Oxidation and reduction

Nature of science:

How evidence is used—changes in the definition of oxidation and reduction from one involving specific elements (oxygen and hydrogen), to one involving electron transfer, to one invoking oxidation numbers is a good example of the way that scientists broaden similarities to general principles. (1.9)

• Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in oxidation number.
• An oxidizing agent is reduced and a reducing agent is oxidized.
• Variable oxidation numbers exist for transition metals and for most main-group non-metals.
• The activity series ranks metals according to the ease with which they undergo oxidation.
• The Winkler Method can be used to measure biochemical oxygen demand (BOD), used as a measure of the degree of pollution in a water sample.

Applications and skills:

• Deduction of the oxidation states of an atom in an ion or a compound.
• Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers represented by Roman numerals.
• Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox reactions.
• Deduction of redox reactions using half-equations in acidic or neutral solutions.
• Deduction of the feasibility of a redox reaction from the activity series or reaction data.
• Solution of a range of redox titration problems.
• Application of the Winkler Method to calculate BOD.

• Access to a supply of clean drinking water has been recognized by the United Nations as a fundamental human right, yet it is estimated that over one billion people lack this provision. Disinfection of water supplies commonly uses oxidizing agents such as chlorine or ozone to kill microbial pathogens.

Theory of knowledge:

• Chemistry has developed a systematic language that has resulted in older names becoming obsolete. What has been lost and gained in this process?
• Oxidation states are useful when explaining redox reactions. Are artificial conversions a useful or valid way of clarifying knowledge?

Utilization:

• Aerobic respiration, batteries, solar cells, fuel cells, bleaching by hydrogen peroxide of melanin in hair, household bleach, the browning of food exposed to air, etc.
• Driving under the influence of alcohol is a global problem which results in serious road accidents. A redox reaction is the basis of the breathalyser test.
• Natural and synthetic antioxidants in food chemistry.
• Photochromic lenses.
• Corrosion and galvanization.

• Oxidation number and oxidation state are often used interchangeably, though IUPAC does formally distinguish between the two terms. Oxidation numbers are represented by Roman numerals according to IUPAC.
• Oxidation states should be represented with the sign given before the number, eg +2 not 2+.
• The oxidation state of hydrogen in metal hydrides (-1) and oxygen in peroxides (-1) should be covered.
• A simple activity series is given in the data booklet in section 25.

• Syllabus and cross-curricular links:
• Topic 1.3—experimental determination of amounts, masses, volumes and concentrations of solutions
• Topic 3.2—halogen reactivity
• Topics 4.1 and 4.2—difference between ionic and covalent bonding
• Topic 10.2—oxidation of alcohols
• Biology topics 8.2 and 8.3—redox reactions in physiology

Aims:

• Aim 6: Experiments could include demonstrating the activity series, redox titrations and using the Winkler Method to measure BOD.
• Aim 8: Oxidizing agents such as chlorine can be used as disinfectants. Use of chlorine as a disinfectant is of concern due to its ability to oxidize other species forming harmful by-products (eg trichloromethane).