Acid-Base Equilibria (2) Defining pH and acid strength

Edexcel A level Chemistry (2017)

Topic 12: Acid–Base Equilibria:

Here are the next five learning objectives:

12/4. To be able to define the term ‘pH’

12/5. To be able to calculate pH from hydrogen ion concentration

12/6. To be able to calculate the concentration of hydrogen ions, in mol dm-3, in a solution from its pH, using the expression [H+] = 10–pH

12/7. To understand the difference between a strong acid and a weak acid in terms of degree of dissociation

12/8. To be able to calculate the pH of a strong acid

It was, I think, Gustav Carlsberg of lager fame who wanted a measure of acidity that could be used in his brewery and was simple and effective. 

It’s thanks to him that we have pH as a measure of the acidity of solutions.

pH stands for the power of the hydrogen ion.

pH   =    — log₁₀[H⁺]

or to put that equation into words ‘pH is the negative logarithm to base 10 of the hydrogen ion concentration in mol.dm^—3’

If [H⁺]  =  1M  then the pH is 0

If [H⁺]  =  7.3× 10^–5 M  then the pH is 4.13

So the greater the [H⁺] the lower the pH or as we have known from using the pH scale and Full Range Indicator strong acids have a lower pH than weak acids.

So a 0.001M solution of hydrochloric acid has a hydrogen concentration of 10^–3   or a pH of 3. 

Therefore, we can also write that its hydrogen concentration is 10^—pH

[H⁺]    =   10^—pH

So what makes a strong acid different from a weak acid?

In strong acids the dissociation of the hydrogen ions is complete.

All the acid molecules have split into hydrogen ions and the acid anion.

We can put a full one way arrow between the reactants and products thus:

HCl(g)    +    H₂O(l)      ⟶   H₃O⁺ (aq)    +   Cl^— (aq)

But a weak acid does not contain all the acid molecules split into hydrogen ions and acid anions.

The reactants and products are in equilibrium.

CH₃COOH(l)    +     H₂O(l)    ⇌   H₃O⁺(aq)   +   CH₃COO^—(aq)

So for a given concentration the pH of a weak acid will be higher than that of a strong acid. 

However as a strong acid is diluted its pH increases!!!

Think of this:

Hydrochloric acid concentrations and pH:

Hydrochloric acid concentration (mol.dm—3)	[H+] (mol.dm—3)	pH
0.1	0.1	1
0.01	0.01	2
0.001	0.001	3
0.0001	0.0001	4
0.00001	0.00001	5

But this is ridiculous!!!

How can HCl hydrochloric acid have a pH of 5 when it is a strong, fully dissociated acid?

What this little analysis proves, if nothing else, is that pH is an unreliable measure of acid strength. 

There needs to be a better measure of acid strength that does not leave open the possibility of dilution affecting the result.