Word and Chemical Equations

Section 1 of 7Conservation 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.

Section 2 of 7Word 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.

Section 3 of 7Balancing 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.

Section 4 of 7Worked 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)

Section 5 of 7Common 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.

Section 6 of 7Key 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.

Section 7 of 7Exam technique

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.