Acids have a pH below 7 and are usuallyformed 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:
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.
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:
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 neutralisehydrochloric 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:
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 metalnitrate as shown in the equations below:
The products of a neutralisation reaction are salt and water.
All acids contain hydrogen ions (H+(aq) and 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 and sulfuric acid produces salts
called sulfates and phosphoric acid produces salts called phosphates.
