3.1.1Quantitative Chemistry
Conservation of Mass and Balancing Chemical Equations — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 2 of 10
Section 1 of 6Conservation of mass
The law
- The law No atoms are created or destroyed in a chemical reaction, so the total mass of the products equals the total mass of the reactants.
Explanation
- Explanation Atoms are rearranged into new substances.
- The same atoms are present before and after, so the mass cannot change.
Why this requires balanced equations
- Why this requires balanced equations A symbol equation shows the same number of atoms of each element on both sides.
- If it does not, it is unbalanced.
Section 2 of 6Balancing an equation
1
Write the correct formulae for all reactants and products. Never change a formula to balance an equation.
2
Count the atoms of each element on both sides.
3
Add a large number (coefficient) in front of a formula to increase the number of that substance. The number multiplies every atom in the formula.
4
Recount all the elements after each change, since adjusting one can unbalance another.
5
Check each element is equal on both sides and add state symbols if required.
Section 3 of 6Worked examples
| Unbalanced | Balanced | Check |
|---|---|---|
| Mg + O2 → MgO | 2Mg + O2 → 2MgO | 2 Mg and 2 O on each side |
| H2 + O2 → H2O | 2H2 + O2 → 2H2O | 4 H and 2 O on each side |
| Na + Cl2 → NaCl | 2Na + Cl2 → 2NaCl | 2 Na and 2 Cl on each side |
| CH4 + O2 → CO2 + H2O | CH4 + 2O2 → CO2 + 2H2O | 1 C, 4 H, 4 O on each side |
Section 4 of 6Reading an equation
2H2 + O2 → 2H2O
Coefficient
- Coefficient The large number in front shows how many units of that substance take part.
Subscript
- Subscript The small number in a formula shows how many atoms of that element are in one unit and must never be changed during balancing.
Arrow
- Arrow The arrow means 'reacts to make' and separates reactants from products.
Mass check
- Mass check Add up the relative masses on each side and they will match, confirming conservation of mass.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Law of conservation of mass | The total mass of reactants equals the total mass of products in a chemical reaction. |
| Reactant | A substance that is used up in a chemical reaction. |
| Product | A substance made in a chemical reaction. |
| Coefficient | The large number written in front of a formula in an equation. |
| Balanced equation | An equation with equal numbers of each type of atom on both sides. |
Section 6 of 6Exam technique
1
Changing a subscript, such as writing H2O2 instead of H2O, to make an equation balance. That changes the substance.
2
Only balancing one element and forgetting to recount the others.
3
Forgetting that the coefficient multiplies the whole formula, including every element in it.
4
Explaining conservation of mass as 'the mass stays the same' without mentioning atoms being rearranged, not lost.
5
Writing the formula of an element like oxygen as O instead of O2.
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