Common Mistakes and How to Avoid Them

Every year, examiner reports highlight the same recurring mistakes. Knowing what these are — and checking for them — can add several marks to your total.

Energetics Mistakes

Sign Errors

The most common energetics mistake is getting the sign of ΔH wrong. Remember:

Exothermic reactions release energy: ΔH is negative
Endothermic reactions absorb energy: ΔH is positive

When using q = mcΔT, remember that ΔH = -q/n (the negative sign accounts for the convention that energy released by the system is negative).

Hess's Law Direction Errors

When constructing enthalpy cycles, pay attention to the direction of arrows. If you reverse a reaction, you reverse the sign of ΔH. Students frequently forget to reverse the sign when going "against the arrow" in a cycle.

Equation Mistakes

Not Balancing Equations

Always check that your equation balances for atoms and charge (for ionic equations). An unbalanced equation receives zero marks.

Forgetting State Symbols

State symbols (s, l, g, aq) are required in thermochemistry, equilibrium, and redox equations. Missing state symbols can cost 1 mark per equation.