Chemical reactions Study Guide

Oxidation and reduction are fundamental chemical processes often referred to together as redox reactions. Historically, these terms were defined by the transfer of oxygen atoms. Oxidation is defined as the gain of oxygen by a substance during a chemical reaction. When a substance gains oxygen, it becomes oxidised. For example, when magnesium metal reacts with oxygen gas to form magnesium oxide, the magnesium has been oxidised because it gained oxygen. The magnesium atoms transform from Mg to MgO. Conversely, reduction is defined as the loss of oxygen from a substance during a chemical reaction. When a substance loses oxygen, it is described as being reduced. For instance, in the reaction where copper(II) oxide is heated with hydrogen to produce copper metal and water, the copper(II) oxide is reduced because it loses oxygen to become copper. It is crucial to understand that oxidation and reduction always occur simultaneously; one substance cannot gain oxygen without another substance losing oxygen, or vice versa. Therefore, in any redox reaction, there will always be one species that is oxidised and another that is reduced. Identifying which species gains or loses oxygen allows us to determine if it has been oxidised or reduced.

Electrolysis is a crucial industrial process that uses electrical energy to drive non-spontaneous chemical reactions. It specifically refers to the decomposition of an ionic compound when molten or in aqueous solution, using direct electric current. For electrolysis to occur, the ionic compound must be able to conduct electricity, meaning its ions must be free to move. This is why solid ionic compounds do not undergo electrolysis; their ions are held in a fixed lattice structure. When molten or dissolved in water, the ions become mobile. The apparatus for electrolysis consists of an electrolyte (the molten or dissolved ionic compound), electrodes (conductors, usually made of graphite or an inert metal) immersed in the electrolyte, and a power supply. There are two types of electrodes: the anode and the cathode. The anode is the positive electrode, attracting negatively charged ions (anions). Oxidation, which is the loss of electrons, occurs at the anode. The cathode is the negative electrode, attracting positively charged ions (cations). Reduction, which is the gain of electrons, occurs at the cathode. The type of products formed at each electrode depends on the specific ions present in the electrolyte and their relative reactivity. For example, in the electrolysis of molten lead bromide, lead ions (Pb²⁺) are attracted to the cathode and gain electrons to form lead metal (Pb), while bromide ions (Br⁻) are attracted to the anode and lose electrons to form bromine gas (Br₂). In aqueous solutions, water molecules can also be involved in the reactions, complicating the outcome. For instance, if hydrogen ions and a less reactive metal ion are present, hydrogen gas might be produced at the cathode instead of the metal.