IB Chemistry Paper 2 — How to Earn Full Marks (Method & ECF)

• Method (M) marks reward the correct approach — the right equation, the right rearrangement, the right substitution, the right mole ratio.
• Accuracy (A) marks reward the correct result — the final number, correctly rounded, with correct units.

The crucial mechanism sitting on top of this is error-carried-forward (ECF), sometimes called the own-figure rule. If your method is sound but you carried in a wrong value from an earlier part, the later method mark — and often the later accuracy mark — is still awarded on your figure. You are penalised once for the original slip, not repeatedly all the way down the question.

Here is what that looks like on a two-part calculation:

The student above loses one mark, not two. The student who panics after part (a) and leaves part (b) blank loses two. That single difference — pressing on instead of abandoning — is worth real grade points across a whole paper.

• Write the equation in symbols first, then substitute numbers, then give the answer — three lines, every time.
• Keep each part's answer on its own line so the examiner can follow the chain from part to part.
• Never overwrite a value you carry forward — the marker needs to trace where your later numbers came from.

Do the whole thing silently in your calculator and copy down only the result, and you have thrown away the protection ECF was designed to give you.

• Units: carry them through your working, not just onto the final line. Watch the classic traps — enthalpy in kJ mol⁻¹ not J, concentration in mol dm⁻³ not mol, and volumes converted to dm³ before use.
• Significant figures: decide your figure count from the least precise data in the question, and be consistent. A calculator-length answer (six or seven digits) and an over-rounded one can both cost the sig-fig mark — and sometimes the accuracy mark with it.

A right number wearing the wrong units, or eight digits of false precision, is a mark quietly handed back. Fixing this needs no new chemistry — only discipline in the last ten seconds of each answer.

The trap is under-answering an "explain" as if it were a "state," and over-writing a "state" into a paragraph that wastes time you need elsewhere. For the full list, read IB command terms explained.

• Moles and stoichiometry — n = m ÷ M, mole ratios from a balanced equation, limiting reagent, and percentage yield. State the ratio explicitly; it is often its own method mark.
• Energetics from calorimetry — q = mcΔT, then divide by moles and attach the sign to get ΔH in kJ mol⁻¹. Show the conversion from J to kJ on its own line.
• Hess's law and bond enthalpies — draw the cycle or list bonds broken minus bonds formed. Tidy layout is what earns the method mark here.
• Equilibrium (Kc) — write the correct Kc expression first (powers matching coefficients), then substitute. The expression itself is a mark.
• Acids and bases — pH = −log[H⁺] and, at HL, Ka/pKa work. Keep the concentration units visible throughout.
• Gibbs free energy (HL) — ΔG = ΔH − TΔS, converting ΔS from J to kJ before combining, then reading spontaneity from the sign.

For every one of these, the accuracy mark and the units/sig-fig mark are separate opportunities. Land both.

• States only (partial): "The boiling point increases down group 17."
• States and explains (full): "The boiling point increases down group 17 because the number of electrons increases, strengthening London (dispersion) forces, which require more energy to overcome."

Name the actual interaction — covalent bonding, ionic attraction, hydrogen bonding, London forces — rather than reaching for vague phrases like "stronger bonds" or "more reactive." A named mechanism reads as understanding; a vague one reads as a guess and scores like one.

• Missing or wrong units on the final answer — forfeits the unit mark automatically.
• Sig-fig slips — calculator-length or over-rounded answers.
• Vague explanations that never name the interaction.
• Unbalanced equations or missing state symbols — easy marks dropped in a rush.
• Abandoning a calculation after an early error, throwing away every ECF mark that was still live.
• Badly drawn structures — an ambiguous displayed or skeletal formula loses the mark even when you "meant" the right thing.

What are ECF marks in IB Chemistry?

ECF stands for error-carried-forward (the own-figure rule). If your method in a later part is correct but you carried in a wrong value from an earlier part, the later method mark — and usually the accuracy mark too — is still awarded on your own figure. You are penalised once for the original slip, not again and again. This is exactly why you must show full working: ECF can only credit a method the examiner can see.

Do units matter in Paper 2?

Yes — units are explicitly assessed, and a correct number with missing or wrong units loses the mark. Carry units through your working and check the final line every time. Common losses are enthalpy given in J instead of kJ mol⁻¹ and concentration written as mol instead of mol dm⁻³.

How many significant figures should I give?

Match the least precise data in the question and stay consistent. Over-precise calculator answers and over-rounded ones both risk the significant-figures mark, and sometimes the accuracy mark with it.

Should I keep going if I think an earlier answer is wrong?

Almost always, yes. Because of ECF, a correct method applied to your own (wrong) figure still earns the later marks. Leaving a part blank guarantees you lose those marks; carrying on keeps them in reach.

What is the difference between "state" and "explain"?

"State" wants the fact only — one correct point. "Explain" (and "state and explain") wants the fact plus the reason or mechanism behind it. Answering an "explain" with a bare fact leaves half the marks on the table.