Equations and stoichiometry

A chemical equation shows what reacts (the reactants, on the left) and what forms (the products, on the right). Atoms are only rearranged, never made or destroyed, so a balanced equation has the same number of each kind of atom on both sides. Methane burning: CH₄ + 2O₂ → CO₂ + 2H₂O. Count them: 1 C, 4 H and 4 O on each side.

To balance an equation, change only the numbers in front (the coefficients), never the small numbers inside a formula, which would change the substance. Start with an element that appears in just one compound on each side, leave hydrogen and oxygen for last, and check every element at the end. For propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O.

The coefficients are mole ratios. CH₄ + 2O₂ → CO₂ + 2H₂O says one mole of methane uses two moles of oxygen and makes one of carbon dioxide and two of water. That's stoichiometry: grams → moles, use the ratio, moles → grams. Burn 16 g of methane (1 mol) and you get 1 mol of CO₂, which is 44 g.

Reactants are rarely mixed in exactly the right ratio. The one that runs out first is the limiting reactant, and it decides how much product you get; the other is in excess. In 2H₂ + O₂ → 2H₂O, 3 mol of H₂ needs only 1.5 mol of O₂, so with 2 mol of O₂ the hydrogen runs out first: you make 3 mol of water and 0.5 mol of oxygen is left over.

The amount the equation predicts is the theoretical yield. Real reactions lose some to side reactions, incomplete reaction and spills, so percent yield = actual ÷ theoretical × 100. Make 33 g of CO₂ where 44 g was possible and the yield is 75%.