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 is less than 100% ionized in aqueous solution.

An example of a weak acid is acetic acid, CH3COOH Acetic Acid

A Weak Acid is an acid that is less than 100% ionized in aqueous solution. It reacts with water Acetic Acid

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

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

A Weak Acid is an acid that is less than 100% ionized in aqueous solution. And an acetate or ethanoate ion. Acetic Acid Acetate ion

A Weak Acid is an acid that is less than 100% ionized in aqueous solution. Unlike strong acids, which ionize to completion, weak acids exist as equilibrium mixtures, as shown by the double arrow. Acetic Acid Acetate ion An Equilibrium

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

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

So a solution of acetic acid consists mostly of neutral CH 3 COOH molecules. A solution of acetic acid is mostly neutral CH 3 COOH molecules

And the concentrations of the ions in this solution are very low. The concentrations of the ions are very low A solution of acetic acid is mostly neutral CH 3 COOH 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

The acetic acid molecules spread out to fill the solution.

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

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. A small number of ions are present in the solution

So weak acids that start out as neutral molecules, like CH3COOH are weak electrolytes. Molecular Weak Acids are Weak Electrolytes

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 O 2- and NH 2 – Strong Acids Weak Acids NOT Acids bases conjugate 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 ion ization decr eases 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. K a values get smaller G e t w e a k e r Deg ree of ion ization decr eases

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 Degree o f I o ni za ti 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 Degree o f I o ni za ti on Ka val ues get lar ger

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, Molecular Weak Acids Deg ree of ion ization decr eases

It means that the number of dissolved ions present in 1.0 Molar solutions of these molecular acids will decrease as we move down. Molecular Weak Acids Deg ree of ion ization decr eases Number of dissolved ions decr eases

So what do you think the trend in conductivity of molecular acids will be as we move down the column? Molecular Weak Acids Deg ree of ion ization decr eases Number of dissolved ions decr eases 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, Number of dissolved ions decr eases

the conductivity of molecular acids also decreases. Conductivity of molecular acids decreases Number of dissolved ions decr eases

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 Conductivity of molecular acids decreases

We can also see that higher conductivity correlates with a higher value for the ionization constant Ka. Higher conductivity Lower conductivity Higher K a value Lower K a value Conductivity of molecular acids decreases

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 weaker as acids

This is reflected by a decrease in their Ka values as we move down. G e t weaker as acids K a values get smaller

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. NaHSO 4  Na + + HSO 4 –

For example, the HSO4 minus ion could be accompanied by the spectator cation Na+. NaHSO 4  Na + + HSO 4 – Spectator cation

These two ions would result from the dissociation of the salt sodium bisulphate, NaH2SO4. NaHSO 4  Na + + HSO 4 – The source of HSO 4 – could be a salt like NaHSO 4

Similarly, the hydrogen oxalate ion HC2O4 minus could come from the dissociation of the salt potassium hydrogen oxalate, KHC2O4 NaHSO 4  Na + + HSO 4 – KHC 2 O 4  K + + HC 2 O 4 – The source of HC 2 O 4 – could be a salt like KHC 2 O 4

And the positive ion NH4+ could come from the dissociation of an ammonium salt like NH4Cl. The Cl minus ion is a spectator here. NaHSO 4  Na + + HSO 4 – KHC 2 O 4  K + + HC 2 O 4 – NH 4 Cl  NH Cl – The source of NH 4 + could be a salt like NH 4 Cl

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… NH 4 Cl water

To produce a solution of 0.1 molar NH4Cl. 0.1 M NH 4 Cl

Because NH 4 Cl 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 M

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 M

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. 0.1 M

So the total ion concentration in this solution is 0.1 plus M Total ion concentration = 0.1 M M = 0.2 M

Which is 0.2 molar, high enough to account for the high conductivity. 0.1 M Total ion concentration = 0.1 M M = 0.2 M

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

So if we compare acetic acid with the ammonium ion.

We see that NH4+ is a much weaker acid than CH3COOH. NH 4 + is a weaker acid than CH 3 COOH

However 0.10 M NH4+ would have a higher conductivity than CH3COOH. 0.1 M NH 4 + has a higher conductivity than 0.10 M CH 3 COOH

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 M CH 3 COOH 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 M NH 4 + is an ionic weak acid 0.10 M CH 3 COOH is a molecular weak acid