weak acids acids and bases can be classified as either strong or weak. here we’ll deal with weak...

Weak Acids

Acids and bases can be classified as either strong or weak. Here we’ll deal with weak acids

A Weak Acid is an acid that isless than 100% ionized in aqueous solution.

A Weak Acid is an acid that is less than 100% ionized in aqueous solution.


An example of a weak acid is acetic acid, CH3COOH

3 (aq) 2 ( ) 3 (aq) 3 (aq)CH COOH H O H O CH COOl

Acetic Acid


It reacts with water

2 ( ) 3 (aq) 3 (a3 ( q) qa )CH CO H O H O H CH OO COl

Acetic Acid


To produce

Acetic Acid



A hydronium ion.

33 (aq) 2 ( ) 3( a) )q qa (CH COOH H O CH COOH Ol

Acetic Acid


And an acetate or ethanoate ion.

3 (aq) 2 ( ) 3 (aq) 3 (aq)CH COO CH H O H O H COOl

Acetic Acid Acetate ion


Unlike strong acids, which ionize to completion, weak acids exist as equilibrium mixtures, as shown by the double arrow.


Acetic Acid Acetate ionAn

Equilibrium

For most of the weak acids dealt with in Chemistry 12 (click), the molecular form is highly favoured at equilibrium

3 (aq) 2 ( )CH CO HO OH l

The molecular form is favoured

3 (aq) 3 (aq)H O CH COO

And the concentrations of the ions are very low compared to that of the molecules.

3 (aq) 2 ( )CH COOH H O l 3 (aq) 3 (aq) H O CH COO

The concentrations of the ions are very

low

So a solution of acetic acid consists mostly of neutral CH3COOH molecules.

3 (aq) 2 ( )CH COOH H O l 3 (aq) 3 (aq)H O CH COO

3CH COOH

3CH COOH

3CH COO

3CH COOH3CH COOH

3CH COOH

3CH COOH

3CH COOH

3CH COOH

3H O

A solution of acetic acid is mostly

neutral CH3COOH molecules

And the concentrations of the ions in this solution are very low.


The concentrations of the ions are very

low

3CH COOH

3CH COOH

3CH COO

3CH COOH3CH COOH

3CH COOH

3CH COOH

3CH COOH

3CH COOH

3H O

A solution of acetic acid is mostly

neutral CH3COOH molecules

If we insert a conductivity apparatus into pure water, it does not conduct enough to make the bulb glow.


Now, we’ll add some acetic acid (click) to the water

3 (aq) 2 ( )CH CO HO OH l 3 (aq) 3 (aq)H O CH COO

3CH COOH

The acetic acid molecules spread out to fill the solution.


3CH COOH

3CH COOH

3CH COOH3CH COOH

3CH COOH

3CH COOH

3CH COOH

3CH COOH

A small number of the acetic acid molecules ionize, and the bulb glows dimly.


3CH COOH

3CH COOH

3CH COO

3CH COOH3CH COOH

3CH COOH

3CH COOH

3CH COOH

3CH COOH

3H O

Because there are a small number of ions present in the solution of a weak acid, the conductivity is not zero, but it is low.


3CH COOH

3CH COO

3CH COOH3CH COOH

3CH COOH 3CH COOH

3H O

3CH COOH3CH COOH

A small number of ions are present in the

solution

So weak acids that start out as neutral molecules, like CH3COOH are weak electrolytes.


3CH COOH

3CH COO

3CH COOH3CH COOH

3CH COOH 3CH COOH

3H O

3CH COOH3CH COOH

Molecular Weak Acids are Weak

Electrolytes

Where do we find

Weak Acids?

So now that we know what weak acids are, where do we find them?

You may recall that the 6 acids on the top left of the Acid table are classified as strong acids. Remember these are 100% ionized in aqueous solution.

Strong Acids

The species below hydronium on the left, all the way down to water can act as weak acids. Some of these, including water are amphiprotic so they can also act as weak bases, as we’ll see later.

Strong Acids

Weak Acids

Just a quick word about the two species on the very bottom of the left side, hydroxide and ammonia. These cannot acts as acids in aqueous solution.

Strong Acids

Weak Acids

NOT Acids

They are found on the right side of the acid table and are classified as bases.

Strong Acids

Weak Acids

NOT Acids

The only reason they are written here,

Strong Acids

Weak Acids

NOT Acids

Is that they happen to be conjugate acids of the bases O2- and NH2–

Strong Acids

Weak Acids

NOT Acids basesconjugate

acids

Notice they both have a single arrow pointing toward them, which is further verification that these are NOT acids. These reactions go only in reverse, not forward.

NOT Acids

Single arrow pointing

toward them

Looking at the weak acids, it is important to understand that (click) they get progressively weaker as we go down the left side of the table, from HIO3 to H2O.

G e

t w e

a k

e r

Because acids get weaker as we move down the table, we can also say that the degree of ionization decreases.

Deg

ree

of io

niz

atio

n d

ecre

ases

G e

t w e

a k

e r

This is indicated by the fact that the values of their ionization constant, Ka, get smaller as we move down the table. Take a look at these to verify this to yourself.

Ka

valu

es g

et s

malle

r

G e

t w e

a k

e r

Deg

ree

of io

niz

atio

n d

ecre

ases

Of course, we can also say that (click) weak acids progressively get stronger as we more Up on the left side, from water at the bottom, to HIO3 at the top .

G

e t

s t

r o

n

g

e

r

This means the degree of ionization increases as we move up.

G

e t

s t

r o

n

g

e

r

Deg

ree

of

Io

niz

ati

on

And again, this is reflected by the increase in Ka values as we move up.

G

e t

s t

r o

n

g

e

r

Deg

ree

of

Io

niz

ati

on

Ka

valu

es

g

et

la

rger

Molecular weak acids are indicated here by the red arrows. These are acids that are neutral molecules before they ionize.

Molecular Weak Acids

Because the degree of ionization decreases as we move down the table,


Deg

ree

of io

niz

atio

n d

ecre

ases

It means that the number of dissolved ions present in 1.0 Molar solutions of these molecular acids will decrease as we move down.


Deg

ree

of io

niz

atio

n d

ecre

ases

Nu

mb

er o

f dis

solv

ed

ion

s d

ecre

ases

So what do you think the trend in conductivity of molecular acids will be as we move down the column?


Deg

ree

of io

niz

atio

n d

ecre

ases

Nu

mb

er o

f dis

solv

ed

ion

s d

ecre

ases

Trend in Conductivity of Molecular Acids?

?

electrical conductivity depends on the number of dissolved ions in solution, so as we move down the column and the number of dissolved ions decreases,

Nu

mb

er o

f dis

solv

ed

ion

s d

ecre

ases

the conductivity of molecular acids also decreases. C

on

du

ctiv

ity o

f mole

cu

lar a

cid

s d

ecre

ases

Nu

mb

er o

f dis

solv

ed

ion

s d

ecre

ases

So if we were to compare the conductivity of phosphoric acid with that of boric acid, we would predict that phosphoric acid has higher conductivity than boric acid.

Higher conductivity

Lower conductivity

Con

du

ctiv

ity o

f mole

cu

lar a

cid

s d

ecre

ases

We can also see that higher conductivity correlates with a higher value for the ionization constant Ka.

Higher conductivity

Lower conductivity

Higher Ka value

Lower Ka value

Con

du

ctiv

ity o

f mole

cu

lar a

cid

s d

ecre

ases

Notice that many of the species that act as weak acids are ions to begin with.

Ionic Weak Acids

As expected, the ability of each of these to act as an acid (click) decreases as we move down the left side of the table

G e

t w

eaker a

s a

cid

s

This is reflected by a decrease in their Ka values as we move down. G

e t

weaker a

s a

cid

s

Ka

valu

es g

et s

malle

r

Because these are ions, they do not occur as substances themselves in nature. If its an anion, it would need to have an accompanying cation and if it’s a cation it would need to have an accompanying anion.

NaHSO4 Na+ + HSO4–

For example, the HSO4 minus ion could be accompanied by the spectator cation Na+.


Spectator cation

These two ions would result from the dissociation of the salt sodium bisulphate, NaH2SO4.


The source of HSO4

– could be a salt like NaHSO4

Similarly, the hydrogen oxalate ion HC2O4 minus could come from the dissociation of the salt potassium hydrogen oxalate, KHC2O4


KHC2O4 K+ + HC2O4–

The source of HC2O4–

could be a salt like KHC2O4

And the positive ion NH4+ could come from the dissociation of an ammonium salt like NH4Cl. The Cl minus ion is a spectator here.


KHC2O4 K+ + HC2O4–

NH4Cl NH4+ + Cl–

The source of NH4+

could be a salt like NH4Cl

Notice that the ammonium ion, NH4+ is low on the left side of the table and its Ka value, of 5.6 × 10–10 is quite small. This means NH4+ is quite a weak acid.

Quite a Weak Acid

Because NH4+ is quite a weak acid, we might expect its conductivity to be weak. So let’s try it. We’ll set up a conductivity apparatus and (click) add enough of the salt NH4Cl…

NH4Cl

water

To produce a solution of 0.1 molar NH4Cl.

0.1 MNH4Cl

Because NH4Cl is ionic, we know that it actually consists of a crystal lattice of NH4+ and Cl minus ions. We show one of each ion here. Now we’ll see what happens.

0.1 M4NH Cl

The salt NH4Cl will quickly and completely dissociate into free ammonium and chloride ions. And notice the light bulb glows brightly to show that we now have HIGH conductivity.

0.1 MCl4NH

As far as conductivity is concerned, it doesn’t matter that NH4+ is a weak acid. Because NH4Cl is a highly soluble ionic salt, it dissociates completely into ammonium and chloride ions, both with a concentration of 0.1 molar.

4NH

Cl0.1 M

0.1 M

So the total ion concentration in this solution is 0.1 plus 0.1

4NH

Cl0.1 M

0.1 M

Total ion concentration= 0.1 M + 0.1 M = 0.2 M

Which is 0.2 molar, high enough to account for the high conductivity.

4NH

Cl0.1 M

0.1 M

Total ion concentration= 0.1 M + 0.1 M = 0.2 M

Because NH4+ is quite a weak acid, most of it will remain as 0.1M NH4+ in solution.

4NH

Cl0.1 M

And only a tiny fraction of it will ionize into hydronium ions and ammonia molecules. This occurs to a very limited extent, so it will have no significant effect on already high total ion concentration in this solution.

4 2NH H O Cl

3 3H O NH

So if we compare acetic acid with the ammonium ion.

We see that NH4+ is a much weaker acid than CH3COOH.

NH4+ is a

weaker acid than CH3COOH

However 0.10 M NH4+ would have a higher conductivity than CH3COOH.

0.1 M NH4+ has a

higher conductivity than

0.10 M CH3COOH

This is because CH3COOH is a MOLECULAR weak acid. The only ions it produces in solution come from its limited ionization as a weak acid. Because 0.1 M CH3COOH has few ions, it is a poor conductor or weak electrolyte.

0.10 M CH3COOH is a

molecular weak acid

And although NH4+ is a weaker acid than CH3COOH, NH4+ is an IONIC weak acid. 0.1 M NH4+ ALREADY HAS a high ion concentration of ions, even before it undergoes acid ionization to form hydronium. Because it has a high ion concentration, it is a good conductor, or strong electrolyte.

0.10 M NH4+ is an

ionic weak acid

0.10 M CH3COOH is a

molecular weak acid

weak acids acids and bases can be classified as either strong or weak. here we’ll deal with weak...

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