Chemistry unit guide
What drives chemical reactions
Enthalpy, Hess cycles, bond enthalpy, entropy, spontaneity and equilibrium.
Reactivity 1 carries a heavy share of the calculation marks in Paper 2, and it is where sign conventions cost more marks than any concept. Enthalpy and entropy questions are usually structured so that a sign error early on propagates to the end, and a Gibbs free energy answer with the right magnitude and the wrong sign earns nothing for the conclusion. The equilibrium half of the unit is mostly explanation rather than calculation, and rewards precision about what shifts and why.
Topics in this unit
What gets examined
- Calculating enthalpy change from experimental temperature data using q = mcΔT, including which mass to use.
- Hess's law cycles, both as equations to combine and as diagrams to construct.
- Bond enthalpy calculations - bonds broken minus bonds formed - and why they give approximate answers.
- Predicting the sign of ΔS from the change in moles of gas.
- Using ΔG = ΔH - TΔS, including finding the temperature at which a reaction becomes spontaneous.
- Applying Le Chatelier's principle to concentration, pressure and temperature changes, and knowing that a catalyst changes neither position nor K.
How to revise it
Fix the sign convention before you calculate
Exothermic is negative. Write that at the top of the page in the exam. Most lost marks in this unit are a correct magnitude with the wrong sign, and the conclusion mark depends entirely on the sign.
Use the mass of the solution, not the solute
In q = mcΔT the m is the mass of water or solution absorbing the heat. Using the mass of the reagent is one of the most frequent errors examiners report.
Draw the Hess cycle even when combining equations
The diagram makes the direction of each arrow explicit, which is where the sign errors come from. It also earns method marks when the arithmetic goes wrong.
Say which direction and why, separately
Le Chatelier answers need the direction of shift and the reason in terms of opposing the change. Stating only the direction routinely scores half.
Where marks get lost
- Reversing the sign when a Hess cycle equation is used backwards.
- Using bond enthalpies for a reaction involving liquids or solids, where they only apply to gases.
- Forgetting to convert ΔH from kJ to J, or ΔS from J to kJ, before combining them in ΔG.
- Claiming a catalyst increases yield - it changes rate only.
- Saying equilibrium shifts "to the right" without saying what change it is opposing.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.