Higher and foundation tier


Acids have a pH below 7. Acids are made 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 in the water. If a solution has a pH of 7 we say it is neither acidic or alkaline but it is neutral. The table below lists acids and alkalis you will probably have used in the lab. You should note that all acids have an excess of hydrogen ions (H+) and all alkali have an excess of hydroxide ions (OH-) when they dissolve in water.

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. It produces a salt and water, we can represent this as:

equations for neutralisation

Neutralisation equations and reactions

The neutralisation reactions shown above involve adding acids to alkalis. The products of this neutralisation reaction are a salt and water e.g.
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)
What is left over now forms the salt:
Cl- + Na+ → NaCl
You can think of the salt as the acid where the hydrogen 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 in the hydrochloric acid is replaced by a metal, 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 .

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:
2H+(aq) + 2OH-(aq) 2H2O(l)
What what is left will be the metal from the alkali and the sulfate from the acid. This will form the salt as shown 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, potssium sulfate and calcium sulfate are formed. There is an obvious pattern here, it's the same pattern for the example with hydrochloric acid. The hydrogen in the acid is replaced by a metal, this means that with sulfuric acid, the salt 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 contains hydrogen ions and nitrate ions. So the hydrogen ions will be neutralised by the hydroxide ions and the salt formed will be a metal nitrate.
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