If an equation will not balance, changing a small number inside a formula can look like a quick fix. It also changes the substance. H₂O and H₂O₂ are different compounds; you cannot swap one for the other to repair an atom count.
The task is to change how many particles take part while keeping their identities fixed. That means changing coefficients in front of formulae, not subscripts within them.
The examples here are original learning exercises for chemical equations. Check the full requirements for your tier and examination year on the Cambridge Chemistry 0620 syllabus page.
Count one element at a time
Start with magnesium reacting with oxygen:
Mg + O₂ → MgO
There is one magnesium atom on each side, but two oxygen atoms on the left and one on the right. Place a 2 before MgO to make two oxygen atoms on the right. That also creates two magnesium atoms on the right, so place a 2 before Mg:
2Mg + O₂ → 2MgO
Check both elements again. Left: Mg = 2 and O = 2. Right: Mg = 2 and O = 2. The coefficients are in their smallest whole-number ratio.
Remember that a coefficient multiplies the whole formula
In 2Al₂O₃, there are four aluminium atoms and six oxygen atoms. The coefficient 2 multiplies both subscripts. If you count two aluminium atoms and three oxygen atoms, you have counted only one formula unit.
Try balancing aluminium reacting with oxygen:
Al + O₂ → Al₂O₃
Oxygen appears in groups of two on the left and three on the right. Six is a useful common total: use 3O₂ and 2Al₂O₃. Now the right side contains four aluminium atoms, so the completed equation is:
4Al + 3O₂ → 2Al₂O₃
You can reach the same result in another order. The important check is the final count, not whether you used an identical sequence of steps.
Balance before interpreting reacting amounts
The equation 2Mg + O₂ → 2MgO describes a particle ratio and a mole ratio of 2:1:2. It does not say that two grams of magnesium react with one gram of oxygen. Different substances have different molar masses.
This distinction becomes useful when you move to reacting-mass calculations. First establish the correct equation, then use the amount of substance and the coefficients. Do not use coefficients as mass ratios by themselves.
Try these equations
- H₂ + O₂ → H₂O
- Na + Cl₂ → NaCl
- CaCO₃ → CaO + CO₂
Draw a two-column atom count for each question. Leave formulae untouched. If the equation already balances, say so instead of adding unnecessary coefficients.
Answers and checks
- 2H₂ + O₂ → 2H₂O. Each side contains four hydrogen atoms and two oxygen atoms.
- 2Na + Cl₂ → 2NaCl. Each side contains two sodium atoms and two chlorine atoms.
- CaCO₃ → CaO + CO₂ is already balanced. There is one calcium atom, one carbon atom and three oxygen atoms on each side; on the right, those oxygen atoms are split between two products.
These are symbolic exercises, not instructions to carry out reactions at home.
Make your next correction specific
If you changed a subscript, rewrite the starting formulae in a box before balancing. If your final answer has the right atoms but unnecessary common factors, reduce the coefficients. If you missed an element in a product, count every element once more after your last change.
Continue with StudyForA's Chemistry 0620 lessons. Explain why a coefficient changes the quantity but not the identity of a substance before moving to a harder equation.