Chemical Reactions and Equations
Chemical Symbols and Formulae
Every element has a unique chemical symbol, shown on the periodic table. Compounds are represented by chemical formulae that show which elements are present and how many atoms of each. For example, \(H_2O\) tells you water contains 2 hydrogen atoms and 1 oxygen atom. \(CaCO_3\) tells you calcium carbonate contains 1 calcium, 1 carbon and 3 oxygen atoms. Small subscript numbers apply to the element directly before them; a number before a formula (a coefficient) multiplies every atom in that formula.
Working out the formula of a compound from its name requires knowledge of ion charges. For example, sodium chloride is \(NaCl\) because \(Na^+\) and \(Cl^-\) balance in a 1:1 ratio. Magnesium oxide is \(MgO\) (\(Mg^{2+}\) and \(O^{2-}\)).
Chemical formulae show which elements are present and the ratio of atoms. Subscript numbers apply to the preceding element; coefficients multiply the whole formula. \(H_2O\) = 2 hydrogen + 1 oxygen.
Writing and Balancing Equations
A chemical equation shows reactants on the left and products on the right, separated by an arrow (\(\rightarrow\)). A word equation uses names: magnesium + oxygen → magnesium oxide. A symbol equation uses formulae: \(2Mg + O_2 \rightarrow 2MgO\).
Symbol equations must be balanced so that the number of atoms of each element is the same on both sides. This reflects the conservation of mass — atoms are rearranged in reactions but never created or destroyed (C3.1k). To balance an equation: count atoms on each side, add coefficients (large numbers in front of formulae) until both sides match, and never change the subscript numbers in a formula.
Balance the reaction between hydrogen and oxygen to form water:
Unbalanced: \(H_2 + O_2 \rightarrow H_2O\)
Count atoms: Left — 2 H, 2 O. Right — 2 H, 1 O. Oxygen is unbalanced.
Put a 2 in front of \(H_2O\): \(H_2 + O_2 \rightarrow 2H_2O\). Now right side has 4 H and 2 O. Left has 2 H and 2 O. Hydrogen is unbalanced.
Put a 2 in front of \(H_2\): \(2H_2 + O_2 \rightarrow 2H_2O\). Now both sides have 4 H and 2 O. Balanced.
Equations must be balanced: same number of atoms on each side. Add coefficients to balance — never change subscript numbers. Conservation of mass: total mass of reactants = total mass of products.
State Symbols
State symbols are written after each formula to show the physical state: (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous (dissolved in water). For example:
$$2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$$
State symbols give extra information about reaction conditions and help you understand whether substances are dissolved, gaseous, etc.
Conservation of mass can be demonstrated experimentally. If you carry out a reaction in a sealed container and measure the total mass before and after, it remains the same. In open containers, reactions that produce gas may appear to lose mass (because the gas escapes), but the total mass of all products — including the gas — equals the total mass of reactants.
An important application of balanced equations is calculating reacting masses. Using relative atomic masses from the periodic table, you can work out how much product will form from a given amount of reactant. For example, from \(2Mg + O_2 \rightarrow 2MgO\): 2 moles of magnesium (48 g) react with 1 mole of oxygen (32 g) to form 2 moles of magnesium oxide (80 g). This confirms conservation of mass: 48 + 32 = 80.
You should also be aware of the difference between exothermic reactions (which release energy to the surroundings, causing a temperature rise, e.g. combustion and neutralisation) and endothermic reactions (which absorb energy from the surroundings, causing a temperature drop, e.g. thermal decomposition and dissolving ammonium nitrate in water). Energy changes during reactions are linked to the breaking and forming of chemical bonds.
Breaking bonds requires energy (endothermic process), while forming bonds releases energy (exothermic process). In an exothermic reaction, the energy released by forming new bonds exceeds the energy needed to break existing bonds, so the overall reaction releases energy to the surroundings and the temperature rises. In an endothermic reaction, the opposite is true — more energy is needed to break bonds than is released when new bonds form.
Types of Chemical Reaction
OCR Gateway requires knowledge of several reaction types:
• Combustion: a substance reacts with oxygen, releasing energy. \(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)
• Displacement: a more reactive element replaces a less reactive one. \(Zn + CuSO_4 \rightarrow ZnSO_4 + Cu\)
• Neutralisation: acid + base → salt + water. \(HCl + NaOH \rightarrow NaCl + H_2O\)
• Oxidation: gaining oxygen (or losing electrons). Reduction: losing oxygen (or gaining electrons). Redox reactions involve both.
• Thermal decomposition: a substance breaks down when heated. \(CaCO_3 \rightarrow CaO + CO_2\)
Key reaction types: combustion (fuel + oxygen), displacement (reactive metal replaces less reactive one), neutralisation (acid + base → salt + water), thermal decomposition (breaks down with heat). All must be balanced.