Chemical Formulae, Equations and Calculations

Conservation of mass

• During a chemical reaction, atoms are rearranged but never created or destroyed
• This is the law of conservation of mass: the total mass of reactants equals the total mass of products
• It is why every chemical equation must be balanced

Word equations

• A word equation uses the full chemical names of the substances involved:

reactants → products

• The left of the arrow shows the reactants (the starting substances)
• The right of the arrow shows the products (the new substances formed)
• The arrow (→) is read as "produces" or "reacts to give"
• Reaction conditions (e.g. heat, the name of a catalyst) can be written above the arrow

Example. Magnesium burns in oxygen to form magnesium oxide:

magnesium + oxygen → magnesium oxide

Symbol (chemical) equations

• A symbol equation uses chemical formulae instead of full names
• Three rules to remember:
  1. The diatomic non-metals must be written as molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂
  2. State symbols can be written in brackets after each formula:
     • (s) = solid
     • (l) = liquid
     • (g) = gas
     • (aq) = aqueous (dissolved in water)
  3. The equation must be balanced — the same number of each kind of atom must appear on both sides

Example (combustion of sulfur):

S (s) + O₂ (g) → SO₂ (g)

Balancing equations

• Work across the equation from left to right, checking each element in turn
• Adjust the coefficient (the number written in front of each formula) until the counts match on both sides
• Never change the subscripts inside a formula — that would change what the substance is. Adding a 2 to make H₂O into H₂O₂ turns water into hydrogen peroxide
• Treat polyatomic groups that survive the reaction unchanged (NO₃⁻, SO₄²⁻, CO₃²⁻, etc.) as single units rather than counting their individual atoms
• Balance any element that appears on its own last

Example. Balancing the combustion of methane:

CH₄ + O₂ → CO₂ + H₂O

1. Carbon: 1 on each side ✓
2. Hydrogen: 4 on the left, only 2 on the right → put a 2 in front of H₂O:

CH₄ + O₂ → CO₂ + 2H₂O

1. Oxygen: 2 on the left, now 4 on the right (2 from CO₂ + 2 from 2H₂O) → put a 2 in front of O₂:

CH₄ + 2O₂ → CO₂ + 2H₂O

1. Re-check: C 1 = 1, H 4 = 4, O 4 = 4 ✓