Balancing chemical equations
If the names of substances are simply replaced by their symbols and formulaA combination of symbols that indicates the chemical composition of a substance., most chemical equations are not correct. This is because the numbers of atomThe smallest part of an element that can exist. of each elementA substance made of one type of atom only. in the reactantA substance that reacts together with another substance to form products during a chemical reaction. may differ from those in the productA substance formed in a chemical reaction..
To make the chemical equation correct it must be balanced.
| Step | Action |
| 1 | Write a word equation for the reaction. |
| 2 | Write a simple equation by writing the name of each substance with its symbol or formula. |
| 3 | Check whether the equation is balanced by counting the atoms of each element. |
| 4 | If necessary, change the number of one of the molecules to make the atoms of at least one element balance. You may need to multiply both sides if the number of an atom on the right is not a multiple of the atoms on the left. |
| 5 | Repeat steps 3 and 4 until the chemical equation is balanced. |
| Step | 1 |
|---|---|
| Action | Write a word equation for the reaction. |
| Step | 2 |
|---|---|
| Action | Write a simple equation by writing the name of each substance with its symbol or formula. |
| Step | 3 |
|---|---|
| Action | Check whether the equation is balanced by counting the atoms of each element. |
| Step | 4 |
|---|---|
| Action | If necessary, change the number of one of the molecules to make the atoms of at least one element balance. You may need to multiply both sides if the number of an atom on the right is not a multiple of the atoms on the left. |
| Step | 5 |
|---|---|
| Action | Repeat steps 3 and 4 until the chemical equation is balanced. |
Example
Magnesium reacts with oxygen to produce magnesium oxide. Write the balanced chemical equation for this reaction.
| Step | Result |
| 1 | magnesium + oxygen → magnesium oxide |
| 2 | Mg + O2 → MgO |
| 3 | reactants: 1 × Mg, 2 × O products: 1 × Mg, 1 × O Not balanced. |
| 4 | Mg + O2 → 2MgO |
| Step 3 (repeat) | reactants: 1 × Mg, 2 × O products: 2 × Mg, 2 × O Not balanced. |
| Step 4 (repeat) | 2Mg + O2 → 2MgO |
| Step 3 (repeat) | reactants: 2 × Mg, 2 × O products: 2 × Mg, 2 × O This chemical equation is now balanced. |
| Step | 1 |
|---|---|
| Result | magnesium + oxygen → magnesium oxide |
| Step | 2 |
|---|---|
| Result | Mg + O2 → MgO |
| Step | 3 |
|---|---|
| Result | reactants: 1 × Mg, 2 × O products: 1 × Mg, 1 × O Not balanced. |
| Step | 4 |
|---|---|
| Result | Mg + O2 → 2MgO |
| Step | Step 3 (repeat) |
|---|---|
| Result | reactants: 1 × Mg, 2 × O products: 2 × Mg, 2 × O Not balanced. |
| Step | Step 4 (repeat) |
|---|---|
| Result | 2Mg + O2 → 2MgO |
| Step | Step 3 (repeat) |
|---|---|
| Result | reactants: 2 × Mg, 2 × O products: 2 × Mg, 2 × O This chemical equation is now balanced. |
Question
Write a balanced chemical equation for the reaction of calcium with oxygen.
2Ca + O2 → 2CaO