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 the two acids hydrochloric and chloric (I) acid. 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 disinfect it. The
chloric(I) acid
or hypochlorous acid as it is often called is an oxidising agent and also a bleach, it is the active disinfecting agent responsible for killing any harmful organisms present in the water to ensure it is safe to
drink. The chloric (I) acid is a more
effective disinfectant than chlorine alone. It is highly effective at penetrating the cell walls of microorganisms, which allows it to easily diffuse through the cell membrane, once inside the cell, the chloric (I) acid (HOCl) quickly oxidises the essential components of the cell, such as proteins, lipids, and nucleic acids, leading to cell death. This process effectively inactivates bacteria, viruses, and some protozoa.
The concentration of chlorine in drinking water
is around 0.7mg/dm3 though its concentration is higher in swimming pools. However great 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
also the additional problem that chlorine can react
with organic compounds found in the water and form organochloro compounds
(see below for more detail) which are also very toxic and are also potentially carcinogenic. However if too little chlorine
is added it may not necessarily kill all
the potentially harmful microorganisms which maybe present such as cholera, typhus and E.coli bacterium.
Chlorine water in the lab
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,; as shown in the image above. If universal indicator is added to 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, an equation for this disproportionation reaction is shown below:
Br2(aq) + H2O(l) ⇌ HBr(aq) + HBrO (aq)
The products of this reaction are hydrobromic acid (HBr) and hypobromous acid or bromic (I) acid (HOBr). Iodine is for all practical purposes insoluble in water and does not undergo the reactions that occur when chlorine or bromine are added to water. Although iodine is often dissolved in ethanol or a potassium iodide solution to form a solution called a tincture. This solution typically contains about 2-7% iodine and 2.4% sodium iodide or potassium iodide in alcohol and water. This solution is commonly used for disinfecting wounds and as a general antiseptic for skin infections.
Chlorine and drinking water
Chlorine is added to drinking water primarily for its disinfectant properties. It helps to kill or inactivate harmful microorganisms that can cause diseases, making water safe to drink. The main reason why chlorine is added to drinking water includes:
- Bacteria: Chlorine for example is highly effective at killing
many types of bacteria; such as Escherichia coli (E. coli), Salmonella, and Campylobacter. These bacteria can cause serious illnesses if ingested.
- Viruses: Chlorine can also inactivate many viruses, including those that can cause gastrointestinal illnesses like norovirus and hepatitis A.
- Protozoa: Some protozoa, like Giardia lamblia and Cryptosporidium parvum, can also be controlled with chlorine. However, these are more resistant to chlorine than bacteria and viruses, requiring higher concentrations and longer contact times for effective disinfection.
Benefits of adding chlorine to drinking water
We probably take it for granted that the water we get from ours tap is safe to drink, however in many countries around the world this is not the case. There are many benefits to adding chlorine to drinking water, for example
chlorine is highly effective at preventing many waterborne diseases such as cholera, dysentery, and typhoid fever. Chlorine also provides a residual effect, meaning it continues to disinfect the water as it travels through pipes and so preventing contamination of the water during distribution. The chlorination of drinking water is also a relatively inexpensive method of disinfecting our drinking water.
Problems and dangers of adding chlorine to drinking water
While chlorine is extremely beneficial for disinfecting water, there are some potential problems and dangers associated with its use, these include:
- The formation of unwanted by-products, for example when chlorine reacts with organic matter in water such as decaying leaves, sewage and other plant material, it can form unwanted and harmful by-products some of which are potentially carcinogens.
Some of these unwanted by-products have been linked to health issues, including an increased risk of cancer as mentioned above as well as reproductive problems with long-term exposure.
- Chlorine can impart an unpleasant taste and smell to drinking water, which some people find off-putting.
- Some individuals may experience mild irritation or sensitivity to chlorine which can cause skin rashes, respiratory issues or eye irritation, especially for those with existing conditions like skin conditions such as eczema or respiratory issues such as asthma.
-
Over time chlorine can corrode pipes, particularly those made of metals like iron and lead. This can lead to leaks, reduced water quality, and the release of harmful metals being released into the water supply.
-
Chlorine is toxic to fish and other aquatic life. If improperly managed, chlorinated water released into natural bodies of water can harm the local ecosystem.
Chlorine water and sunlight
If you plan to use chlorine water in the lab it has to be freshly prepared.
The reason for this is simply 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 to ensure that any potential pathogens are effectively removed from the water.
Bleach manufacture and disproportionation reactions
We saw above that chlorine (Cl2(g)) dissolves in cold water
it forms a mixture of two acids; the strong acid hydrochloric acid and
weak chloric (I) acid.
Cl2(g) + H2O(l) ⇌ HCl(aq) + HClO (aq)
This is a reversible 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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