Chemistry - chemical reactions and equations
What the exam tests
Chemical reactions, balanced equations, oxidation and reduction, acids and bases, and neutralisation. Expect word and symbol equations and simple mole or concentration reasoning.
Core rules
- Mass is conserved; atoms are rearranged. Balance equations by changing coefficients, never subscripts.
- Oxidation is gain of oxygen, loss of electrons or increase in oxidation number. Reduction is the reverse. They occur together in redox.
- Acid + base → salt + water is neutralisation. Acid + metal → salt + hydrogen. Acid + carbonate → salt + water + carbon dioxide.
- pH below 7 is acidic, 7 neutral, above 7 basic. Indicators show the endpoint but not the pH value precisely.
- Exothermic reactions release energy; endothermic reactions absorb it.
Worked example
Hydrochloric acid reacts with sodium hydroxide: HCl + NaOH → NaCl + H₂O. One mole of HCl neutralises one mole of NaOH. If 25.0 cm³ of 0.10 mol/L NaOH is used, it contains 0.0025 mol, so 0.0025 mol HCl is needed. If the acid concentration is 0.20 mol/L, volume = n/c = 0.0025/0.20 = 0.0125 L = 12.5 cm³.
Traps
- Water is H₂O, not H₂O₂, and formulas must be correct before balancing.
- Neutralisation does not always give pH 7; it depends on acid and base strengths and amounts.
- In redox, the substance oxidised loses electrons and is the reducing agent.