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Higher and foundation tiers

Neutralisation using acid alkali reactions

Students carrying out a chemistry experiment in the lab

Acids have a pH below 7 and are usually formed by dissolving substances (usually non-metal oxide) in water that cause an excess of hydrogen ions (H+) to be released into the water. Alkalis on the other hand have a pH above 7 and are formed when substances, usually metal oxides and metal hydroxides dissolve in water to cause an excess of hydroxide ions (OH-) to be released into the water. If a solution has a pH of 7 we say it is neither acidic nor alkaline but it is neutral.

The table below lists some common acids and alkalis you will probably have used in the lab. You should note that all acids are solutions that contain an excess of hydrogen ions (H+) while all alkalis are solutions which have an excess of hydroxide ions (OH-) present .

acid molecular formula alkali molecular formula
hydrochloric acid HCl sodium hydroxide NaOH
sulfuric acid H2SO4 potassium hydroxide KOH
nitric acid HNO3 calcium hydroxide Ca(OH)2
ethanoic acid CH3COOH ammonium hydroxide NH4 OH

The reaction between an acid and an alkali or base is called neutralisation, the products of this neutralisation reaction are a salt and water, we can represent this as:

equations for neutralisation

Neutralisation equations and reactions

The neutralisation reactions shown below involve adding acids to alkalis and bases. The products of these neutralisation reactions are a salt and water.
hydrochloric acid(aq) + sodium hydroxide(aq) sodium chloride(aq) + water(l)
HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l)
If you study the equations above you will see that the hydrogen ion (H+) in the acid and the hydroxide ion (OH-) in the alkali simply react together to form water, we can show this as:
H+(aq) + OH-(aq) H2O(l)
and what is left over now simply forms the salt:
Cl- + Na+ → NaCl
You can think of the salt as the acid where the hydrogen in the acid is replaced by the metal present in the alkali e.g. If we use hydrochloric acid and neutralise it with 3 different alkalis then the salt produced is always a chloride; as shown below:
hydrochloric acid(aq) + lithium hydroxide(aq) lithium chloride(aq) + water(l)
HCl(aq) + LiOH(aq)LiCl(aq) + H2O(l)
hydrochloric acid(aq) + potassium hydroxide(aq)potassium chloride(aq) + water(l)
HCl(aq) + KOH(aq)KCl(aq) + H2O(l)
hydrochloric acid(aq) + calcium hydroxide(aq)calcium chloride(aq) + water(l)
2HCl(aq) + Ca(OH)2(aq)CaCl2(aq) + 2H2O(l)
In each example the hydrogen ion (H+) in the hydrochloric acid is replaced by the metal, present in the alkali, that is the metals lithium, potassium and finally calcium in the last example. In each case the salt formed is a metal chloride. The metal comes from the alkali and the chloride from the acid. This means every time you neutralise hydrochloric acid you will make a salt called a chloride .

Neutralisation using sulfuric acid

Sulfuric acid (H2SO4) contains two acidic hydrogen ions (2H+(aq)), it is called a diprotic acid. Whereas hydrochloric acid which can only donate one H+(aq) is called a monoprotic acid. Sulfuric acid contains two hydrogen ions and a sulfate ion (S042-). When sulfuric acid is neutralised it will need two hydroxide ions (OH-) to neutralise the two hydrogen ions in the acid an equation for this reaction is shown below:
2H+(aq) + 2OH-(aq) 2H2O(l)
What is left will be the metal ion from the alkali and the sulfate ion from the acid. This will form the salt as shown in the equations below:
sulfuric acid(aq) + lithium hydroxide(aq)lithium sulfate(aq) + water(l)
H2SO4(aq) + 2LiOH(aq) Li2SO4(aq) + 2H2O(l)
sulfuric acid(aq) + potassium hydroxide(aq) potassium sulfate(aq) + water(l)
H2SO4(aq) + 2KOH(aq) → K2SO4(aq) + 2H2O(l)
sulfuric acid(aq) + calcium hydroxide(aq) calcium sulfate(aq) + water(l)
H2SO4(aq)+ Ca(OH)2(aq)Li2SO4(aq) + 2H2O(l)
In these three examples the salts lithium sulfate, potassium sulfate and calcium sulfate are formed. There is an obvious pattern here, it's the same pattern for the examples with hydrochloric acid. The hydrogen in the acid is replaced by the metal present in the alkali; this means that with sulfuric acid the salt formed will always be a metal sulfate. Finally if we use nitric acid (HNO3), then exactly the same pattern as above occurs e.g. nitric acid has the formula HNO3 and it contains one hydrogen ion and nitrate ions (NO-3). So the hydrogen ions will be neutralised by the hydroxide ions and the salt formed will be a metal nitrate as shown in the equations below:
nitric acid(aq) + sodium hydroxide(aq)sodium nitrate(aq) + water(l)
HNO3 (aq) + NaOH(aq) NaNO3 (aq) + H2O(l)
nitric acid(aq) + lithium hydroxide(aq)lithium nitrate(aq) +water(l)
HNO3 (aq) + LiOH(aq) LiNO3 (aq) + H2O(l)
nitric acid(aq) + sodium hydroxide(aq)calcium nitrate(aq) + water(l)
2HNO3 (aq) + Ca(OH)2 (aq) Ca(NO3)2 (aq) + 2H2O(l)

Key Points

Practice questions

Check your understanding - Questions on neutralisation

Check your understanding - Additional questions on neutralisation

Check your understanding - Quick Quiz on neutralisation