Introduction
Chemical reaction may be classified according to what is happening to the reactants during the process, or the type of products formed. Oxidation state, or number, is a theoretical construct used to provide a rationale for certain chemical processes, which do not fit easily into other, more mainstream, types of reaction.
In redox reactions it is suggested that electrons are 'more attached' to electronegative atoms than to electropositive atoms. We will never know for sure whether or not this is the case, but it provides a useful model to help us explain the driving force behind one class of reactions, oxidation and reduction.
Reactivity 3.2.1 - Oxidation and reduction can be described in terms of electron transfer, change in oxidation state, oxygen gain/loss or hydrogen loss/gain.
- Deduce oxidation states of an atom in a compound or an ion.
- Identify the oxidized and reduced species and the oxidizing and reducing agents in a chemical reaction.
Guidance
- Include examples to illustrate the variable oxidation states of transition element ions and of most main group non-metals.
- Include the use of oxidation numbers in the naming of compounds
Tools and links
- Structure 3.1 - What are the advantages and limitations of using oxidation states to track redox changes?
- Structure 2.3 - The surface oxidation of metals is often known as corrosion. What are some of the consequences of this process?
Reactivity 3.2.2 - Half-equations separate the processes of oxidation and reduction, showing the loss or gain of electrons.
- Deduce redox half-equations and equations in acidic or neutral solutions.
Guidance
Tools and links
- Tool 1, Inquiry 2 - Why are some redox titrations described as “self-indicating”?
Reactivity 3.2.3 - The relative ease of oxidation and reduction of an element in a group can be predicted from its position in the periodic table.
- The reactions between metals and aqueous metal ions demonstrate the relative ease of oxidation of different metals.
- Predict the relative ease of oxidation of metals.
- Predict the relative ease of reduction of halogens.
- Interpret data regarding metal and metal ion reactions.
Guidance
- The relative reactivity of metals observed in metal/ metal ion displacement reactions does not need to be learned; appropriate data will be supplied in examination questions.
Tools and links
- Structure 3.1 - Why does metal reactivity increase, and non-metal reactivity decrease, down the main groups of the periodic table?
- Tool 1, Inquiry 2 - What observations can be made when metals are mixed with aqueous metal ions, and solutions of halogens are mixed with aqueous halide ions?
Reactivity 3.2.4 - Acids react with reactive metals to release hydrogen.
- Deduce equations for reactions of reactive metals with dilute HCl and H2SO4.
Guidance
Tools and links
Reactivity 3.2.5 - Oxidation occurs at the anode and reduction occurs at the cathode in electrochemical cells.
- Identify electrodes as anode and cathode, and identify their signs/polarities in voltaic cells and electrolytic cells, based on the type of reaction occurring at the electrode.
Guidance
Tools and links
Reactivity 3.2.6 - A primary (voltaic) cell is an electrochemical cell that converts energy from spontaneous redox reactions to electrical energy.
- Explain the direction of electron flow from anode to cathode in the external circuit, and ion movement across the salt bridge.
Guidance
- Construction of primary cells should include: half-cells containing metal/metal ion, anode, cathode, electric circuit, salt bridge.
Tools and links
- Reactivity 1.3 - Electrical energy can be derived from the combustion of fossil fuels or from electrochemical reactions. What are the similarities and differences in these reactions?
Reactivity 3.2.7 - Secondary (rechargeable) cells involve redox reactions that can be reversed using electrical energy.
- Deduce the reactions of the charging process from given electrode reactions for discharge, and vice versa.
Guidance
- Include discussion of advantages and disadvantages of fuel cells, primary cells and secondary cells.
Tools and links
- Reactivity 2.3 - Secondary cells rely on electrode reactions that are reversible. What are the common features of these reactions?
Reactivity 3.2.8 - An electrolytic cell is an electrochemical cell that converts electrical energy to chemical energy by bringing about non-spontaneous reactions.
- Explain how current is conducted in an electrolytic cell.
- Deduce the products of the electrolysis of a molten salt.
Guidance
- Construction of electrolytic cells should include: DC power source connected to anode and cathode, electrolyte.
Tools and links
- Structure 2.1 - Under what conditions can ionic compounds act as electrolytes?
Reactivity 3.2.9 - Functional groups in organic compounds may undergo oxidation.
- Deduce equations to show changes in the functional groups during oxidation of primary and secondary alcohols, including the two-step reaction in the oxidation of primary alcohols.
Guidance
- Include explanation of the experimental set-up for distillation and reflux.
- Include the fact that tertiary alcohols are not oxidized under similar conditions.
- Names and formulas of specific oxidizing agents, and the mechanisms of oxidation, will not be assessed.
Tools and links
- Structure 3.2 - How does the nature of the functional group in a molecule affect its physical properties, such as boiling point?
- Reactivity 1.3 - What is the difference between combustion and oxidation of an alcohol?
- AHL Structure 3.1 - Why is there a colour change when an alcohol is oxidized by a transition element compound?
Reactivity 3.2.10 - Functional groups in organic compounds may undergo reduction.
- Deduce equations to show reduction of carboxylic acids to primary alcohols via the aldehyde, and reduction of ketones to secondary alcohols.
Guidance
- Include the role of hydride ions in the reduction reaction.
- Names and formulas of specific reducing agents, and the mechanisms of reduction, will not be assessed.
Tools and links
- Structure 3.1 - How can oxidation states be used to show that the following molecules are given in increasing order of oxidation: CH4, CH3OH, HCHO, HCOOH, CO2?
Reactivity 3.2.11 - Reduction of unsaturated compounds by the addition of hydrogen lowers the degree of unsaturation.
- Deduce the products of the reactions of hydrogen with alkenes and alkynes.
Guidance
Tools and links
- Reactivity 3.4 - Why are some reactions of alkenes classified as reduction reactions while others are classified as electrophilic addition reactions?
Reactivity 3.2.12 - The hydrogen half-cell H+(aq) + e- ⇌ 1/2H2(g) is assigned a standard electrode potential of zero by convention. It is used in the measurement of standard electrode potential, E⦵. (HL)
- Interpret standard electrode potential data in terms of ease of oxidation/reduction.
Guidance
- Standard reduction potentials are given in the data booklet.
Tools and links
Reactivity 3.2.13 - Standard cell potential, E⦵cell, can be calculated from standard electrode potentials. E⦵cell has a positive value for a spontaneous reaction. (HL)
- Predict whether a reaction is spontaneous in the forward or reverse direction from E⦵ data.
Guidance
Tools and links
Reactivity 3.2.14 - The equation ΔG⦵ = − nFE⦵cell shows the relationship between standard change in Gibbs energy and standard cell potential for a reaction. (HL)
- Determine the value for ΔG⦵ from E⦵ data.
Guidance
- The equation and the value of F in C mol–1 are given in the data booklet.
Tools and links
- Reactivity 1.4 - How can thermodynamic data also be used to predict the spontaneity of a reaction?
Reactivity 3.2.15 - During electrolysis of aqueous solutions, competing reactions can occur at the anode and cathode, including the oxidation and reduction of water. (HL)
- Deduce from standard electrode potentials the products of the electrolysis of aqueous solutions.
Guidance
- Electrolytic processes should include the electrolysis of water and of aqueous solutions.
- The effects of concentration and the nature of the electrode are limited to the electrolysis of NaCl(aq) and CuSO4(aq).
Tools and links
Reactivity 3.2.16 - Electroplating involves the electrolytic coating of an object with a metallic thin layer. (HL)
- Deduce equations for the electrode reactions during electroplating.
Guidance
Tools and links
- Tool 1 - How is an electrolytic cell used for electroplating?
In Reactivity 3.2 - Electron transfer reactions
- 3.2.1 - Oxidation and reduction
- 3.2.2 - Half-equations
- 3.2.3 - Redox and the periodic table
- 3.2.4 - Reactive metals and acids
- 3.2.5 - Electrochemical cells
- 3.2.6 - Primary (voltaic) cells
- 3.2.7 - Secondary (rechargeable) cells
- 3.2.8 - Electrolytic cells
- 3.2.9 - Oxidation in organic compounds
- 3.2.10 - Reduction in organic compounds
- 3.2.11 - Reduction of unsaturated compounds
- 3.2.12 - The hydrogen half-cell
- 3.2.13 - Standard cell potential
- 3.2.14 - Gibbs energy and standard cell potential
- 3.2.15 - Electrolysis of aqueous solutions
- 3.2.16 - Electroplating