Neutralisation and titration curves
Neutralisation and titration curves
- Neutralisation reacts an acid with an alkali.
- A titration follows the pH as you add one to the other.
- The curve guides which indicator to use.
Neutralisation
- The hydrogen ions from the acid react with the hydroxide ions from the alkali:
$$\text{H}^{+}(\text{aq}) + \text{OH}^{-}(\text{aq}) \rightarrow \text{H}_2\text{O}(\text{l})$$
- A salt is also formed, from the rest of the acid and base.
Practice
The ionic equation for neutralisation is:
Hydrogen ions from the acid combine with hydroxide ions from the alkali to make water (plus a salt).
The titration curve
- The curve has a steep, almost vertical jump near the end point.
- Its exact shape depends on whether each reactant is strong or weak.
Practice
A titration curve has:
Near the end point the pH changes very rapidly, giving a steep jump on the curve.
Practice
For a weak acid with a strong base, the indicator should change colour in the:
The weak-acid/strong-base jump sits at higher pH, so the indicator must change in that range.
Choosing an indicator
- Pick an indicator whose colour change falls inside the steep jump.
- For a weak acid with a strong base, you need one that changes in the higher pH range.
Practice
A suitable indicator is one whose colour change:
The indicator must change colour within the steep jump so the end point is shown sharply.
You've got it
Key idea
- neutralisation: $\text{H}^{+} + \text{OH}^{-} \rightarrow \text{H}_2\text{O}$, also forming a salt
- a titration curve has a steep pH jump at the end point (shape depends on strong/weak)
- choose an indicator that changes colour inside the jump (higher pH for weak acid + strong base)