Writing & Balancing Equations
- A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of each atom in the reactants and products
- A word equation is a longer way of describing a chemical reaction using only words to show the reactants and products
Balancing equations
- During chemical reactions, atoms cannot be created or destroyed
- The number of each atom on each side of the reaction must therefore be the same
- E.g. the reaction needs to be balanced
- When balancing equations remember:
- Not to change any of the formulae
- To put the numbers used to balance the equation in front of the formulae
- To balance firstly the carbon, then the hydrogen and finally the oxygen in combustion reactions of organic compounds
- When balancing equations follow the following the steps:
- Write the formulae of the reactants and products
- Count the numbers of atoms in each reactant and product
- Balance the atoms one at a time until all the atoms are balanced
- Use appropriate state symbols in the equation
- The physical state of reactants and products in a chemical reaction is specified by using state symbols
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous
Formulae for Ionic Compounds
- The formulae of simple ionic compounds can be calculated if you know the charge on the ions
- Below are some common ions and their charges:
Common Ions & Their Charges Table
- For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0
- Example: what is the formula of aluminium sulfate?
- Write out the formulae of each ion, including their charges
- Al3+ SO42-
- Balance the charges by multiplying them out:
Al3+ x 2 = +6 and SO42- x 3 = -6; so +6 – 6 = 0
- So the formula is Al2(SO4)3
Examiner Tip
Another method that also works is to 'swap the numbers'.
In the example above the numbers in front of the charges of the ions (3 and 2) are swapped over and become the multipliers in the formula (2 and 3).
Easy when you know how!
Worked example
Balance the following equation:
magnesium + oxygen → magnesium oxide
Answer:
Step 1: Write out the symbol equation showing reactants and products
Mg + O2 → MgO
Step 2: Count the numbers of atoms in each reactant and product
Step 3: Balance the atoms one at a time until all the atoms are balanced
2Mg + O2 → 2MgO
This is now showing that 2 moles of magnesium react with 1 mole of oxygen to form 2 moles of magnesium oxide
Step 4: Use appropriate state symbols in the fully balanced equation
2Mg (s) + O2 (g) → 2MgO (s)
Ionic equations
- In aqueous solutions ionic compounds dissociate into their ions
- Many chemical reactions in aqueous solutions involve ionic compounds, however only some of the ions in solution take part in the reactions
- The ions that do not take part in the reaction are called spectator ions
- An ionic equation shows only the ions or other particles taking part in a reaction, and not the spectator ions
Worked example
1. Balance the following equation
zinc + copper(II) sulfate → zinc sulfate + copper
2. Write down the ionic equation for the above reaction
Answer 1:
Step 1: To balance the equation, write out the symbol equation showing reactants and products
Zn + CuSO4 → ZnSO4 + Cu
Step 2: Count the numbers of atoms in each reactant and product. The equation is already balanced
Step 3: Use appropriate state symbols in the equation
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Answer 2:
Step 1: The full chemical equation for the reaction is
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
Step 2: Break down reactants into their respective ions
Zn (s) + Cu2+ + SO42- (aq) → Zn2++ SO42- (aq) + Cu (s)
Step 3: Cancel the spectator ions on both sides to give the ionic equation
Zn (s) + Cu2+ + SO42- (aq) → Zn2++ SO42- (aq) + Cu (s)
Zn (s) + Cu2+(aq) → Zn2+ (aq) + Cu (s)