Redox Reactions

The One Thing

Redox reactions are coupled electron-transfer processes: oxidation and reduction occur simultaneously because electrons lost by one species are gained by another. Their equations must conserve both atoms and total electrical charge.

Reactions, Processes and Experiments

Key Terms

• Oxidation: A process involving loss of electrons, increase in oxidation number, addition of oxygen, or removal of hydrogen.
• Reduction: A process involving gain of electrons, decrease in oxidation number, removal of oxygen, or addition of hydrogen.
• Redox reaction: A reaction in which oxidation and reduction occur simultaneously.
• Oxidising agent: A substance that causes oxidation of another substance and is itself reduced by accepting electrons.
• Reducing agent: A substance that causes reduction of another substance and is itself oxidised by donating electrons.
• Oxidation number: The apparent charge assigned to an atom in a compound or ion according to rules for electron distribution.
• Disproportionation: A redox reaction in which the same element in one oxidation state is simultaneously oxidised and reduced.
• Comproportionation: A reaction in which two forms of the same element with different oxidation numbers combine to form a product with an intermediate oxidation number.
• Half-reaction: One of the two parts of a redox reaction showing either oxidation or reduction separately.
• Electrochemical cell: A system in which a redox reaction produces electrical energy or electrical energy drives a chemical reaction.
• Equivalent mass: The mass of an oxidising or reducing substance corresponding to the number of electrons accepted or donated per formula unit in a particular reaction.
• n-factor: The number of electrons gained or lost by one formula unit of a redox substance during a reaction.
• Molarity: The concentration represented by M in the relationship N = M x n-factor.
• Normality: The concentration represented by N in the relationship N = M x n-factor.
• Redox titration: A titration using an electron-transfer reaction to determine the concentration of an oxidising or reducing agent.

Easily Confused

• Oxidation and reduction: Oxidation is loss of electrons or an increase in oxidation number; reduction is gain of electrons or a decrease in oxidation number.
• Oxidising agent and reducing agent: An oxidising agent accepts electrons and is reduced; a reducing agent donates electrons and is oxidised.
• Disproportionation and comproportionation: Disproportionation produces both oxidation and reduction of the same element from one oxidation state; comproportionation combines two oxidation states to produce an intermediate oxidation state.
• Oxidation number and ionic charge: Oxidation number is an assigned apparent charge based on electron-distribution rules; the charge of a monoatomic ion is its actual ionic charge.
• Balancing atoms and balancing charge: Ionic equations require conservation of both every element and total electrical charge; balancing elements alone is insufficient.
• Oxidation-number method and ion-electron method: The oxidation-number method equalises oxidation-number changes, whereas the ion-electron method balances separate half-reactions using species such as H2O, H+, OH-, and electrons.
• Molarity and normality: Molarity is related to normality through N = M x n-factor; normality depends additionally on the reaction-specific n-factor.

What Gets Asked

• Identify which species is oxidised and which is reduced in Zn + Cu2+ -> Zn2+ + Cu; the common error is reversing the electron changes.
• Identify the oxidising agent and reducing agent in Zn + Cu2+ -> Zn2+ + Cu; the oxidising agent is reduced, while the reducing agent is oxidised.
• Assign oxidation numbers using the stated rules, including the exceptions for oxygen in peroxides and compounds with fluorine, and hydrogen in metal hydrides such as NaH and CaH2.
• Balance a redox equation by the oxidation-number method; marks are lost when the total increase and decrease in oxidation number are not equalised.
• Balance an ionic equation by the ion-electron method in acidic or basic medium; the relevant distinction is the use of H+ and H2O in acidic medium and OH- followed by cancellation of excess water in basic medium.
• Calculate equivalent mass, n-factor, molarity, or normality using the reaction-specific number of electrons and N = M x n-factor; using an inappropriate n-factor loses marks.