C3-1 : Acids and Bases
Proton Donors & Acceptors
When an acid dissolves in water, it forms H+ ions. This is a hydrogen atom which has lost an electron – in other words, it is a proton. These produced protons become surrounded by water molecules to keep them in solution – we call it hydrated. Hydrated hydrogen ions are shown with H+ (aq). An alkali is a base which dissolves in water, and produces OH- ions (hydroxide ions).
Because acids act as a source of protons, we call them proton donors. The hydroxide ions from an alkali combine with protons to form water:
OH- (aq) + H+ (aq) → H2O (l)
And because alkalis behave like this, we call them proton acceptors.
Strength of Acids and Alkalis
The strength of an acid depends on the extent to which it ionises in water. A strong acid or alkali is one which is 100% ionised in water. Hydrochloric acid, sulphuric acid and nitric acid are all strong acids. Sodium hydroxide and potassium hydroxide are both strong alkalis. A weak acid or alkali is only partly ionised in water. Ethanoic acid, citric acid and carbonic acid are all weak acids; and ammonia solution is a weak alkali.
We can detect strong and weak acids using their pH. This scale is a measure of the concentration of hydrogen ions in a solution.
A strong acid, e.g. hydrochloric, will be completely ionised, so the concentration of hydrogen ions is 1 mol/dm³. However, a weak acid, such as citric acid is only partly ionised, so the concentration of hydrogen ions will be much lower than 1 mol/dm³
Adding an acidic solution to an alkaline solution will produce a neutralisation reaction. They react together and neutralise each other, producing a salt in the process. When a neutralisation reaction takes place, the quantities of each solution used must be correct, because if a very strong acid and a very strong alkali were mixed, if there was more acid solution, the whole alkali solution would be neutralised, but not all of the acid solution would be – so the mixture would become slightly acidic overall. We can measure precise volumes of acids and alkalis needed to react with each other using titrations.
In the neutralisation reaction, the point at which the acid and the alkali have completely reacted is called the end point. We can show the end point using a chemical indicator. Indicators change colour over different pH ranges. We have to choose suitable indicators when carrying out titrations with different combinations of acids and alkalis:
- strong acid + strong alkali – use any indicator
- weak acid + strong alkali – use phenolphthalein
- strong acid + weak alkali – use methyl orange
These are the steps to carry out a titration to calculate how much acid is needing to react with an alkaline solution:
- Measure an already-known volume of the alkali…