The neutralisation reaction
Neutralisation is the reaction between an acid and a base (or alkali) to produce salt and water only. The essential ionic equation is:
Key idea / 要点
H+(aq) + OH−(aq) → H2O(l)
The H+ from the acid combines with the OH− from the alkali to form water.
An example in full: HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l). At the exact end point, the acid and alkali have completely cancelled, giving a neutral solution.
Titration calculation
Neutralisation is used in titration to find an unknown concentration. For an acid and alkali reacting in mole ratio a : b:
Key idea / 要点
(MaVa) / (MbVb) = a / b
where M is molarity, V is volume, a and b are the mole coefficients from the balanced equation.
Worked example
25.0 cm3 of NaOH is neutralised by 20.0 cm3 of 0.10 mol dm−3 HCl. Find the molarity of NaOH. (Ratio 1:1)
Moles HCl = 0.10 × 20.0/1000 = 0.0020 mol. Moles NaOH = 0.0020 mol.
MNaOH = 0.0020 ÷ (25.0/1000) = 0.080 mol dm−3
Everyday uses: antacids (a base) relieve excess stomach acid; lime CaO neutralises acidic soil; toothpaste neutralises acid made by mouth bacteria; a baking soda paste soothes an acidic bee sting.
In a titration the acid is usually placed in a burette and added slowly to a fixed volume of alkali (measured with a pipette) containing a few drops of indicator. The volume of acid added when the indicator just changes colour is the titre. Repeating until consistent (concordant) titres are obtained improves reliability, and the mean titre is then used in the calculation of the unknown concentration.
Remember
- Acid + base → salt + water; ionically H+ + OH− → H2O.
- Use the balanced mole ratio in titration calculations.
- An indicator (e.g. phenolphthalein) shows the end point.