3.1.2Formulae, Stoichiometry and the Mole Concept
Writing Equations — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 3 of 6 Unit quiz
Part 1 of 5Word equations
Reactants and products
- Reactants and products A word equation names every substance in a reaction.
- The reactants go on the left and the products (the new substances formed) go on the right, joined by an arrow that means 'react to make'.
- (the starting substances)
- The reactants go on the left and the products (the new substances formed) go on the right, joined by an arrow that means 'react to make'.
- Example: sodium hydroxide + hydrochloric acid → sodium chloride + water.
Conditions and catalysts
- Conditions and catalysts Anything that is not a reactant or product,
- such as a catalyst or a temperature, is written above or below the arrow rather than in the equation itself.
Reading the question carefully
- Reading the question carefully Questions sometimes list the products first,
- so decide which substances are being used up and which are being made before you write anything down.
- Some reactants are only hinted at. 'Burns in air' means the substance reacts with oxygen,
- so oxygen belongs on the left even though the question never names it.
Part 2 of 5Building a symbol equation
1
Write the word equation first, so that you know exactly which substances are involved.
2
Replace each name with its correct formula, remembering that hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as diatomic molecules: H2, N2, O2, F2, Cl2, Br2 and I2.
3
Count the atoms of each element on both sides of the arrow.
4
Balance by placing whole-number coefficients in front of formulae. Never change a subscript inside a formula, because that would turn it into a different substance.
5
Re-count every element, then add state symbols to every formula.
Part 3 of 5State symbols
| Symbol | Meaning | Clue for choosing it |
|---|---|---|
| (s) | Solid | Metals and most ionic compounds are solid unless dissolved or molten. |
| (l) | Liquid | Water and molten substances; some covalent compounds such as bromine. |
| (g) | Gas | Hydrogen, oxygen, carbon dioxide, ammonia and other small covalent molecules. |
| (aq) | Aqueous (dissolved in water) | The question mentions a solution, or an acid or alkali. |
Part 4 of 5Worked example: magnesium burning
1Word equation
- Word equation magnesium + oxygen → magnesium oxide.
2Formulae
- Formulae Mg + O2 → MgO.
3Balance
- Balance There are two oxygen atoms on the left but only one on the right, so put 2 in front of MgO.
- That makes two magnesium on the right, so put 2 in front of Mg.
4Final equation
- Final equation 2Mg(s) + O2(g) → 2MgO(s).
Part 5 of 5Balancing hints
Start with the awkward element
- Start with the awkward element Balance elements that appear in only one substance on each side first,
- and leave hydrogen and oxygen until last if they appear in several places.
Treat groups as a unit
- Treat groups as a unit If a group such as OH or SO4 appears unchanged on both sides, balance it as a whole group instead of as separate atoms.
Why balancing matters
- Why balancing matters Atoms are never created or destroyed in a reaction, so the same number of each type of atom must appear on both sides.
- This is the law of conservation of mass.
Key terms
| Term | Meaning |
|---|---|
| Reactant | A substance that is used up in a chemical reaction; it appears on the left of the arrow. |
| Product | A new substance made in a chemical reaction; it appears on the right of the arrow. |
| Coefficient | The large number in front of a formula showing how many units of that substance take part. |
| State symbol | A bracketed letter after a formula showing whether it is solid, liquid, gas or aqueous. |
| Diatomic molecule | A molecule made of two atoms of the same element, such as O2 or Cl2. |
Avoid these mistakes
1
Balancing by changing a subscript, for example writing MgO2 instead of 2MgO. Only coefficients may be changed.
2
Writing oxygen as O instead of O2; the diatomic elements must be written as molecules.
3
Leaving out oxygen when a question says a substance 'burns in air' or 'in oxygen'.
4
Forgetting state symbols, or using (aq) for a pure liquid such as water; water is (l) unless something is dissolved in it.
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