Balanced chemical equations
Word equations
A chemical reaction can be summarised using a word equationAn equation in which only the names of the reactants and products are used to model a reaction..
sulfur + oxygen 鈫 sulfur dioxide
Chemical equations
In a chemical equation the names of substances are replaced by symbols and formulaA combination of symbols that indicates the chemical composition of a substance..
S + O2 鈫 SO2
Conservation of atoms
atomThe smallest part of an element that can exist. cannot be created or destroyed. This means the number of atoms in the reactantA substance that reacts together with another substance to form products during a chemical reaction. at the start of the reaction must equal the number of atoms in the productA substance formed in a chemical reaction. the end.
The equation above (S + O2 鈫 SO2) is correct.
| Atoms in the reactants | Atoms in the products |
| 1 脳 S | 1 脳 S |
| 2 脳 O | 2 脳 O |
| Atoms in the reactants | 1 脳 S |
|---|---|
| Atoms in the products | 1 脳 S |
| Atoms in the reactants | 2 脳 O |
|---|---|
| Atoms in the products | 2 脳 O |
This means that the mass of the reactants must equal the mass of the products. This is known as conservation of massThe total mass of reactants before a reaction is equal to the total mass of products after a reaction, because no atoms can be created or destroyed so the mass must remain unchanged..
Question
How many of each type of atom are there in the following?
| C atoms | O atoms | |
| CO | ||
| CO2 | ||
| 2CO | ||
| 2CO2 |
| CO | |
|---|---|
| C atoms | |
| O atoms | |
| CO2 | |
|---|---|
| C atoms | |
| O atoms | |
| 2CO | |
|---|---|
| C atoms | |
| O atoms | |
| 2CO2 | |
|---|---|
| C atoms | |
| O atoms | |
| C atoms | O atoms | |
| CO | 1 | 1 |
| CO2 | 1 | 2 |
| 2CO | 2 | 2 |
| 2CO2 | 2 | 4 |
| CO | |
|---|---|
| C atoms | 1 |
| O atoms | 1 |
| CO2 | |
|---|---|
| C atoms | 1 |
| O atoms | 2 |
| 2CO | |
|---|---|
| C atoms | 2 |
| O atoms | 2 |
| 2CO2 | |
|---|---|
| C atoms | 2 |
| O atoms | 4 |
Balancing chemical equations
In a chemical equation, if the names of substances are replaced by their symbols and formulae, most reactions are not correct - there is a different number of atoms in the reactants than in the products.
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
Carbon monoxide reacts with oxygen to produce carbon dioxide. Write the balanced chemical equation for this reaction.
| Step | Result | Equation balanced? |
| 1 | carbon monoxide + oxygen 鈫 carbon dioxide | |
| 2 | CO + O2 鈫 CO2 | |
| 3 | Reactants: 1 脳 C, (1 脳 O) + (2 脳 O) = 3 脳 O. Products: 1 脳 C, 2 脳 O | Not balanced. |
| 4 | 2CO + O2 鈫 CO2 | |
| Step 3 (repeat) | Reactants: 2 脳 C, (2 脳 O) + (2 脳 O) = 4 脳 O. Products: 1 脳 C, 2 脳 O. | Not balanced. |
| Step 4 (repeat) | 2CO + O2 鈫 2CO2 | |
| Step 3 (repeat) | Reactants: 2 脳 C, 4 脳 O. Products: 2 脳 C, (2 脳 2 脳 O) = 4 脳 O. | This chemical equation is now balanced. |
| Step | 1 |
|---|---|
| Result | carbon monoxide + oxygen 鈫 carbon dioxide |
| Equation balanced? |
| Step | 2 |
|---|---|
| Result | CO + O2 鈫 CO2 |
| Equation balanced? |
| Step | 3 |
|---|---|
| Result | Reactants: 1 脳 C, (1 脳 O) + (2 脳 O) = 3 脳 O. Products: 1 脳 C, 2 脳 O |
| Equation balanced? | Not balanced. |
| Step | 4 |
|---|---|
| Result | 2CO + O2 鈫 CO2 |
| Equation balanced? |
| Step | Step 3 (repeat) |
|---|---|
| Result | Reactants: 2 脳 C, (2 脳 O) + (2 脳 O) = 4 脳 O. Products: 1 脳 C, 2 脳 O. |
| Equation balanced? | Not balanced. |
| Step | Step 4 (repeat) |
|---|---|
| Result | 2CO + O2 鈫 2CO2 |
| Equation balanced? |
| Step | Step 3 (repeat) |
|---|---|
| Result | Reactants: 2 脳 C, 4 脳 O. Products: 2 脳 C, (2 脳 2 脳 O) = 4 脳 O. |
| Equation balanced? | This chemical equation is now balanced. |
Question
Write a balanced chemical equation for the complete combustion of methane (CH4) with oxygen (O2).
Word equation: methane + oxygen 鈫 carbon dioxide + water
Simple chemical equation: CH4 + O2 鈫 CO2 + H2O
Reactants: 1 脳 C, 4 脳 H, 2 脳 O
Products: 1 脳 C, 3 脳 O, 2 脳 H not balanced
Try: CH4 + O2 鈫 CO2 + 2H2O
Reactants: 1 脳 C, 4 脳 H, 2 脳 O
Products: 1 脳 C, 4 脳 H, 4 脳 O not balanced
Try: CH4 + 2O2 鈫 CO2 + 2 H2O
Reactants: 1 脳 C, 4 脳 H, 4 脳 O
Products: 1 脳 C, 4 脳 H, 4 脳 O balanced
Balanced chemical equation: CH4 + 2O2 鈫 CO2 + 2H2O