Chemistry Reference
In-Depth Information
In the laboratory, if you wish to neutralise a
common acid such as hydrochloric acid you can use
an alkali such as sodium hydroxide. If the pH of the
acid is measured as some sodium hydroxide solution is
added to it, the pH increases. If equal volumes of the
same concentration of hydrochloric acid and sodium
hydroxide are added to one another, the resulting
solution is found to have a pH of 7. The acid has been
neutralised and a neutral solution has been formed.
Solutions of weak acids are poorer conductors of
electricity and have slower reactions with metals,
bases and metal carbonates.
All acids when in aqueous solution produce
hydrogen ions, H
+
(
aq
). To say an acid is a strong
acid does not mean it is concentrated. The
strength
of an acid tells you how easily it dissociates (ionises)
to produce hydrogen ions. The
concentration
of
an acid indicates the proportions of water and acid
present in aqueous solution. It is important to
emphasise that a strong acid is still a strong acid
even when it is in dilute solution and a weak acid is
still a weak acid even if it is concentrated.
hydrochloric + sodium → sodium + water
acid hydroxide chloride
HCl(
aq
) + NaOH(
aq
) → NaCl(
aq
) + H
2
O(
l
)
As we have shown, when both hydrochloric acid and
sodium hydroxide dissolve in water the ions separate
completely. We may therefore write:
Strong and weak bases
An alkali is a base which produces hydroxide ions,
OH
−
(
aq
), when dissolved in water. Sodium hydroxide
is a
strong alkali
because when it dissolves in water its
lattice breaks up completely to produce ions.
H
+
(
aq
)Cl
−
(
aq
) + Na
+
(
aq
)OH
−
(
aq
) → Na
+
(
aq
) + Cl
−
(
aq
) + H
2
O(
l
)
You will notice that certain ions are unchanged on
either side of the equation. They are called
spectator
ions
and are usually taken out of the equation. The
equation now becomes:
H
+
(
aq
) + OH
−
(
aq
) → H
2
O(
l
)
This type of equation is known as an
ionic equation
.
The reaction between any acid and alkali in aqueous
solution can be summarised by this ionic equation. It
shows the ion which causes acidity (H
+
(
aq
)) reacting
with the ion which causes alkalinity (OH
−
(
aq
)) to
produce neutral water (H
2
O(
l
)).
water
sodium hydroxide
sodium ions + hydroxide ions
Na
+
(
aq
) + OH
−
(
aq
)
NaOH(
s
)
These substances which are strong alkalis produce
large quantities of hydroxide ions and have a high
pH. Other common strong soluble bases include
potassium hydroxide.
A
weak alkali
, such as ammonia, produces fewer
hydroxide ions when it dissolves in water than a
strong soluble base of the same concentration. It
is only partially ionised. It has a lower pH than a
strong base, but still above 7.
Questions
1
Complete the following equations:
a
CH
3
COOH
+
NaOH
→
b
H
2
SO
4
+
KOH
→
c
NH
3
+
HBr
→
In each case name the acid and the base. Also in parts
a
and
b
write the ionic equation for the reactions.
2
Explain the terms 'concentration' and 'strength' as applied
to acids.
3
Explain what part water plays in the acidity of a solution.
4
Alongside the names of various chemicals below are shown
their respective pH values in aqueous solution.
ammonia + water
ammonium ions + hydroxide ions
NH
4
+
(
aq
)
+ OH
−
(
aq
)
NH
3
(
g
) + H
2
O(
l
)
The ammonia molecules react with the water
molecules to form ammonium ions and hydroxide
ions. However, fewer ammonia molecules do this
so only a low concentration of hydroxide ions is
produced.
potassium hydroxide
pH 13
Neutralising an acid
A common situation involving neutralisation of
an acid is when you suffer from indigestion. This
is caused by a build-up of acid in your stomach.
Normally you treat it by taking an indigestion remedy
containing a substance which will react with and
neutralise the acid.
hydrogen bromide
pH 2
calcium hydroxide
pH 11
sodium chloride
pH 7
hydrogen chloride
pH 2
magnesium hydroxide
pH 10
citric acid
pH 4
