Question

What do chemical equations tell us?


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2. Why do we need to balance chemical equations?

Answer 1

Short answers first, then a quick example.

1) What do chemical equations tell us?
- Which substances react (reactants) and which are formed (products).
- The physical states (s, l, g, aq) and sometimes conditions (heat, catalyst, pressure) or energy change (ΔH).
- The relative amounts needed/produced — the stoichiometric coefficients give mole, molecule, or (for gases at same T and P) volume ratios.
- The direction of the process (→ for forward, ⇌ for reversible).
- They do NOT automatically give rates or mechanism (how fast or the stepwise pathway) unless extra information is added.

2) Why must we balance chemical equations?
- Because of the law of conservation of mass: atoms (and charge) cannot be created or destroyed in a chemical reaction, so the number of each type of atom must be the same on both sides.
- Coefficients are adjusted to reflect the correct mole ratios. Those ratios are essential for quantitative work: calculating amounts of reactant needed, predicting product yield, finding limiting reagents, and relating gas volumes.
- You must change coefficients (numbers in front of formulas), not subscripts (which change the identity of the substances).

Example
Unbalanced: H2 + O2 → H2O
Atom counts on left: H = 2, O = 2; on right: H = 2, O = 1 — oxygen isn’t conserved.
Balanced: 2 H2 + O2 → 2 H2O
Now H: 4 on both sides, O: 2 on both sides. Coefficients tell you 2 moles H2 react with 1 mole O2 to make 2 moles H2O.

That’s why balancing is required: to respect conservation laws and to give correct stoichiometric proportions for calculations and practical use.