Redox Reactions

The One Thing

Redox reactions are simultaneous electron-transfer processes: oxidation releases electrons while reduction accepts them. Oxidation-number changes, conservation of atoms and charge, and electrode definitions provide systematic ways to identify and balance these reactions.

Reactions, Processes and Experiments

Key Terms

• Oxidation: Loss of electrons, addition of oxygen, removal of hydrogen, or increase in oxidation number.
• Reduction: Gain of electrons, removal of oxygen, addition of hydrogen, or decrease in oxidation number.
• Redox reaction: A chemical reaction in which oxidation and reduction take place simultaneously.
• Oxidising agent: A substance that accepts electrons and causes oxidation of another substance; it undergoes reduction.
• Reducing agent: A substance that donates electrons and causes reduction of another substance; it undergoes oxidation.
• Oxidation number: The apparent charge assigned to an atom in a compound or ion by assuming that bonding electrons belong to the more electronegative atom.
• Disproportionation: A redox reaction in which the same element is simultaneously oxidised and reduced.
• Comproportionation: A redox reaction in which the same element in two different oxidation states forms a product with an intermediate oxidation state.
• Electrode: A conducting surface through which electrons enter or leave an electrochemical cell.
• Anode: The electrode where oxidation occurs.
• Cathode: The electrode where reduction occurs.
• Half-reaction: An equation showing either the oxidation or reduction part of a redox reaction.
• Electrochemical cell: A system in which chemical and electrical energy are interconverted through redox reactions.

Easily Confused

• Oxidation and reduction: Oxidation involves electron loss or an increase in oxidation number; reduction involves electron gain 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.
• Anode and cathode: Oxidation always occurs at the anode, whereas reduction always occurs at the cathode, regardless of cell type.
• Galvanic and electrolytic cells: The galvanic-cell anode is negative and cathode positive; the electrolytic-cell anode is positive and cathode negative.
• Disproportionation and comproportionation: Disproportionation produces simultaneous oxidation and reduction of the same element; comproportionation combines two oxidation states to form an intermediate oxidation state.
• Oxidation number and ionic charge: The oxidation number is an assigned value based on bonding-electron assumptions, whereas the charge of a monoatomic ion is its actual ionic charge.

What Gets Asked

• Identify oxidation and reduction in Zn + Cu2+ → Zn2+ + Cu. The mark-losing error is assigning zinc as the oxidising agent; zinc is oxidised and is therefore the reducing agent, while Cu²⁺ is reduced and is the oxidising agent.
• Calculate oxidation numbers, including manganese in MnO4−. Use x + 4(−2) = −1 to obtain manganese as +7.
• Apply oxidation-number rules. Common exceptions include oxygen at −1 in peroxides and positive in compounds with fluorine, and hydrogen at −1 in metal hydrides such as NaH.
• Balance redox equations by the ion-electron method. Balance atoms other than oxygen and hydrogen first, then oxygen with H₂O, hydrogen with H⁺ in acidic medium, and charge with electrons.
• Adapt the ion-electron method to basic medium. Add an equal number of OH⁻ ions to both sides to neutralise excess H⁺, then cancel water molecules.
• Determine electrode signs and electron direction. Oxidation is at the anode and reduction at the cathode; the anode is negative in a galvanic cell but positive in an electrolytic cell, and electrons flow from anode to cathode.