Chemical Reactions and Equations

The One Thing

Chemical reactions transform reactants into products with different properties, and chemical equations represent these transformations while obeying the law of conservation of mass. Reactions can be classified by how substances combine, decompose, displace one another, or exchange ions, with oxidation and reduction occurring together in redox reactions.

Reactions, Processes and Experiments

Key Terms

• Chemical reaction: A process in which reactants change into one or more new substances called products.
• Reactants: The substances present at the beginning of a chemical reaction.
• Products: The new substances formed during a chemical reaction.
• Chemical equation: A symbolic representation of a chemical reaction using chemical formulae and symbols.
• Balanced chemical equation: An equation in which the number of atoms of each element is equal on both sides.
• Law of conservation of mass: Mass is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.
• Combination reaction: A reaction in which two or more substances combine to form a single product.
• Decomposition reaction: A reaction in which one compound breaks down into two or more simpler substances.
• Thermal decomposition: Decomposition caused by heat.
• Electrolytic decomposition: Decomposition caused by electricity.
• Photochemical decomposition: Decomposition caused by light.
• Displacement reaction: A reaction in which a more reactive element displaces a less reactive element from its compound.
• Double displacement reaction: A reaction in which two compounds exchange ions to form two new compounds.
• Precipitation reaction: A reaction in which an insoluble solid forms from solutions.
• Oxidation: Gain of oxygen, loss of hydrogen, loss of electrons, or increase in oxidation state.
• Reduction: Loss of oxygen, gain of hydrogen, gain of electrons, or decrease in oxidation state.
• Redox reaction: A reaction in which oxidation and reduction take place simultaneously.
• Oxidising agent: A substance that causes oxidation of another substance and itself undergoes reduction.
• Reducing agent: A substance that causes reduction of another substance and itself undergoes oxidation.
• Exothermic reaction: A reaction that releases heat.
• Endothermic reaction: A reaction that absorbs energy from the surroundings.
• Corrosion: The slow damage of metals due to reactions with air, moisture, or chemicals.
• Rancidity: The oxidation of fats and oils that causes unpleasant smell and taste.
• Catalyst: A substance that changes the rate of a reaction without being permanently consumed.
• State symbols: Symbols showing physical state: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution.
• Precipitate: An insoluble solid formed during a reaction in solution.
• Coefficient: A number placed before a chemical formula to balance an equation.

Easily Confused

• Reactants and products: Reactants are present at the beginning; products are formed during the reaction.
• Oxidation and reduction: Oxidation involves gain of oxygen, loss of hydrogen or electrons, or an increase in oxidation state; reduction involves the opposite changes.
• Combination and decomposition: Combination forms one product from two or more substances; decomposition breaks one compound into simpler substances.
• Displacement and double displacement: Displacement involves one element replacing another; double displacement involves exchange of ions between two compounds.
• Exothermic and endothermic reactions: Exothermic reactions release energy; endothermic reactions absorb energy.
• Oxidising and reducing agents: An oxidising agent is itself reduced, whereas a reducing agent is itself oxidised.
• Coefficients and subscripts: Coefficients may be changed to balance an equation; subscripts must not be changed because they alter the substances.
• Corrosion and rancidity: Corrosion affects metals, whereas rancidity is oxidation of fats and oils.
• Thermal, electrolytic and photochemical decomposition: Thermal decomposition is caused by heat, electrolytic decomposition by electricity, and photochemical decomposition by light.
• Double displacement and precipitation: Double displacement describes ion exchange; precipitation specifies that an insoluble solid forms.

What Gets Asked

• Writing and balancing equations: Students may be asked to represent a reaction as Reactants → Products and balance it using the smallest whole-number coefficients. Changing subscripts instead of coefficients changes the substances and loses marks.
• Identifying reaction types: Questions may use the patterns A + B → AB, AB → A + B, A + BC → AC + B, and AB + CD → AD + CB. The specific arrangement of reactants and products determines the classification.
• Explaining redox reactions: In CuO + H2 → Cu + H2O, students must identify that CuO is reduced to Cu and H2 is oxidised to H2O, rather than describing only one half of the redox process.
• Explaining displacement: In Zn + CuSO4 → ZnSO4 + Cu, zinc displaces copper because zinc is more reactive. Omitting the reactivity explanation loses the key reason for the reaction.
• Interpreting observations: A change in colour, temperature, state, gas formation, or precipitate formation may indicate a chemical reaction. A precipitate must be identified as an insoluble solid formed in solution.
• Applying oxidation concepts: Questions may ask about rusting and rancidity, including their causes and prevention. Rusting requires iron, oxygen, and water, while rancidity results from oxidation of fats and oils.