Chapter 9: Oxidation and Reduction Glossary

Worksheet 9.1

Chapter 9: Oxidation and reduction – glossary

Adsorption The process where a gas, liquid or solid forms a layer on the surface of a solid. As it is a surface phenomenon, it is increased when the solid has a large surface area.

Anode The electrode where oxidation occurs; it is negatively charged in a galvanic cell and positively charged in an electrolytic cell.

Cathode The electrode where reduction occurs; it is positively charged in a galvanic cell and negatively charged in an electrolytic cell.

Displacement reactions A redox reaction in which a more reactive element displaces a less reactive element from a solution of its ions, often in aqueous solution.

Disproportionation A redox reaction in which different atoms of the same element are simultaneously oxidized and reduced.

Electrode Where an electric current enters and leaves an electrolyte. The two electrodes are known as the anode and cathode, and are defined by where the electrons enter (cathode) or leave (anode) the electrolyte. Oxidation always occurs at the anode and reduction always occurs at the cathode.

Electrode potential The potential difference produced between the electrode and the solution in a half-cell. This can only be measured in reference to another half-cell. The electrode potential is usually measured relative to the hydrogen half-cell, when it is known as the standard electrode potential.

Electrolysis The process when the passage of an external source of voltage through an electrolyte generates chemical change leading to the breakdown of the electrolyte.

Electrolyte A liquid that conducts electricity as a result of the presence of ions. Electrolytes are either molten salts or aqueous solutions of salts, acids or alkalis.

Electromotive force (emf) The maximum potential difference that can be generated by a particular source of electric current. Due to internal resistance, this can be measured in practice only when the source is not supplying current.

Electroplating The process where electrolysis is used to deposit a layer of a metal on the surface of another metal. The electrolyte contains the ion of the metal to be plated, and the cathode consists of the object to be plated.

Galvanized iron Iron that is protected from corrosion by a thin layer of a more reactive metal, such as zinc, on its surface. The zinc is preferentially oxidized, and so protects the iron below from oxidation.

Half-cells Also known as electrodes. Each half-cell consists of a metal in contact with a solution of its ions (or a platinum electrode in contact with a solution of ions or gas). When the two half-cells are joined by a salt bridge and an external circuit, an electric voltage is generated. The magnitude of this depends on the difference between the reduction potentials of the two half-cells.

Half-equations An equation that shows only one of the two processes – oxidation or reduction – which occur in a redox reaction. Each half-equation is balanced using electrons: the oxidation reaction has electrons on the right and the reduction half-equation has electrons on the left. The sum of the two half-equations gives the net reaction, with the electrons cancelling out.

Ion discharge The process of releasing a neutral product from an ion as a result of redox change at an electrode during electrolysis. Discharge involves oxidation of anions at the anode, and reduction of cations at the cathode.

Ionic equation An equation showing the ions and species taking part in a reaction, usually with the omission of spectator ions. Ionic equations are commonly used to describe redox reactions.

Oxidation The loss of electrons from a species during a chemical reaction.

Oxidation number or state A value assigned to an atom in a compound, which is a measure of the electron control it has relative to the pure element, which is assigned a value of zero. It is a way of tracking changes in electron density during a reaction. The oxidation number is shown using Roman numerals in brackets after the symbol for the element.

Oxidizing agent A species that brings about oxidation by removing electrons from another reactant. During the reaction the oxidizing agent gains electrons and so is reduced.

Platinized platinum Platinum metal that is coated with a thin layer of finely divided platinum pieces. The larger surface area of the metal increases the adsorption of gases. It is found in the standard hydrogen electrode.

Reactivity series A list of metal ions in order of their relative tendency to become oxidized. The series shows the order in which metals can displace one another from their salts.

Redox reaction A reaction in which oxidation and reduction occur. It is characterized by changes in the oxidation numbers of the atoms taking part in the reaction.

Reducing agent A species that brings about reduction by giving electrons to another reactant. During the reaction the reducing agent loses electrons and so is oxidized.

Reduction The gain of electrons by a species during a chemical reaction.

Sacrificial protection The process where a more reactive metal is used to coat the surface of a less reactive metal so it will be preferentially oxidized and so protect the inner metal. A layer of zinc on iron, known as galvanization, is a common example.

Salt bridge A connection between two half-cells in a galvanic cell that completes the circuit and allows current to flow by preventing a build-up of charge. A salt bridge is usually constructed from a glass tube or filter paper, containing a solution of ions which will not interfere with the redox reactions occurring at the electrodes.

Selective discharge This occurs when there is more than one ion carrying the same charge (positive or negative) present in an electrolyte during electrolysis. Both ions will then be attracted to the same electrode and the selection of which ion is discharged depends on their relative electrode potentials, their concentrations, and in some cases the chemical nature of the electrode.

Standard conditions The conditions used when measuring the standard electrode potential of a half-cell. They are:25°C, 1 atmosphere pressure, 1 mol dm–3 solutions, pure substances.

Standard electrode potential E° Also known as standard reduction potential. This is the electrode potential of a half-cell relative to the hydrogen half-cell, which is assigned a value of zero. It is measured at standard conditions.

Standard hydrogen electrode Used to measure standard electrode potentials of a half-cell when they are connected together. The hydrogen electrode consists of platinum metal with a platinized surface, in contact with a solution of H + ions with H2 gas at 1 atmosphere pressure bubbling through. The hydrogen electrode is assigned a voltage of 0V. It will be reduced by half-cells with a lower electrode potential, and oxidized by half-cells with a higher electrode potential.

Voltaic cell Also known as a galvanic cell. It consists of two half-cells, each of which is composed of an electrode in contact with an electrolyte, joined together by a salt bridge and an external circuit. It generates an electric voltage as the result of spontaneous redox change.

Voltmeter An instrument used to measure the voltage generated, for example, in a galvanic cell.