Writing and balancing equations

Formulas are the words of chemistry

A chemical formula shows which elements are in a substance and the ratio of their atoms. A small number written below the line (a subscript) applies only to the atom just before it. Brackets group atoms that travel together, and a subscript after the bracket multiplies everything inside it.

In Year 9 you named ionic compounds (a metal with a non-metal): the metal comes first and keeps its name, and the non-metal ending changes to -ide, as in sodium chloride. Many ionic compounds contain polyatomic ions, groups of atoms that carry a charge and behave as one unit in reactions.

Molecular compounds (non-metals only) are named with prefixes that count the atoms: mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6). So CO is carbon monoxide, CO₂ is carbon dioxide, N₂O₄ is dinitrogen tetroxide and SF₆ is sulfur hexafluoride. "Mono" is left off the first element. A few compounds are always called by their everyday names: H₂O is water, NH₃ is ammonia, CH₄ is methane, and HCl gas is hydrogen chloride (not "hydrogen monochloride").

Reading an equation

A chemical equation is a short, exact description of a reaction. The reactants are written on the left and the products on the right, with an arrow that means "react to form". Large numbers in front of a formula are coefficients: they count whole molecules or formula units, so 2H₂O means two water molecules, which hold 4 H atoms and 2 O atoms. State symbols show whether each substance is a solid (s), liquid (l), gas (g) or dissolved in water (aq, short for aqueous).

Balancing equations

Atoms are never created or destroyed in a chemical reaction; they are only rearranged. So a correct equation must have the same number of each kind of atom on both sides. This is called a balanced equation. You balance it by changing the coefficients only. Changing a subscript would change the substance: H₂O₂ is hydrogen peroxide, a bleach, not water.

1. Write the word equation, then the correct formula for every substance.
2. Count the atoms of each element on each side. Treat a polyatomic ion that appears unchanged on both sides (such as NO₃ or SO₄) as a single unit.
3. Start with an element that appears in only one substance on each side. Leave elements that appear on their own (such as O₂, H₂ or Fe) until last.
4. Add coefficients one at a time, recounting after each change.
5. Check that every element balances and the coefficients are the smallest whole numbers.

Sometimes the oxygen count comes out odd. For example, ethane gives C₂H₆ + O₂ → 2CO₂ + 3H₂O, which needs 7 O atoms, or 3½ O₂. You can't have half a molecule, so double everything: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.

Conservation of mass

In the 1770s and 1780s Antoine Lavoisier heated metals in sealed glass vessels and weighed them very carefully. The total mass never changed, even though the metal turned into a powdery new substance. He stated the law of conservation of mass: in a chemical reaction, the total mass of the products equals the total mass of the reactants. A balanced equation is the reason: the same atoms, with the same masses, are on both sides.

A reaction can seem to gain or lose mass when a gas enters or escapes. In an open flask, carbon dioxide from vinegar and bicarbonate soda escapes into the air, so the reading drops. When the gas is trapped in a balloon, the reading stays the same.

Balanced equations also let you predict masses. Using the relative masses of the atoms (H = 1, C = 12, O = 16), one propane molecule has a mass of 44 units and five O₂ molecules have 160 units. They make three CO₂ (132 units) and four H₂O (72 units). So 44 g of propane reacts with 160 g of oxygen to make 132 g of carbon dioxide and 72 g of water: 204 g on each side. Scale that up and 9 kg of propane gives 9 × 132 ÷ 44 = 27 kg of carbon dioxide.