IB Chemistry Paper 2 tips: calculations, explanations and mechanisms that earn the marks

IB Chemistry Paper 2 rewards working you can see. Marks are split between method and answer, so a calculation laid out step by step can earn most of its marks even with a slip, and a bare final number earns nothing if it is wrong. The other big scorers are explanations written in terms of particles, structures and mechanisms drawn to the conventions, and pacing that gets you to the last question. This guide covers each, with original worked examples.

What does Chemistry Paper 2 look like at SL and HL?

Both levels contain short-answer and extended-response questions, and calculators and a clean data booklet are allowed. One number to get right: HL Chemistry Paper 2 is out of 90, not 80 as in HL Biology, so don't borrow timing plans from a Biology friend. The subject guide does not divide the Chemistry paper into named sections, so look at a specimen or recent paper with your teacher to see the layout. For the whole exam structure, see what the Chemistry exams look like now.

How should you lay out a calculation to earn method marks?

Mark schemes usually award marks for intermediate steps (a correct amount in mol, a correct substitution) as well as for the final answer. Make every step visible:

1. Write the equation or relationship you are using (n = m ÷ M, pV = nRT).
2. Convert units first, on their own line.
3. Substitute, with units.
4. Write each intermediate value, keeping extra digits in the calculator.
5. Give the final answer with units and sensible significant figures, set apart so the examiner can find it.

Worked example 1: molar mass from gas data

Original example written for this guide; it is not an IB question.

0.500 g of a volatile liquid is vaporised. The vapour occupies 155 cm³ at 373 K and 101 kPa. Calculate the molar mass, assuming ideal behaviour (R = 8.31 J K⁻¹ mol⁻¹).

The first answer used Pa with dm³, so it is out by a factor of 1000; a molar mass below 1 g mol⁻¹ is impossible and should have been a warning. The second makes the unit conversion its own line, which is where the method mark sits. (Using kPa with dm³ gives the same correct answer, because kPa × dm³ = J; what fails is mixing Pa with dm³.)

How do you make error carried forward work for you?

Error carried forward (ECF) lets a later part earn its marks when you use your own earlier wrong value correctly, so one slip costs one mark instead of a whole chain. The full explanation, with a worked titration chain and the limits on when ECF applies, is in IB mark schemes explained. In Chemistry Paper 2 it comes down to three habits:

• Put each intermediate value on its own line, with its label and units (n(HCl), then n(Na₂CO₃), then the concentration), so the examiner can find the value you carried forward.
• Use your own earlier answer, even if you doubt it. Switching to a "nicer" number you guessed breaks the chain the examiner is trying to follow.
• If you couldn't get an earlier value at all, state an assumed one ("assuming n = 2.00 × 10⁻³ mol") and carry on. Correct method on it may still earn marks; a blank cannot.

How do you write an "explain" answer at particle level?

Most Chemistry "explain" marks go to three moves: name the particles, name the interaction or energy change involved, and link it to the observation. Count the marks and make sure each move is a separate statement.

Worked example 2: comparing boiling points

Original example written for this guide; it is not an IB question.

Explain why ethanol (C₂H₅OH, boiling point 78 °C) has a much higher boiling point than propane (C₃H₈, −42 °C), although they have similar molar masses. [3]

"Stronger bonds" loses the marks twice: it doesn't say which forces, and it implies covalent bonds break on boiling. The same structure works for rates (collision frequency and energy relative to E_a), conductivity (mobile ions or delocalised electrons) and equilibrium shifts. Our list of common IB Chemistry mistakes has more wrong-versus-right pairs.

For extended responses worth more marks, plan in the margin first: one short note per mark, in the order you will write them. Clear numbered points are fine if each is a full statement; the examiner is looking for distinct, correct chemistry, not style.

How do you draw structures and mechanisms that score?

Structures

• Lewis formulas show every lone pair, not just the bonding pairs: one on N in NH₃, two on O in H₂O, three on Cl in HCl.
• State the shape and the angle together: CH₄ tetrahedral, 109.5°; NH₃ trigonal pyramidal, about 107°; H₂O bent, about 104.5°. Then say why: lone pairs repel more than bonding pairs.
• A full structural formula shows every bond, including the O–H in an alcohol and each C–H. Writing CH₃ is a condensed formula, so check which one the question asks for.
• Stereoisomers need the 3D or cis/trans arrangement actually drawn, not just named.

Mechanisms

Draw each mechanism your level requires the same way every time, ticking off this list:

• Every curly arrow starts at a lone pair or a bond and ends where the electron pair goes.
• The lone pair on the nucleophile is drawn, and so is the charge on any ion.
• Partial charges (δ+, δ−) are shown on the polar bond being attacked.
• Where a transition state or intermediate is required, it is drawn with its charge (in square brackets for a transition state).
• The products, including the leaving group, are shown.

A mechanism with every arrow correct but no products, or with an arrow starting from the minus sign of OH⁻ rather than its lone pair, can lose marks that were otherwise yours.

How should you use the data booklet in Paper 2?

• Know the layout before the exam. Practise every past question with the same edition you will be given, so finding a bond enthalpy or a constant takes seconds.
• Quote the value you used in your working ("C–H = 414 kJ mol⁻¹"). It shows the examiner where the number came from.
• Use the question's values over the booklet's whenever the question supplies them.
• Know what isn't there. Relationships you are expected to know, and the reasoning behind every value, still have to come from you.

How should you pace Paper 2?

At 1.8 minutes per mark (SL) or about 1.7 (HL), a 6-mark question deserves around 10 minutes and a 1-mark "state" deserves a few seconds. Three habits:

• Glance through the whole paper first (a minute or two) so the hardest-looking question doesn't ambush you at the end.
• Set checkpoints. At HL, aim to be about one third of the way through the marks after 50 minutes, two thirds after 100.
• Move on when stuck, leaving space to come back. An unfinished calculation with working shown can still earn method marks; a blank cannot.

Our guide to time management in IB exams covers recovery when you fall behind, and the Paper 1A guide covers the multiple-choice half of the course.

What changes between SL and HL?

The skills are the same; the scale and content differ. HL Paper 2 is 60 minutes longer and worth 40 more marks, and it draws on the additional HL content too: for example, Reactivity 1.4 (entropy and spontaneity, which is HL only) and the extended acid–base and electrochemistry calculations in Reactivity 3.1 and 3.2. Both levels give Paper 2 the same 44% weight. If you are HL, practise sustained calculation chains, because a long paper rewards stamina as much as speed.