9 / 19 Redox Chemistry

9.1.NoS 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)

9.1.U1 Oxidation and reduction can be considered in terms of oxygen gain/hydrogen loss, electron transfer or change in oxidation number.

9.1.U2 An oxidizing agent is reduced and a reducing agent is oxidized.

9.1.U3 Variable oxidation numbers exist for transition metals and for most main-group

9.1.U4 The activity series ranks metals according to the ease with which they undergo oxidation.

9.1.U5 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.

9.1.AS1 Deduction of the oxidation states of an atom in an ion or a compound.

9.1.AS2 Deduction of the name of a transition metal compound from a given formula, applying oxidation numbers represented by Roman numerals.

9.1.AS3 Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox reactions.

9.1.AS4 Deduction of redox reactions using half-equations in acidic or neutral solutions.

9.1.AS5 Deduction of the feasibility of a redox reaction from the activity series or reaction data.

9.1.AS6 Solution of a range of redox titration problems.

9.1.AS7 Application of the Winkler Method to calculate BOD.

9.1.G1 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.

9.1.G2 Oxidation states should be represented with the sign given before the number, eg +2 not 2+.

9.1.G3 The oxidation state of hydrogen in metal hydrides (-1) and oxygen in peroxides (-1) should be covered.

9.1.G4 A simple activity series is given in the data booklet in section 25.

9.1.IM1 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.

9.1.ToK1 Chemistry has developed a systematic language that has resulted in older names becoming obsolete. What has been lost and gained in this process?

9.1.ToK2 Oxidation states are useful when explaining redox reactions. Are artificial conversions a useful or valid way of clarifying knowledge?

9.1.Uz1 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.

9.1.Uz2 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.

9.1.Uz3 Natural and synthetic antioxidants in food chemistry.

9.1.Uz4 Photochromic lenses.

9.1.Uz5 Corrosion and galvanization.

9.1.Aims1 Aim 6: Experiments could include demonstrating the activity series, redox titrations, and using the Winkler method to measure BOD.

9.1.Aims2 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).

9.2.NoS Ethical implications of research—the desire to produce energy can be driven by social needs or profit. (4.5)

9.2.U1 Voltaic cells convert energy from spontaneous, exothermic chemical processes to electrical energy.

9.2.U2 Oxidation occurs at the anode (negative electrode) and reduction occurs at the cathode (positive electrode) in a voltaic cell.

9.2.U3 Electrolytic cells convert electrical energy to chemical energy, by bringing about non-spontaneous processes.

9.2.U4 Oxidation occurs at the anode (positive electrode) and reduction occurs at the cathode (negative electrode) in an electrolytic cell.

9.2.AS1 Construction and annotation of both types of electrochemical cells.

9.2.AS2 Explanation of how a redox reaction is used to produce electricity in a voltaic cell and how current is conducted in an electrolytic cell.

9.2.AS3 Distinction between electron and ion flow in both electrochemical cells.

9.2.AS4 Performance of laboratory experiments involving a typical voltaic cell using two metal/metal-ion half-cells.

9.2.AS5 Deduction of the products of the electrolysis of a molten salt.

9.2.G1 For voltaic cells, a cell diagram convention should be covered.

9.2.IM1 Research in space exploration often centres on energy factors. The basic hydrogen–oxygen fuel cell can be used as an energy source in spacecraft, such as those first engineered by NASA in the USA. The International Space Station is a good example of a multinational project involving the international scientific community.

9.2.ToK1 Is energy just an abstract concept used to justify why certain types of changes are always associated with each other? Are concepts such as energy real?

9.2.Uz1 Fuel cells.

9.2.Uz2 Heart pacemakers.

9.2.Aims1 Aim 6: Construction of a typical voltaic cell using two metal/metal-ion half-cells.

9.2.Aims2 Aim 6: Electrolysis experiments could include that of a molten salt. A video could also be used to show some of these electrolytic processes.

9.2.Aims3 Aim 8: Although the hydrogen fuel cell is considered an environmentally friendly, efficient alternative to the internal combustion engine, storage of hydrogen fuel is a major problem. The use of liquid methanol, which can be produced from plants as a carbon neutral fuel (one which does not contribute to the greenhouse effect), in fuel cells has enormous potential. What are the current barriers to the development of fuel cells?

19.1.NoS1 Employing quantitative reasoning—electrode potentials and the standard hydrogen electrode. (3.1)

19.1.NoS2 Collaboration and ethical implications—scientists have collaborated to work on electrochemical cell technologies and have to consider the environmental and ethical implications of using fuel cells and microbial fuel cells. (4.5)

19.1.U1 A voltaic cell generates an electromotive force (EMF) resulting in the movement of electrons from the anode (negative electrode) to the cathode (positive electrode) via the external circuit. The EMF is termed the cell potential (Eº).

19.1.U2 The standard hydrogen electrode (SHE) consists of an inert platinum electrode in contact with 1 mol dm-3 hydrogen ion and hydrogen gas at 100 kPa and 298 K. The standard electrode potential (Eº) is the potential (voltage) of the reduction half-equation under standard conditions measured relative to the SHE. Solute concentration is 1 mol dm-3 or 100 kPa for gases. Eº of the SHE is 0 V.

19.1.U3 When aqueous solutions are electrolysed, water can be oxidized to oxygen at the anode and reduced to hydrogen at the cathode

19.1.U4 ∆Gº = -nFEº. When Eº is positive, ∆Gº is negative indicative of a spontaneous process. When Eº is negative, ∆Gº is positive indicative of a non-spontaneous process. When Eº is 0, then ∆Gº is 0.

19.1.U5 Current, duration of electrolysis and charge on the ion affect the amount of product formed at the electrodes during electrolysis.

19.1.U6 Electroplating involves the electrolytic coating of an object with a metallic thin layer.

19.1.AS1 Calculation of cell potentials using standard electrode potentials.

19.1.AS2 Prediction of whether a reaction is spontaneous or not using Eº values

19.1.AS3 Determination of standard free-energy changes (∆Gº) using standard electrode potentials.

19.1.AS4 Explanation of the products formed during the electrolysis of aqueous solutions

19.1.AS5 Perform lab experiments that could include single replacement reactions in aqueous solutions.

19.1.AS6 Determination of the relative amounts of products formed during electrolytic processes.

19.1.AS7 Explanation of the process of electroplating.

19.1.G1 Electrolytic processes to be covered in theory should include the electrolysis of aqueous solutions (eg sodium chloride, copper(II) sulfate etc) and water using both inert platinum or graphite electrodes and copper electrodes. Explanations should refer to Eº values, nature of the electrode and concentration of the electrolyte.

19.1.G2 ∆G° = −nFE° is given in the data booklet in section 1.

19.1.G3 Faraday’s constant = 96 500 C mol-1 is given in the data booklet in section 2

19.1.G4 The term “cells in series” should be understood

19.1.IM1 Many electrochemical cells can act as energy sources alleviating the world’s energy problems but some cells such as super-efficient microbial fuel cells (MFCs) (also termed biological fuel cells) can contribute to clean-up of the environment. How do national governments and the international community decide on research priorities for funding purposes?

19.1.ToK1 The SHE is an example of an arbitrary reference. Would our scientific knowledge be the same if we chose different references?

19.1.Uz1 Electroplating.

19.1.Uz2 Electrochemical processes in dentistry

19.1.Uz3 Rusting of metals.

19.1.Aims1 Aim 8: Biological fuel cells can produce electrical energy to power electrical devices, houses, factories etc. They can assist in environmental clean-up. Microbial fuel cells (MFCs) powered by microbes in sewage can clean up sewage which may result in cost-free waste water treatment.