Neutralisation Reactions

Neutralisation is the reaction between an acid and a base to form a salt and water. It is one of the most important reaction types in 0620, underpinning salt preparation, titrations, and real-world applications.

The general pattern

acid + base -> salt + water

A base is a substance that neutralises an acid. An alkali is a soluble base.

Specific patterns

Acid + metal oxide

acid + metal oxide -> salt + water

HCl + CuO: 2HCl + CuO -> CuCl2 + H2O Observation: black solid dissolves, blue solution forms.

H2SO4 + MgO: H2SO4 + MgO -> MgSO4 + H2O Observation: white solid dissolves, colourless solution.

Acid + metal hydroxide

acid + metal hydroxide -> salt + water

HCl + NaOH -> NaCl + H2O Observation: no visible change (both solutions are colourless). Temperature rises.

H2SO4 + Cu(OH)2 -> CuSO4 + 2H2O (after balancing) Observation: blue solid dissolves, blue solution forms.

Acid + metal carbonate

acid + metal carbonate -> salt + water + carbon dioxide

This produces an extra product (CO2) compared to the other patterns.

2HCl + CaCO3 -> CaCl2 + H2O + CO2 Observation: effervescence, solid dissolves, gas turns limewater milky.

Acid + ammonia

HCl + NH3 -> NH4Cl HNO3 + NH3 -> NH4NO3 H2SO4 + 2NH3 -> (NH4)2SO4

These produce ammonium salts, important as fertilisers.

The ionic equation

For all reactions between a soluble acid and a soluble alkali:

H+(aq) + OH-(aq) -> H2O(l)

All other ions (e.g. Na+, Cl-) are spectator ions and cancel out.

Energy change

Neutralisation is always exothermic. The temperature of the mixture increases. This can be measured using a polystyrene cup, thermometer, and known volumes/concentrations of acid and alkali. See energy changes in reactions.

pH changes during neutralisation

• Strong acid (pH ~1) + alkali: pH rises toward 7
• At exactly the right proportions: pH = 7 (neutral)
• Excess alkali: pH > 7

An indicator shows the endpoint. Universal indicator shows the continuous pH change.

Titration

Titration is the quantitative method for performing neutralisation:

1. Measure a known volume of alkali into a conical flask using a pipette
2. Add indicator (e.g. methyl orange or phenolphthalein)
3. Add acid from a burette until the indicator changes colour (endpoint)
4. Record the volume of acid used

See titration and solubility.

Practical applications

Application	Detail
Antacids	Neutralise excess stomach acid (HCl) with bases like Mg(OH)2 or CaCO3
Treating acidic soil	Add lime (CaO or Ca(OH)2) to neutralise acidic soil for farming
Treating acidic lakes	Add powdered limestone (CaCO3) to counteract acid rain
Wastewater treatment	Neutralise industrial waste before discharge
Toothpaste	Basic substance neutralises acids produced by bacteria

How to recognise in exams

Look for: acid reacting with a metal oxide, hydroxide, carbonate, or ammonia. Key words: “neutralise”, “pH changes”, “salt and water”, “titration”.

25 cm3 of 0.1 mol/dm3 sodium hydroxide is exactly neutralised by hydrochloric acid. Write the equation and calculate the moles of NaOH used. [3 marks]

• NaOH + HCl -> NaCl + H2O [1]
• Volume in dm3 = 25/1000 = 0.025 dm3 [1]
• Moles = 0.1 x 0.025 = 0.0025 mol [1]

Explain why farmers add calcium hydroxide to acidic soil. Write an equation to represent the reaction if the soil acidity is due to sulfuric acid. [3 marks]

• Calcium hydroxide is a base/alkali and neutralises the acid in the soil [1]
• The pH of the soil increases towards neutral, making it suitable for crops [1]
• Ca(OH)2 + H2SO4 -> CaSO4 + 2H2O [1]