Chemical reactions Study Guide

Oxidation and reduction are fundamental chemical processes that often occur together in what are known as redox reactions. When we consider these reactions in terms of oxygen transfer, the definitions are quite straightforward. Oxidation is defined as the gain of oxygen by a substance. For example, when a metal reacts with oxygen to form a metal oxide, the metal has been oxidised because it has gained oxygen atoms. Conversely, reduction is defined as the loss of oxygen from a substance. A common example is the extraction of metals from their ores, where a metal oxide loses its oxygen to become a pure metal; in this process, the metal oxide has been reduced. It's crucial to remember that oxidation and reduction always happen simultaneously. If one substance gains oxygen (is oxidised), another substance must lose oxygen (is reduced). In a reaction, the species that causes another species to be oxidised is called the oxidising agent, and in terms of oxygen transfer, the oxidising agent is the one that provides the oxygen and thus gets reduced itself. The species that causes another species to be reduced is called the reducing agent, and it is the one that removes oxygen, getting oxidised in the process. For instance, in the reaction between copper(II) oxide and hydrogen gas to produce copper and water, copper(II) oxide loses oxygen, so it is reduced. Hydrogen gains oxygen, so it is oxidised. Copper(II) oxide acts as the oxidising agent, and hydrogen acts as the reducing agent.

The Law of Conservation of Mass is a fundamental principle in chemistry. It states that during any chemical reaction, no atoms are created or destroyed. This means that the total mass of the reactants before a chemical reaction must be exactly equal to the total mass of the products after the reaction. Think of it like Lego bricks: you can take apart a Lego house and build a Lego car, but you still have the same number and type of Lego bricks. Similarly, in a chemical reaction, atoms are rearranged to form new substances, but their total number and mass remain constant. For example, if 10 grams of substance A reacts completely with 5 grams of substance B, then the total mass of the products formed must be 15 grams. This law is crucial for understanding and balancing chemical equations, as well as for performing quantitative chemistry calculations. It also explains why an open system might appear to lose or gain mass if gases are involved, but in a closed system, the mass will always remain constant.