In a chemical reaction, the atoms of the reacting molecules or elements are rearranged to form new molecules: old chemical bonds are broken and new bonds are formed. Bond breaking absorbs energy, whereas bond formation releases energy. This chapter classifies chemical reactions, explains the rate of a reaction, equilibrium, the ionic product of water and the pH scale.
1. Representing a Chemical Reaction
- Word equation: Methane + Oxygen → Carbon dioxide + Water. It does not give the chemical composition of the reactants and products.
- Chemical equation: the chemicals are shown by their formulas. Reactants are written to the left of the arrow and products to the right; the arrow shows the direction of the reaction.
- The law of conservation of matter states that matter cannot be created or destroyed. So the number of atoms of the reactants and the products must be equal: CH4 + 2O2 → CO2 + 2H2O.
- A balanced chemical equation is the simplified representation of a chemical reaction which describes the chemical composition, physical state of the reactants and the products, and the reaction conditions: CH4(g) + 2O2(g) → CO2(g) + 2H2O(g).