Chemical reactions Study Guide

Understanding chemical formulae and balanced equations is fundamental to chemistry. A chemical formula represents a molecule or compound, showing the types of atoms involved and how many of each. For instance, H₂O indicates two hydrogen atoms and one oxygen atom form a water molecule. Elements are represented by standard symbols, like 'H' for hydrogen and 'O' for oxygen. Memorising common element symbols and the formulae of simple compounds is essential.

A chemical equation describes a chemical reaction, with reactants on the left and products on the right, separated by an arrow. For example, 2H₂ + O₂ → 2H₂O shows that two molecules of hydrogen gas react with one molecule of oxygen gas to produce two molecules of water. The numbers before the chemical formulae are called stoichiometric coefficients, which indicate the relative number of molecules or moles of each substance involved.

The principle of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. This means the total mass of the reactants must equal the total mass of the products. To satisfy this principle, chemical equations must be 'balanced'. Balancing an equation involves adjusting the stoichiometric coefficients so that the number of atoms of each element is the same on both sides of the equation. For example, in the reaction 2H₂ + O₂ → 2H₂O, there are four hydrogen atoms on both sides (2 × 2) and two oxygen atoms on both sides (1 × 2 on the product side, and 2 × 1 on the reactant side). You cannot change the small numbers within a chemical formula (subscripts), as that would change the identity of the compound itself. Balancing is a trial-and-error process, often starting with elements that appear in only one reactant and one product.

Half equations are used to represent changes in individual species, typically in redox reactions or at electrodes. They show the loss or gain of electrons. While less common in introductory 'balancing equations' contexts, they still adhere to the principle of conservation of mass where atoms must balance, although charges must also balance, which is a key distinguishing feature of half-equations.

Balanced ionic equations provide a precise representation of chemical reactions by focusing only on the particles directly involved in the chemical change, excluding spectator ions. Spectator ions are ions that remain in solution unchanged throughout the reaction, appearing on both sides of the full ionic equation. To construct a balanced ionic equation, you first need the balanced symbol equation for the reaction. Next, identify all aqueous ionic compounds and dissociate them into their constituent ions. Solids, liquids, and gases do not dissociate and are written in their molecular or empirical formula. For example, solid precipitates, water, and gaseous products remain intact. Once all applicable species are written as ions, compare both sides of the equation. Any identical ions (in terms of type, number, and charge) present on both the reactant and product sides are spectator ions and can be cancelled out. The remaining ions and neutral species form the balanced ionic equation. The final ionic equation must be balanced for both atoms and charge. The sum of charges on the reactant side must equal the sum of charges on the product side. This method is particularly useful for understanding precipitation reactions, acid-base neutralisations, and redox reactions in aqueous solutions, highlighting the actual chemical transformation occurring. Remember to include state symbols for all species in the ionic equation to indicate whether they are aqueous (aq), solid (s), liquid (l), or gas (g).