Disproportionation reactions
As an example to explain what a disproportionation reaction is
consider the reaction that occurs when chlorine gas is dissolved in water.
An equation for this equilibrium reaction is shown below:
chlorine(g) + water(l) ⇌ hydrochloric acid(aq) + chloric (I) acid (aq)
Cl2(g) + H2O(l) ⇌ HCl(aq) + HClO (aq)
The products of this reaction are two acids. Hydrochloric acid
which is a strong acid and chloric (I) acid which is a weak acid (note in chloric(I) acid
the I refers to the oxidation state/number of the chlorine). So we could
rewrite the above equation to show all the ions produced when the chlorine goes into solution:
Cl2(g) + H2O(l) ⇌ 2H+(aq) + Cl-(aq) + ClO-(aq)
In this equation chlorine has been both oxidised and reduced.
The equation below shows the oxidation number of chlorine before
and after it reacts. To begin with being an element means the oxidation state
of chlorine is 0, however in one of the products
namely the hydrochloric acid the oxidation number of chlorine
is -1, this means it has gained 1 electron and been reduced.
In the other
product; the chloric(I) acid the oxidation number
of the chlorine is +1; this means it has lost an electron
and been oxidised. Reactions like this where
one substance is both oxidised and reduced
are called disproportionation reactions.
This is the reaction which takes place when chlorine is added to
drinking water to kill any bacteria present. The
chloric(I) acid
is an oxidising agent and also a bleach, it is responsible for killing any harmful organisms present in water to ensure it is safe to
drink. The concentration of chlorine in drinking water
is around 0.7mg/dm3 though it is higher in the water in swimming pools. Care needs to be taken to carefully
control the
amount of chlorine added to water
simply because chlorine is very toxic even in small amounts. There is
the additional problem that it can react
with organic compounds in the water and form organochloro compounds
which are also very toxic. However if too little chlorine
is added it may not necessarily kill all
the potentially harmful organisms which maybe present such as cholera, typhus and E.coli bacterium.
You may have used chlorine water in your chemistry lessons; this is
simply made by bubbling chlorine gas through water. It is a pale green
coloured solution. If universal indicator is added to a chlorine water, the solution initially turns red due to the presence
of the hydrochloric
acid, which of course is a strong acid. However the indicator quickly
turns colourless due to the bleaching action
of the chloric(I) acid present.
A similar reaction happens with bromine though the position of
equilibrium lies much more to the left.
Br2(aq) + H2O(l) ⇌ HBr(aq) + HBrO (aq)
while
iodine is for all practical purposes insoluble in water.
If you plan to use chlorine water in the lab it has to be freshly prepared.
The reason for this is because if a bottle
of chlorine water is left exposed to sunlight its pale green colour
fades and oxygen gas is released according to the equation below:
2Cl2(g) + 2H2O(l) → 4H+(aq) + 4Cl-(aq) + O2(g)
This unwanted reaction can create problems, for example chlorine
is added to drinking water to kill unwanted pathogens. Some
of these pathogens can be a particular
problem in swimming pools which is why chlorine is added to water used in public baths. However sunlight
can cause the chlorine to leave the water
reducing the amounts present. Care has to be taken to ensure that the levels of chlorine are
maintained at the correct level.
Bleach manufacture and disproportionation reactions
We saw above that chlorine (Cl2(g)) dissolves in cold water
it forms a mixture of 2 acids; hydrochloric and
chloric (I) acid.
Cl2(g) + 2H2O(l) ⇌ HCl(aq) + HClO (aq)
This is an equilibrium reaction, so what would happen if we added cold sodium hydroxide
(a strong alkali) to this
equilibrium mixture? Adding hydroxide ions will reduce the number of H+ ions
in the equilibrium mixture so according
to Le Chatelier's principle the equilibrium should shift to the left to give:
Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaClO (aq) + H2O(l)
This is also a disproportionation reaction, initially the oxidation state
of the chlorine is 0 however on the products side of the equation the oxidation
state of chlorine is -1 in NaCl and +1 in NaClO [sodium chlorate(I)]. The
mixture of sodium chloride and sodium chlorate in solution is sold as bleach. The
chlorate ion (ClO3-) is responsible for the disinfecting and bleaching properties associated with household
bleaches.
If the solution containing chlorate (I) ions is heated then a further disproportionation reaction occurs and chlorate(V) (ClO3-) ions are formed
3NaOCl(aq) → 2NaCl(aq) + NaClO3(aq)
Here the chlorine has a +1 oxidation state in the chlorate(I) ion
(ClO-) while it is reduced to Cl- in NaCl and
oxidised to chlorine with a +5 oxidation state in the chlorate(V) ion (ClO3-).
If hot alkali sodium hydroxide is used instead of cold alkali the
a similar disproportionation reaction occurs but the halogen is
oxidised directly to the chlorate(V) ion (ClO3-) missing out the chlorate(I) stage completely; an equation for
this reaction is shown below:
Cl2(g) + 6NaOH(aq) → 5NaCl(aq) + NaClO (aq) + 3H2O(l)
Key points
- A disproportionation reaction is one where a substance is both
oxidised and reduced.
- Chlorine gas dissolves in water
to form a mixture of 2 acids, a weak acid called chloric(I) acid and the strong
hydrochloric acid.
- Bleach is mixture of sodium chloride and sodium chlorate.
Practice questions
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