Redox reactions in action
Read the information on zinc, which is a reducing agent, and the use of oxidising agents in a range of bleaches. Then complete the task that follows.
Zinc
Zinc is naturally present in over 70 enzymes in our bodies. It is known to act as an antioxidant in our bodies.
A deficiency in zinc can lead to a number of health issues, such as brain disorders, sexual health problems, anorexia and depression. Much is still not known about the specific role of zinc in the biochemical processes in our bodies. However, it is thought that zinc is able to prevent reactive species called free radicals 
from oxidising proteins in the body.
A reduction in the concentration of zinc in the body can cause these destructive processes to occur. An example of such process is damage caused to the 'blood brain barrier' which protects the brain from toxins that may be present in the blood.
The recommended dose (RDA) of zinc is 15 mg per day. A number of factors, such as soil depletion, environmental pollution and an increased reliance on processed food have reduced the amount of zinc in a normal diet. For this reason zinc is present in many nutritional supplements such as multivitamins, often in the form of zinc acetate dihydrate.
As with most chemicals, too high a concentration of zinc can also cause problems, such as copper deficiency which is caused when the body is not able to absorb copper in the form of Cu2+ ions.
The top three producers of zinc in the world are China, Australia and Peru. Zinc is extracted from ores by a series of redox reactions, including electrolysis.
Bleaches
Click on the headings below to see the different ways bleach is used.
Coloured chemicals contain parts of the molecules called chromophores. The chromophore is the part of the molecule that causes the substance to be coloured. Most bleaches work by oxidation reactions that alter the structure and/or behaviour of the chromophore.
Many coloured substances contain chromophores which consist of metal ions bonded within complex organic structures. An example of this is chlorophyll, which contains magnesium and haemoglobin, where the presence of iron gives it the red colour.
Another common feature of chromophores within coloured substances can be a section of the molecule that has alternating double and single bonds. Examples of this include the azo dyes, such as the indicator methyl orange and anthocyanins as seen in grapes and other fruits.
The production of wood pulp to produce paper and other fibre based products is a major industry.
Bleaching of the pulp is an essential part of the process. Chlorine has been used for this purpose but chlorine dioxide gas (ClO2) is now being used as an alternative.
ClO2 can be prepared by oxidation of sodium chlorite by chlorine, as shown by the reaction below.
2 NaClO2 + Cl2 → 2ClO2 + 2 NaCl
Other alternative bleaches are now being used around the world to bleach wood pulp. One example is ozone (O3), which reacts with the carbon-carbon double bonds in the organic compounds present in the wood. This removes the colour from these compounds.
The most common bleach used for teeth whitening is hydrogen peroxide (H2O2). This is often administered in the form of carbamide peroxide (CH4N2O.H2O2) which is a combination of urea (CH2COCH2) and hydrogen peroxide.
In the mouth the carbamide peroxide decomposes to release hydrogen peroxide. Pure carbamide peroxide is a white crystalline solid, but it is applied in solution form, with concentrations ranging from 10-35% being used in the whitening process.
In September 2009 a Melbourne magistrate ruled that tooth whitening procedures should only be carried out by dental care providers and not by beauty therapists. This was due to concerns about the risk of damage to teeth and chemical burns as a result of using strong oxidising agents for bleaching teeth.
