Acid-Base Equliibria (5) Explaining titration curves (2)

Edexcel A level Chemistry (2017)

Topic 12: Acid–Base Equilibria:

Here are the next two learning objectives:

Explaining Titration Curves

12/16. To be able to draw and interpret titration curves using all combinations of strong and weak monobasic acids and bases

12/17. To be able to select a suitable indicator, using a titration curve and appropriate data

There are four basic types of titration curve, in this post I’m just going to look at the two less interesting:

a) Strong monobasic acid with weak monobasic base

This is the titration curve for a strong monobasic acid (HCl(aq)) titrated with a weak monobasic base (NH₃(aq)).

We notice that:

• the titration curve starts at the pH of the strong acid 0.1M HCl i.e. pH =–log₁₀[H+]  =  -log₁₀ 0.1 = 1.

• only methyl orange could be an acid-base indicator since the colour change occurs when pH change is rapid (vertical part of the “curve”).

• pH changes over 4 units at the end point 25.00ml.

• so we see why only a drop or two at the end point will turn the indicator colour from acid to alkaline—pH change is rapid at the end point.

b) Weak monobasic acid (CH₃COOH) with weak monobasic base (NH₃) titration curve:

What we notice is this:

• the curve starts at the pH of 0.1M ethanoic acid that is pH=2.9

• the curve rises slowly and the pH change levels off because this is a buffer region.

• why is this a buffer region? 

Because [CH₃COOH] =  [CH₃COO^—] the concentration of undissociated weak acid equals the concentration of dissociated acid anion, a typical condition for a buffer solution. 

• at 12.5ml when the acid is half neutralised the pH = pK_a so given the K_a of ethanoic acid we can say that this pH is – log₁₀ (1.58 × 10^-5) or 4.8.

• why does pH = pK_a at 12.50 ml?

at this point as we see [CH₃COOH] =  [CH₃COO^—] so in the K_a equation both terms will cancel:

Therefore  K_a   =   [H⁺]

So pK_a  =   pH

• addition of further base destroys the buffer and the acid is neutralised at 25.00ml the end point of the titration. 

• the pH then does not rise rapidly but another this time alkaline buffer region occurs 

• There is therefore no indicator able to detect the end point of this titration so it is not possible to titrate a weak acid with a weak base.