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1. What is a Chemical Reaction?
A chemical reaction is a process in which one or more substances are transformed into new substances with different properties. The starting substances are called reactants, while the new substances formed are called products.
Signs of a chemical reaction
- Change in state
- Change in colour
- Evolution of a gas
- Change in temperature or energy
- Formation of a precipitate
Examples
2Mg + O₂ → 2MgO
Zn + 2HCl → ZnCl₂ + H₂
2. Writing a Chemical Equation
A chemical equation represents a chemical reaction using symbols and formulae. A word equation is converted into a symbolic equation by writing the correct chemical formulae.
Word equation → Formula equation
The equation above is a skeletal/unbalanced equation because the number of atoms of each element is not equal on both sides.
Making an equation informative
State symbols may be added: (s) solid, (l) liquid, (g) gas and (aq) aqueous solution. Conditions such as heat or light may be shown above the arrow.
3. Balancing Chemical Equations
Chemical equations are balanced according to the law of conservation of mass: atoms are neither created nor destroyed in a chemical reaction. Therefore, the number of atoms of every element must be the same on both sides.
Example
There are 2 oxygen atoms on the left but only 1 on the right. Balance the equation by changing coefficients, not subscripts.
Step-by-step method
- Write the correct formulae of reactants and products.
- Count atoms of each element on both sides.
- Use coefficients to equalise the atom counts.
- Use the smallest whole-number coefficients.
- Recount all atoms and check the final equation.
More balanced equations
4. Types of Chemical Reactions
A. Combination Reaction
Two or more substances combine to form a single product.
B. Decomposition Reaction
A single compound breaks down into two or more simpler substances. It may occur by heating, light or electricity.
C. Displacement Reaction
A more reactive element displaces a less reactive element from its compound.
D. Double Displacement Reaction
Two compounds exchange ions to form two new compounds. A precipitate may form.
BaSO₄ is an insoluble white precipitate.
E. Redox Reaction
In a redox reaction, oxidation and reduction occur together.
| Reaction type | General idea | Example |
|---|---|---|
| Combination | Many reactants → one product | CaO + H₂O → Ca(OH)₂ |
| Decomposition | One reactant → many products | CaCO₃ → CaO + CO₂ |
| Displacement | One element replaces another | Zn + CuSO₄ → ZnSO₄ + Cu |
| Double displacement | Exchange of ions | Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl |
| Redox | Oxidation and reduction together | CuO + H₂ → Cu + H₂O |
5. Oxidation and Reduction
Oxidation can be understood as addition of oxygen or removal of hydrogen, while reduction can be understood as removal of oxygen or addition of hydrogen.
Oxidation
- Addition of oxygen
- Removal of hydrogen
Reduction
- Removal of oxygen
- Addition of hydrogen
Oxidising and reducing agents
A substance that causes oxidation of another substance is an oxidising agent. A substance that causes reduction of another substance is a reducing agent.
- CuO is reduced, so H₂ acts as the reducing agent.
- H₂ is oxidised, so CuO acts as the oxidising agent.
6. Effects of Oxidation in Everyday Life
Corrosion
Corrosion is the gradual destruction of a metal due to chemical or electrochemical reaction with its environment. Rusting of iron is a common example.
Painting, oiling/greasing, galvanising and using suitable alloys are common ways of protecting metals from corrosion.
Rancidity
Rancidity is the oxidation of fats and oils in food, often causing an unpleasant smell and taste.
It can be reduced by refrigeration, airtight storage, suitable antioxidants and packaging certain foods in an inert gas such as nitrogen.
7. Quick Revision & Exam Focus
Reactants are converted into new products.
Same number of atoms of each element on both sides.
Two or more reactants form one product.
One compound breaks into simpler substances.
More reactive element displaces less reactive element.
Ions exchange between two compounds.
Addition of oxygen / removal of hydrogen.
Removal of oxygen / addition of hydrogen.
Must-learn equations
8. Practice Questions
- What is a chemical reaction? Give two examples.
- List four observations that may indicate a chemical reaction.
- What are reactants and products?
- Why must a chemical equation be balanced?
- State the law of conservation of mass.
- Balance: H₂ + Cl₂ → HCl.
- Balance: Fe + O₂ → Fe₂O₃.
- Balance: Al + HCl → AlCl₃ + H₂.
- What is a combination reaction? Give an equation.
- What is a decomposition reaction? Give one thermal example.
- What is photochemical decomposition?
- What is a displacement reaction?
- Why does zinc displace copper from CuSO₄?
- Define double displacement reaction.
- What is a precipitate? Give an example.
- Define oxidation and reduction.
- Identify oxidation and reduction in CuO + H₂ → Cu + H₂O.
- What is a redox reaction?
- What is corrosion?
- Give two methods of preventing corrosion.
- What is rancidity?
- How does refrigeration help prevent rancidity?
- Why are some food packets flushed with nitrogen?
- Differentiate combination and decomposition reactions.
- Differentiate displacement and double displacement reactions.
- Write the balanced equation for electrolysis of water.
- Write the balanced equation for decomposition of calcium carbonate by heating.
- Write the equation for zinc reacting with dilute hydrochloric acid.
- Why should subscripts not be changed while balancing?
- Why are corrosion and rancidity undesirable effects of oxidation?
9. Interactive Quiz
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