Chemical equations Revision Notes for AQA GCSE Chemistry Combined Science

Chemical equations

Chemical reactions can be written in two main ways: as word equations or as chemical equations using symbols and formulas. Both methods help us understand what happens during a chemical reaction.

Word equations

A word equation shows a chemical reaction using the names of substances only. It's the simplest way to describe what happens.

Structure: Reactants → Products

When hydrogen gas reacts with chlorine gas, it makes hydrogen chloride:

hydrogen + chlorine → hydrogen chloride

The substances on the left (hydrogen and chlorine) are called reactants - these are what you start with. The substance on the right (hydrogen chloride) is called the product - this is what you end up with.

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Word equations are particularly useful when you're first learning about chemical reactions because they use familiar names rather than complex formulas. They help you focus on understanding what substances are involved without getting distracted by chemical symbols.

Chemical equations using symbols

A chemical equation uses chemical formulas and symbols instead of words. This gives us more detailed information about the reaction.

The same reaction as above becomes: $H_2 + Cl_2 \rightarrow 2HCl$

This tells us:

One molecule of hydrogen ( $H_2$ ) reacts with one molecule of chlorine ( $Cl_2$ )
This produces two molecules of hydrogen chloride ( $2HCl$ )

Balancing chemical equations

chatImportant

Key rule: In any chemical reaction, you must have the same number of atoms of each element on both sides of the equation. This reflects the law of conservation of mass - atoms cannot be created or destroyed, only rearranged.

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

Left side: 2 hydrogen atoms, 2 chlorine atoms
Right side: 2 hydrogen atoms (in $2HCl$ ), 2 chlorine atoms (in $2HCl$ )
The equation is balanced because both sides match

State symbols

State symbols show what physical state each substance is in during the reaction. You add these in brackets after each chemical formula:

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

Consider this example where sodium carbonate reacts with sulfuric acid: $Na_2CO_3(s) + H_2SO_4(aq) \rightarrow Na_2SO_4(aq) + CO_2(g) + H_2O(l)$

This shows us:

Sodium carbonate starts as a solid
Sulfuric acid is dissolved in water
Sodium sulphate stays dissolved in water
Carbon dioxide is produced as a gas
Water is produced as a liquid

infoNote

State symbols are especially important in chemistry because the physical state of reactants and products can affect reaction rates, conditions needed, and practical applications. For example, knowing that carbon dioxide is produced as a gas helps explain why you might see bubbling during this reaction.

Worked example: balancing equations

lightbulbExample

Worked Example: Balancing the Combustion of Propane

Question: Write the balanced equation for burning propane ( $C_3H_8$ ) in oxygen.

Step 1: Write the unbalanced equation $C_3H_8 + O_2 \rightarrow CO_2 + H_2O$

Step 2: Count atoms and balance

Propane has 3 carbon atoms → need $3CO_2$
Propane has 8 hydrogen atoms → need $4H_2O$ (each water has 2 hydrogen)
Products need 10 oxygen atoms total → need $5O_2$

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

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Key Points to Remember:

Word equations use names of substances, chemical equations use formulas and symbols
Reactants go on the left, products go on the right of the arrow
Balancing means having the same number of each type of atom on both sides
State symbols show whether substances are solid (s), liquid (l), gas (g), or dissolved in water (aq)
Atoms are never created or destroyed in chemical reactions - they just rearrange

Chemical equations (AQA GCSE Chemistry Combined Science): Revision Notes