Balancing Equations (DP IB Chemistry): Revision Note
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:
Do not change any of the formulae
When polyatomic ions (like SO42- or NO3-) appear on both sides, do not split them into individual atoms unless needed
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
Worked Example
Balance the following equation:
magnesium + oxygen → magnesium oxide
Answer:
Step 1: Write out the symbol equation:
Mg + O2 → MgO
Step 2: Count the numbers of atoms in each reactant and product
| Mg | O |
|---|---|---|
Reactants | 1 | 2 |
Products | 1 | 1 |
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)
Examiner Tips and Tricks
Some elements are diatomic in their natural state and must be written with a subscript 2 in equations:
H2
N2
O2
F2
Cl2
Br2
I2
Worked Example
Write a balanced symbol equation for the following equation
zinc + copper(II) sulfate → zinc sulfate + copper
Answer
Step 1: Write out the symbol equation:
Zn + CuSO4 → ZnSO4 + Cu
Step 2: Count the numbers of atoms in each reactant and product
| Zn | Cu | S | O |
|---|---|---|---|---|
Reactants | 1 | 1 | 1 | 4 |
Products | 1 | 1 | 1 | 4 |
The equation is already balanced
Step 3: Use appropriate state symbols in the equation
Zn (s) + CuSO4 (aq) → ZnSO4 (aq) + Cu (s)
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