Title

Representing Reactions
Quick revise

The balanced chemical equation is the standard way to communicate chemical information but it is only useful if you understand what it means. The best way to do this is to try to see it as a story told in code rather than as a list of symbols.

We can start off by looking at a few chemical equations and “translating” them into words. After all, we think in words so this might make it easier.

CaCO3 → CaO + CO2

At a basic level: Calcium carbonate breaks down to give calcium oxide and carbon dioxide.

Slightly more advanced: One mole of calcium carbonate breaks down to give one mole of calcium oxide and one mole of carbon dioxide. This gives information that is useful to show how much of each chemical reacts and how much is produced. It allows us to link these bits of information in order to do calculations.

2Mg + O2 → 2MgO

At a basic level: Magnesium reacts with oxygen to make magnesium oxide.

Slightly more advanced: Two moles of magnesium atoms react with one mole of oxygen molecules to form one mole of magnesium oxide.

The 2Mg (big and in front of Mg) means “two moles of magnesium”.

The O2 (small and after the O) means that each molecule of oxygen is made of two oxygen atoms joined together.

2O and O2 both contain the same number of O atoms but in O2 we are showing that the atoms are in pairs while in 2O they are in singles.

You can think of this in terms of a box of six eggs, it could have the formula E6. Six eggs on their own would be 6E.

Representing Equations

When we can represent reactions by word and symbol equations:

methane + oxygen → carbon dioxide + water or CH4 + O2 →; CO2 + H2O

To this we can add state symbols to give more information about the substances:

(s) – solid

(l) – liquid

(g) – gas

(aq) – aqueous

Example: CH4(g) + O2(g) → CO2(g) + H2O(g)

This doesn’t tell us the whole story we need to balance the equation to show that we have not destroyed or made new atoms.

Balancing Equations

CH4(g) + O2(g) → CO2(g) + H2O (g)

1. Remember that the formulae for each compound is correct you cannot change CH4 to CH3 just to make the atoms add up.

2. Balance one type of atom at a time:

3. There is one carbon atom on each side so we can leave that alone, however there are 4 H atoms on the left hand side and 2 on the right hand side we can correct this by putting a 2 in front of the water.

CH4(g) + O2(g) → CO2 (g) + 2H2O (g)

4. Now both the carbon and the hydrogen balance, that just leaves us with the oxygen. There are 2 O on the left hand side and 4 on the right hand side. We can correct this by putting a 2 in front of the oxygen on the left hand side:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (g)

This GCSE Chemistry video explains how to balance equations

This video follows on from balancing equations and speaks about how to use the chemical calculations

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