Chemical equations Revision Notes for AQA GCSE Chemistry

Chemical equations

What are chemical equations?

Chemical equations show us what happens in a chemical reaction. They tell us what chemicals react together (the reactants) and what new chemicals are made (the products).

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Chemical equations are like mathematical equations - they show us the relationship between different substances and help us understand what's happening during a chemical reaction.

There are two main ways to write chemical equations:

Word equations - using the names of chemicals
Chemical equations - using symbols and formulas

Word equations

Word equations use the names of chemicals with an arrow showing the direction of the reaction.

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Example: Hydrogen and Chlorine Reaction

hydrogen + chlorine → hydrogen chloride

The chemicals before the arrow are the reactants (what you start with). The chemicals after the arrow are the products (what you end up with).

Chemical equations with symbols

Chemical equations use symbols and formulas instead of names. This makes them shorter and easier to work with.

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Example: Using Chemical Symbols

$H_2 + Cl_2 \rightarrow 2HCl$

This shows:

$H_2$ (hydrogen molecule with 2 H atoms)
$Cl_2$ (chlorine molecule with 2 Cl atoms)
$2HCl$ (2 hydrogen chloride molecules)

Balancing chemical equations

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Key rule: In a chemical reaction, atoms cannot be created or destroyed. The same number of atoms of each element must be on both sides of the equation.

Looking at our example: $H_2 + Cl_2 \rightarrow 2HCl$

Left side: 2 H atoms, 2 Cl atoms
Right side: 2 H atoms, 2 Cl atoms ✓ Balanced!

The numbers in front of formulas (like the 2 in $2HCl$ ) tell us how many molecules we have.

State symbols

State symbols show what physical state each chemical is in:

(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous solution (dissolved in water)

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Example: State Symbols in Action

$Na_2CO_3(s) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + CO_2(g) + H_2O(l)$

This shows sodium carbonate as a solid, sulfuric acid dissolved in water, and so on.

Worked example

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Worked Example: Balancing the Combustion of Propane

To balance the combustion of propane: $C_3H_8 + O_2 \rightarrow CO_2 + H_2O$

Step 1: Count atoms in propane ( $C_3H_8$ )

3 carbon atoms
8 hydrogen atoms

Step 2: Balance the products

Need 3 $CO_2$ for the carbon atoms
Need 4 $H_2O$ for the hydrogen atoms (8 ÷ 2 = 4)

Step 3: Count oxygen atoms needed

3 $CO_2$ needs 6 oxygen atoms
4 $H_2O$ needs 4 oxygen atoms
Total: 10 oxygen atoms needed
So need 5 $O_2$ molecules

Final balanced equation: $C_3H_8 + 5O_2 \rightarrow 3CO_2 + 4H_2O$

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Key Points to Remember:

Chemical equations show reactants → products
Word equations use names, symbol equations use formulas
Atoms are conserved - same number on both sides
Numbers in front of formulas show how many molecules
State symbols show if something is solid (s), liquid (l), gas (g), or dissolved in water (aq)

Chemical equations (AQA GCSE Chemistry): Revision Notes