Simple Model of the Atom (AQA GCSE Chemistry): Revision Notes
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).
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.
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.
Example: Using Chemical Symbols
This shows:
- (hydrogen molecule with 2 H atoms)
- (chlorine molecule with 2 Cl atoms)
- (2 hydrogen chloride molecules)
Balancing chemical equations
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:
- 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 ) 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)
Example: State Symbols in Action
This shows sodium carbonate as a solid, sulfuric acid dissolved in water, and so on.
Worked example
Worked Example: Balancing the Combustion of Propane
To balance the combustion of propane:
Step 1: Count atoms in propane ()
- 3 carbon atoms
- 8 hydrogen atoms
Step 2: Balance the products
- Need 3 for the carbon atoms
- Need 4 for the hydrogen atoms (8 ÷ 2 = 4)
Step 3: Count oxygen atoms needed
- 3 needs 6 oxygen atoms
- 4 needs 4 oxygen atoms
- Total: 10 oxygen atoms needed
- So need 5 molecules
Final balanced equation:
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)