Question

How does the endpoint of a titration differ from the equivalence point?

Answer 1

The endpoint of a titration is the point where the indicator just changes colour.

The equivalence point is when the ratio of the reactants is in the amounts specified by the equation.

Ideally you would want these points to coincide.

For a strong acid and a strong base such as NaOH and HCl the final solution is neutral at pH 7:

`HCl_((aq))+NaOH_((aq))rarrNaCl_((aq))+H_2O_((l))`

Most indicators will change colour at the equivalence point so can be used in a titration.

This is not always the case though. If you neutralise a weak base with a strong acid the final solution will not be neutral e.g:

`NH_(3(aq))+HCl_((aq))rarrNH_4Cl_((aq))+H_2O`

This is because the ammonium ion is slightly acidic:

`NH_4^(+)rightleftharpoonsNH_(3(aq))+H_((aq))^+`

The same problem occurs when a strong base is neutralised by a weak acid. The salt produced is slightly alkaline.

This is known as "salt hydrolysis".

You need to choose an indicator which will change colour at the equivalence point. In this case methyl orange is suitable but phenolphthalein is not.

For more details you can look up "theory of indicators".

Answer 2

They are similar but not identical.....

Explanation:

Equivalence point is the point where equal number of moles of acid and the number of moles of base that have been mixed together are equal.

End point is the point at which the indicator being used in such a reaction changes colour.

Often, the indicator colour change in an acid base titration is not observed AT the equivalence point, but once the equivalence point has been exceeded (i.e. once there is an imbalance of acid or base). This is why equivalence point and end point are not necessarily 100% identical.