Higher and foundation tier
Neutralisation
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:
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
- Alkalis and bases will neutralise acids.
- The products of a neutralisation reaction are salt and water.
- All acids contain, hydrogen ions, H+(aq), all alkalis contain,
hydroxide ions, OH-(aq) , salts
can be thought of as acids where the hydrogen in the acid
is replaced by a metal.
- Hydrochloric acid produces salts called chlorides, nitric acid produces salts called nitrates, sulfuric acids produces salts
called sulfates and phosphoric acid produces salts called phosphates.
Practice questions
Next