1.5.1Principles of Chemistry: Part 1
Word and Chemical Equations — Edexcel International GCSE Science Double Award (Modular) - Chemistry Unit 1 Revision Notes & Topical Questions
5 min read 7 parts Lesson 16 of 23
Part 1 of 5Conservation of mass
1Atoms are rearranged
- Atoms are rearranged In a chemical reaction the atoms in the reactants join up in new ways to make the products.
2None created or destroyed
- None created or destroyed The same atoms are present before and after the reaction.
3Mass stays constant
- Mass stays constant The total mass of the reactants equals the total mass of the products, which is the law of conservation of mass.
4Equations must balance
- Equations must balance A symbol equation therefore needs the same number of each type of atom on both sides of the arrow.
Part 2 of 5Word equations
Layout
- Layout Reactants are written on the left and products on the right, separated by an arrow that means 'reacts to form': reactants → products.
Conditions and catalysts
- Conditions and catalysts A catalyst or a condition such as heat can be written above the arrow.
Example
- Example sodium hydroxide + hydrochloric acid → sodium chloride + water.
Reading the question
- Reading the question Sometimes the products are named first in the question,
- so read carefully to decide which substances are reactants and which are products.
Part 3 of 5Balancing a symbol equation
1
Write the word equation, then replace each name with the correct formula. Never change a formula to help balance.
2
Count the atoms of each element on both sides of the arrow.
3
Add large numbers (coefficients) in front of formulae to make the atom counts equal, adjusting one element at a time.
4
Recount all the elements, leaving hydrogen or oxygen until last if they appear in several substances.
5
Add state symbols where required: (s) solid, (l) liquid, (g) gas, (aq) aqueous solution.
Part 4 of 5Worked example: magnesium and oxygen
| Stage | Working |
|---|---|
| Word equation | magnesium + oxygen → magnesium oxide |
| Unbalanced symbols | Mg + O2 → MgO |
| Count | Left has 2 O atoms, right has 1 O atom |
| Balance oxygen | Put 2 in front of MgO: Mg + O2 → 2MgO |
| Balance magnesium | Put 2 in front of Mg: 2Mg + O2 → 2MgO |
| With states | 2Mg(s) + O2(g) → 2MgO(s) |
Part 5 of 5Common reactions to be able to write
Typical equations
Neutralisation
- Neutralisation NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Burning carbon
- Burning carbon C(s) + O2(g) → CO2(g)
Ammonia and nitric acid
- Ammonia and nitric acid NH3 + HNO3 → NH4NO3, making the fertiliser ammonium nitrate.
Key terms
| Term | Meaning |
|---|---|
| Reactant | A substance that is used up in a chemical reaction. |
| Product | A substance that is made in a chemical reaction. |
| Law of conservation of mass | In a chemical reaction the total mass of the products equals the total mass of the reactants. |
| Balanced equation | An equation with the same number of each type of atom on both sides. |
| State symbol | A symbol (s), (l), (g) or (aq) showing the physical state of a substance. |
| Catalyst | A substance that speeds up a reaction without being used up. |
Avoid these mistakes
1
Changing the small subscript numbers in a formula to balance an equation. Only the large numbers in front may be changed.
2
Writing the word equation with products first, because the question mentioned them first.
3
Forgetting that diatomic elements such as O2, H2 and Cl2 exist as pairs of atoms.
4
Leaving out state symbols when asked, or using (aq) for a pure liquid.
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