The 5 Most Common Mistakes in A-Level Chemistry (And How to Avoid Them)

Every year, exam board examiners reports highlight identical stumbling blocks that prevent talented students from securing their Grade 9 (GCSE) or A* (A-Level). Here is how to eliminate them from your chemistry exam technique:

1. The "Floating" Curly Arrow

In organic mechanisms, a curly arrow signifies the movement of a pair of electrons. It must originate directly from an electron-rich site (either a lone pair or the center of a covalent bond) and terminate directly at an electron-deficient atom. Arrows drawn into vague empty space lose marks immediately.

2. The Unit Pitfall in Gibbs Free Energy (ΔG)

Enthalpy change (ΔH) is almost universally quoted in kJ/mol, while entropy change (ΔS) is given in J/(K·mol). When calculating ΔG = ΔH - TΔS, you must convert both quantities to the same unit before computing. Divide ΔS by 1000 or multiply ΔH by 1000!

3. Standard State Enthalpies

Remember that the standard enthalpy of formation (ΔfH°) of any element in its standard state at 298 K and 100 kPa (such as O2(g), C(graphite), Na(s)) is defined as exactly zero.