Acid-Base Equilibria (4) Explaining titration curves (1)

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 most interesting:

a) Strong monobasic acid with strong monobasic base

This is the titration curve for a strong monobasic acid titrated with a strong monobasic base.

We notice that:

• the titration curve starts at the pH of the strong acid HCl.

• both methyl orange and phenolphthalein could be acid base indicators since both colours changes occur when pH change is rapid (vertical part of the “curve”).

• pH changes over 8 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 with strong monobasic base 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 rises rapidly over about 4 pH units this time not 8. 

• phenolphthalein is one indicator (there are others) that changes colour over this more limited pH range (7—11) and so it can be used in a weak acid–strong base titration.