Word and balanced symbol equations with state symbols, the law of conservation of mass, and explaining apparent mass changes.

What this dot point is asking

WJEC Double Award Unit 2 wants you to write word and balanced symbol equations with state symbols, state the law of conservation of mass, and explain apparent changes in mass.

Word and symbol equations

For example, the reaction of carbon with oxygen:

• Word equation: carbon + oxygen to carbon dioxide.
• Symbol equation: $C + O_2 \rightarrow CO_2$.

State symbols can be added to show the physical state: $(s)$ solid, $(l)$ liquid, $(g)$ gas, $(aq)$ aqueous (dissolved in water).

Balancing equations

For hydrogen burning in oxygen, $H_2 + O_2 \rightarrow H_2O$ is not balanced (2 O on the left, 1 on the right). Balancing gives $2H_2 + O_2 \rightarrow 2H_2O$: now there are 4 hydrogen and 2 oxygen atoms on each side.

The law of conservation of mass

Because the atoms are only rearranged, the total mass cannot change. This is why a balanced equation must have the same atoms on both sides.

Explaining apparent mass changes

Sometimes the mass seems to change because a gas is involved:

• Apparent increase: when a metal burns, it gains mass because it combines with oxygen from the air, which adds to the solid (the gas is taken in).
• Apparent decrease: when a carbonate is heated, it loses mass because carbon dioxide gas escapes into the air.

If the reaction were done in a sealed container, the total mass would not change, showing mass is conserved.

Exothermic and endothermic reactions

Reactions also involve energy, and you may be asked to spot the type from a temperature change. An exothermic reaction gives out energy to the surroundings, so the temperature rises (for example combustion and neutralisation). An endothermic reaction takes in energy from the surroundings, so the temperature falls (for example some dissolving and thermal decomposition reactions). Measuring the temperature before and after a reaction lets you classify it, which links equations to energy changes covered later in the course.

Reactants and products in everyday reactions

Practising with real reactions helps. When magnesium reacts with hydrochloric acid, the reactants are magnesium and hydrochloric acid, and the products are magnesium chloride and hydrogen gas (seen as bubbles). When a fuel burns, the reactants are the fuel and oxygen, and the products are carbon dioxide and water. Being able to name the reactants and products, and then write the word equation, is the first step before balancing a symbol equation, so it is worth doing for many common reactions.

Try this

Q1. Write the state symbol for a substance dissolved in water. [1 mark]

• Cue. (aq).

Q2. In $2H_2 + O_2 \rightarrow 2H_2O$, how many oxygen atoms are on each side? [1 mark]

• Cue. Two.