Balancing Equations (Cambridge (CIE) AS 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:
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
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, without showing the spectator ions
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 number 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)
Worked Example
1. Balance the following equation:
zinc + copper(II) sulfate → zinc(II) sulfate + copper
2. Write 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 number of atoms in each reactant and product. The equation is already balanced
| Zn | Cu | S | O |
|---|---|---|---|---|
Reactants | 1 | 1 | 1 | 4 |
Products | 1 | 1 | 1 | 4 |
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)
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