we're neutral at 7, we're

331
00:23:01,340 --> 00:23:05,200
acidic below 7, and we're
corrosive below 3.

332
00:23:05,200 --> 00:23:08,870
We're basic above 7, and
corrosive above 12 .

333
00:23:08,870 --> 00:23:09,200
5.

334
00:23:09,200 --> 00:23:14,280
So now what I want to do is ask
Dr. Taylor to come up and

335
00:23:14,280 --> 00:23:18,320
we're going to measure some p
h's of things, so we're going

336
00:23:18,320 --> 00:23:22,930
to be interested in knowing
how much danger you're in

337
00:23:22,930 --> 00:23:24,140
around MIT.

338
00:23:24,140 --> 00:23:35,390
So we're going to have
you measure them.

339
00:23:35,390 --> 00:23:38,890
We have these little strips on
it, and so someone will come

340
00:23:38,890 --> 00:23:42,280
around and help you read it, so
you can read off the strip

341
00:23:42,280 --> 00:23:49,620
of what you have. Should
we start with water?

342
00:23:49,620 --> 00:23:51,630
This is random MIT water.

343
00:23:51,630 --> 00:24:00,180
Let's start there.

344
00:24:00,180 --> 00:24:02,690
So, just pick a volunteer.

345
00:24:02,690 --> 00:24:22,190
PROFESSOR: OK, so what we're
going to do is have the TA's

346
00:24:22,190 --> 00:24:25,420
go in and ask you to read off
of a p h strip, what the p h

347
00:24:25,420 --> 00:24:27,550
of various things are, and
actually Marcus, if you could

348
00:24:27,550 --> 00:24:30,920
write on the board
what these are.

349
00:24:30,920 --> 00:24:33,150
So we'll start with MIT water.

350
00:24:33,150 --> 00:24:37,870
So, we know if it's 7 it's
neutral, if it's below 7 we're

351
00:24:37,870 --> 00:24:40,750
talking about acidic, and
above that it's basic.

352
00:24:40,750 --> 00:24:44,050
We actually, also for you to be
able to visualize as well,

353
00:24:44,050 --> 00:24:47,550
what I did is I just boiled up
some cabbage last night and

354
00:24:47,550 --> 00:24:49,530
brought in the extract
with me.

355
00:24:49,530 --> 00:24:54,690
And cabbage actually has
anthocyanins in it, which is a

356
00:24:54,690 --> 00:24:58,660
color indicator, and it changes
color based on whether

357
00:24:58,660 --> 00:25:01,190
it's in an acidic or
a basic solution.

358
00:25:01,190 --> 00:25:03,550
So we'll let you
see this here.

359
00:25:03,550 --> 00:25:06,530
It looks like MIT water,
pretty safe to drink,

360
00:25:06,530 --> 00:25:08,300
which is good news.

361
00:25:08,300 --> 00:25:18,540
We can go ahead and -- so it
looks like if we add MIT water

362
00:25:18,540 --> 00:25:21,530
to cabbage solution, what do you
think's going to happen --

363
00:25:21,530 --> 00:25:24,630
this is neutral right now.

364
00:25:24,630 --> 00:25:25,930
Hopefully not much.

365
00:25:25,930 --> 00:25:29,540
We either have invalid
strips or we'll see

366
00:25:29,540 --> 00:25:31,060
nothing happen here.

367
00:25:31,060 --> 00:25:33,690
All right, so you can see we
still have the purple color

368
00:25:33,690 --> 00:25:34,590
for MIT water.

369
00:25:34,590 --> 00:25:38,270
Two confirmations that it's safe
to drink right out of the

370
00:25:38,270 --> 00:25:39,910
tap when you get home.

371
00:25:39,910 --> 00:25:43,210
So, the next thing is
vinegar did someone

372
00:25:43,210 --> 00:25:44,500
take the, the strip?

373
00:25:44,500 --> 00:25:46,500
STUDENT: We have
a 2 and a 1/2.

374
00:25:46,500 --> 00:25:47,840
PROFESSOR: OK, so are
we drinking vinegar?

375
00:25:47,840 --> 00:25:49,670
STUDENT: No.

376
00:25:49,670 --> 00:25:50,980
PROFESSOR: Probably
not straight.

377
00:25:50,980 --> 00:25:57,490
All right, so we've
got our cabbage

378
00:25:57,490 --> 00:25:59,820
extract here, it's purple.

379
00:25:59,820 --> 00:26:01,950
Does anyone have a guess as to
what color it's going to turn

380
00:26:01,950 --> 00:26:03,860
if we pour in vinegar,
very acidic.

381
00:26:03,860 --> 00:26:07,890
All right, a couple guesses
I hear, blue and pink.

382
00:26:07,890 --> 00:26:09,870
They're both good guesses
because different color

383
00:26:09,870 --> 00:26:12,900
indicators turn different
colors.

384
00:26:12,900 --> 00:26:21,220
But, all right, looks like a
dramatic difference here.

385
00:26:21,220 --> 00:26:24,030
What do we have next
out there?

386
00:26:24,030 --> 00:26:29,780
Baking soda.

387
00:26:29,780 --> 00:26:31,700
Seven for the baking soda.

388
00:26:31,700 --> 00:26:34,130
I'm going to guess we did not
pour in enough or it is not

389
00:26:34,130 --> 00:26:35,550
dissolved here.

390
00:26:35,550 --> 00:26:38,140
All right, so let's do our
secondary test here and see

391
00:26:38,140 --> 00:26:39,870
what happens with
the baking soda.

392
00:26:39,870 --> 00:26:49,426
PROFESSOR: [UNINTELLIGIBLE] the
water we added it to was

393
00:26:49,426 --> 00:26:49,480
also not neutral.

394
00:26:49,480 --> 00:26:50,950
PROFESSOR: All right, so we're
actually pretty basic with the

395
00:26:50,950 --> 00:26:51,640
baking soda.

396
00:26:51,640 --> 00:26:53,230
We won't give it a number
because it was just

397
00:26:53,230 --> 00:26:54,140
dissolved in water.

398
00:26:54,140 --> 00:27:04,250
But we'll remember baking soda,
blue here is basic.

399
00:27:04,250 --> 00:27:06,850
So the next thing we're going to
test is soda that you drink

400
00:27:06,850 --> 00:27:07,520
all the time.

401
00:27:07,520 --> 00:27:08,590
I brought Sprite.

402
00:27:08,590 --> 00:27:11,240
Coke probably would have
been a good pick as

403
00:27:11,240 --> 00:27:13,370
well or Diet Coke.

404
00:27:13,370 --> 00:27:15,780
So, we'll test what that is.

405
00:27:15,780 --> 00:27:26,250
Hopefully it comes out
neutral, right?

406
00:27:26,250 --> 00:27:28,050
So while we're waiting, we'll
start taking a little

407
00:27:28,050 --> 00:27:30,900
bit of a look here.

408
00:27:30,900 --> 00:27:34,110
All right, we've got a 3.

409
00:27:34,110 --> 00:27:34,520
Soda, corrosive.

410
00:27:34,520 --> 00:27:41,290
It's not just the sugar that's
bad for your teeth.

411
00:27:41,290 --> 00:27:46,270
Luckily we see here it is not
quite as bad as vinegar in

412
00:27:46,270 --> 00:27:49,260
terms of how acidic it is, but
we definitely have a color

413
00:27:49,260 --> 00:27:50,070
change here.

414
00:27:50,070 --> 00:28:00,228
PROFESSOR: Has anyone
used soda for

415
00:28:00,228 --> 00:28:01,780
something other than drinking?

416
00:28:01,780 --> 00:28:03,332
What did you use it for?

417
00:28:03,332 --> 00:28:04,500
STUDENT: Cleaning pennies.

418
00:28:04,500 --> 00:28:07,950
PROFESSOR: Cleaning pennies,
what else?

419
00:28:07,950 --> 00:28:12,185
STUDENT: When you have stuff all
over your car battery you

420
00:28:12,185 --> 00:28:13,430
can use Coke.

421
00:28:13,430 --> 00:28:17,580
PROFESSOR: And some of those
other uses make sense.

422
00:28:17,580 --> 00:28:17,720
[UNINTELLIGIBLE]

423
00:28:17,720 --> 00:28:17,960
What else?

424
00:28:17,960 --> 00:28:18,150
STUDENT: Taking the
galvanization

425
00:28:18,150 --> 00:28:18,210
off of a steel wire.

426
00:28:18,210 --> 00:28:18,570
PROFESSOR: OK, so cleaning
steel wire.

427
00:28:18,570 --> 00:28:19,080
How many of you still drink soda
knowing this information?

428
00:28:19,080 --> 00:28:40,230
PROFESSOR: All right, so the
next thing we put out there

429
00:28:40,230 --> 00:28:42,140
was aspirin dissolved in water,
and it's going to

430
00:28:42,140 --> 00:28:44,950
depend what concentration we
did, but we put aspirin in

431
00:28:44,950 --> 00:28:47,580
water and we got a 3 here.

432
00:28:47,580 --> 00:28:53,560
So, aspirin sometimes gives
you an upset stomach, and

433
00:28:53,560 --> 00:28:57,880
that's why Tylenol's an
improvement in some ways --

434
00:28:57,880 --> 00:28:59,640
obviously, that has its
own drawbacks, too.

435
00:28:59,640 --> 00:29:02,340
But you can see what your
stomach on aspirin might be

436
00:29:02,340 --> 00:29:04,190
feeling like here.

437
00:29:04,190 --> 00:29:07,210
So if you're having an upset
stomach, something you might

438
00:29:07,210 --> 00:29:12,880
do is take Tums or Mylanta or
some other kind of a -- yeah,

439
00:29:12,880 --> 00:29:20,200
let's measure the p h here.

440
00:29:20,200 --> 00:29:23,380
So if you decide to take some
Milk of Magnesia after an

441
00:29:23,380 --> 00:29:25,180
upset stomach, are you
hoping it will be

442
00:29:25,180 --> 00:29:27,730
acidic or basic here?

443
00:29:27,730 --> 00:29:28,290
STUDENT: Basic.

444
00:29:28,290 --> 00:29:36,670
PROFESSOR: All right, let's
see what we get.

445
00:29:36,670 --> 00:29:38,920
All right, so this is
kind of thicker.

446
00:29:38,920 --> 00:29:41,800
Let's see how this works.

447
00:29:41,800 --> 00:29:46,380
I think you can start to see
the green at the bottom.

448
00:29:46,380 --> 00:29:50,500
So, it's white in here, so
this is not just color.

449
00:29:50,500 --> 00:29:53,880
So we'll let that
slowly mix in.

450
00:29:53,880 --> 00:29:56,200
What did we get for a p h, if
you can read it on there.

451
00:29:56,200 --> 00:29:57,160
This might be another no-go.

452
00:29:57,160 --> 00:29:59,920
STUDENT: It says 7, but--

453
00:29:59,920 --> 00:30:00,200
PROFESSOR: It's probably
blue--

454
00:30:00,200 --> 00:30:26,630
PROFESSOR: So, how many people
have had lunch yet today?

455
00:30:26,630 --> 00:30:26,720
How many are going to
have lunch soon?

456
00:30:26,720 --> 00:30:27,212
How many are reconsidering what
they're going to eat for

457
00:30:27,212 --> 00:30:27,300
lunch based on this demo?

458
00:30:27,300 --> 00:30:34,410
PROFESSOR: Al right, we'll take
a look at one last thing

459
00:30:34,410 --> 00:30:34,510
that you might be consuming.

460
00:30:34,510 --> 00:30:37,990
Lemon juice, or actually this
is lime juice here.

461
00:30:37,990 --> 00:30:39,820
All right, we're probably ending
with an easy one here.

462
00:30:39,820 --> 00:30:43,690
What do you think, acidic or
basic for the lime juice.

463
00:30:43,690 --> 00:30:47,760
So, really, the question is
probably just the shade that

464
00:30:47,760 --> 00:30:50,310
we're going to get from going
from a purple here.

465
00:30:50,310 --> 00:31:05,130
All right, so what are we
reading for the lime juice?

466
00:31:05,130 --> 00:31:24,850
A 2, OK.

467
00:31:24,850 --> 00:31:28,980
PROFESSOR: OK, so that's the
end of our demo here,

468
00:31:28,980 --> 00:31:33,420
providing you with some tips
about what is corrosive and

469
00:31:33,420 --> 00:31:38,900
what is not corrosive
around MIT.

470
00:31:38,900 --> 00:31:42,570
And I think the title of this
lecture on the syllabus is "is

471
00:31:42,570 --> 00:31:48,340
it safe to drink the water at
MIT," and the answer is a lot

472
00:31:48,340 --> 00:31:50,740
safer than drinking soda.

473
00:31:50,740 --> 00:31:58,030
So let's talk about some acids
in water some more, and

474
00:31:58,030 --> 00:32:03,820
introduce something you'll use
a lot, which is an acid

475
00:32:03,820 --> 00:32:08,750
ionization constant.

476
00:32:08,750 --> 00:32:18,830
So let's look at an example
of an acid in water.

477
00:32:18,830 --> 00:32:28,240
So say we have c h 3, c o o h
aqueous, so it's in water,

478
00:32:28,240 --> 00:32:30,100
which is our solvent.

479
00:32:30,100 --> 00:32:33,580
It's acting as an acid, so it's
giving off a hydrogen ion

480
00:32:33,580 --> 00:32:41,160
to water forming hydronium
ion, and forming its

481
00:32:41,160 --> 00:32:47,480
conjugate, which is missing
its hydrogen ion.

482
00:32:47,480 --> 00:32:51,480
All right, so here we have an
equation, and now we're going

483
00:32:51,480 --> 00:32:54,610
to introduce something which
is the acid ionization

484
00:32:54,610 --> 00:32:59,475
constant, or k a, and you'll
be using k a a lot in this

485
00:32:59,475 --> 00:33:02,680
course, we're also going to
have some k b's for bases.

486
00:33:02,680 --> 00:33:05,660
So the acid ionization
constant.

487
00:33:05,660 --> 00:33:08,040
And it's an equilibrium
constant, so you all know how

488
00:33:08,040 --> 00:33:11,290
to write equilibrium
constants.

489
00:33:11,290 --> 00:33:14,960
So the equilibrium constant is
going to be products, which in

490
00:33:14,960 --> 00:33:21,840
this case is hydronium ions
times the concentration of the

491
00:33:21,840 --> 00:33:32,870
conjugate base of the acid
over the conjugate acid.

492
00:33:32,870 --> 00:33:36,300
And there is no water in that
equation, because here the

493
00:33:36,300 --> 00:33:39,390
water is pretty much pure.

494
00:33:39,390 --> 00:33:42,210
It's the solvent so its
concentration is not going to

495
00:33:42,210 --> 00:33:45,520
change very much, so
it is left off.

496
00:33:45,520 --> 00:33:48,200
And I can tell you that
the ionization

497
00:33:48,200 --> 00:33:50,140
constant here is 1 .

498
00:33:50,140 --> 00:33:53,850
76 times 10 to the minus 5.

499
00:33:53,850 --> 00:33:55,653
Again, that's temperature
dependent, so

500
00:33:55,653 --> 00:33:57,700
that's at 25 degrees.

501
00:33:57,700 --> 00:34:01,570
This is a small number, so that
tells us this is not a

502
00:34:01,570 --> 00:34:04,290
very strong acid.

503
00:34:04,290 --> 00:34:08,150
So it's not ionizing very
much in solution.

504
00:34:08,150 --> 00:34:12,870
That is a definition of a weak
acid, something that doesn't

505
00:34:12,870 --> 00:34:14,310
ionize very much.

506
00:34:14,310 --> 00:34:17,800
The definition of a strong acid
is something that does

507
00:34:17,800 --> 00:34:19,620
ionize quite a bit.

508
00:34:19,620 --> 00:34:26,870
So here then, we can write
equations generically for

509
00:34:26,870 --> 00:34:28,510
acids and bases.

510
00:34:28,510 --> 00:34:31,890
You can write an equation
generically, h a being as your

511
00:34:31,890 --> 00:34:35,030
acid, plus water, goes to
hydronium ions, and your

512
00:34:35,030 --> 00:34:39,530
conjugate of the acid,
which is a minus.

513
00:34:39,530 --> 00:34:42,200
So this is an acid,
h a, in water.

514
00:34:42,200 --> 00:34:47,780
We can also write it as h b plus
as an acid in water going

515
00:34:47,780 --> 00:34:50,140
to hydronium ion concentrations
and the

516
00:34:50,140 --> 00:34:54,460
conjugate, which is base, it's
lost its hydrogen ion as well.

517
00:34:54,460 --> 00:34:56,855
A strong acid is something
that has a k a

518
00:34:56,855 --> 00:34:58,010
greater than one.

519
00:34:58,010 --> 00:35:02,190
More products than reactant
at equilibrium.

520
00:35:02,190 --> 00:35:05,850
So that means it ionizes almost
completely, so it goes

521
00:35:05,850 --> 00:35:10,070
far toward products, ionizes
almost completely.

522
00:35:10,070 --> 00:35:14,590
A weak acid is something with a
k a of less than one, which

523
00:35:14,590 --> 00:35:17,960
means that when you put this
acid in water, it doesn't

524
00:35:17,960 --> 00:35:23,480
ionize very much, when
you have equilibrium.

525
00:35:23,480 --> 00:35:27,760
So you can tell if something
is a strong acid or not by

526
00:35:27,760 --> 00:35:32,410
looking at it's k a value, or
alternatively you can consider

527
00:35:32,410 --> 00:35:34,430
something called p k a.

528
00:35:34,430 --> 00:35:40,780
So the p k a is minus log of the
k a, and if you have a low

529
00:35:40,780 --> 00:35:44,740
value of k a, you'll have a
higher p k a, and the higher

530
00:35:44,740 --> 00:35:47,990
the p k a then, the
weaker the acid.

531
00:35:47,990 --> 00:35:52,350
So you can look it k a or you
can think about p k a in terms

532
00:35:52,350 --> 00:35:55,660
of whether something is
a strong acid or not.

533
00:35:55,660 --> 00:35:59,230
And so we'll just finish
up with this slide.

534
00:35:59,230 --> 00:36:02,360
So, up here we have some
very strong acids.

535
00:36:02,360 --> 00:36:07,670
These k a's are much greater
than one, and you have

536
00:36:07,670 --> 00:36:10,950
extremely low values
for p k a.

537
00:36:10,950 --> 00:36:13,790
And if you keep going from
strong acids, again, a strong

538
00:36:13,790 --> 00:36:17,220
acid has a k a greater than one,
so these are all strong.

539
00:36:17,220 --> 00:36:19,930
And so, then weak acids
are less than one.

540
00:36:19,930 --> 00:36:24,200
And so down here you'd have
small numbers for k a, and so

541
00:36:24,200 --> 00:36:31,350
-- up here we have big k a's,
here we have smaller k a's,

542
00:36:31,350 --> 00:36:34,950
and the corresponding p k a's
are going up, and if we keep

543
00:36:34,950 --> 00:36:39,230
going, this table is very long,
we're going to get some

544
00:36:39,230 --> 00:36:42,320
very high p k a values
when you have some

545
00:36:42,320 --> 00:36:44,960
very, very small numbers.

546
00:36:44,960 --> 00:36:47,180
OK, we'll stop there for
today and continue

547
00:36:47,180 --> 00:36:49,050
acid base next time.