IAL Chemistry Unit 5: 10 Most Common Topics Ranked

TL;DR: I went through every Edexcel IAL Chemistry Unit 5 sitting from October 2020 to January 2026 — 17 papers, 831 mark-carrying question parts, 1,530 marks — and ranked every topic by the marks it earns rather than by how big it looks in the specification. Ten topics carry roughly 49 marks of every 90-mark paper, which is about 55% of all marks in the unit. The top five appear in all 17 papers without a single exception. The table below gives the full ranking, the marks per paper, and which topics carry the 6-mark extended-response questions.

Jump to

• What are the most common topics in Edexcel IAL Chemistry Unit 5?
• How I worked this out
• The five topics that appear in every single paper
• Which Unit 5 topics carry the 6-mark essays?
• The hardest topics aren't the biggest ones
• Why amines lose more marks than any other topic
• Complex ions: the biggest topic, and its classic trap
• Redox titrations: where one early slip costs the whole chain
• How to use this ranking in your revision
• Ranks 10 to 13 are a near-tie
• FAQ

What are the most common topics in Edexcel IAL Chemistry Unit 5?

The ten topics below carry about 49 marks of every 90-mark WCH15 paper — roughly 55% of all marks in the unit. The top five alone carry 35%, and every one of them appeared in all 17 sittings analysed.

Unit 5 is worth 40% of your IA2 and 20% of the whole International A Level, and the paper is 90 marks in 1 hour 45 minutes across three sections, so every mark in this table is expensive. Here is the full ranking.

#	Topic	Marks	Per paper	Papers	Hard marks	6-mark essays
1	Complex ions: ligands, coordination number, shape and isomerism	140	8.2	17/17	44%	4
2	Redox titrations and uncertainty	114	6.7	17/17	34%	6
3	Electrophilic substitution mechanisms	109	6.4	17/17	16%	1
4	Electrode potentials, cells and E°cell	90	5.3	17/17	28%	2
5	Vanadium and chromium redox chemistry	82	4.8	17/17	26%	1
6	Amines: basicity, reactions and preparation	67	3.9	15/17	31%	0
7	Spectra: NMR, IR and MS structure identification	65	3.8	15/17	51%	1
8	Empirical and molecular formula calculations	62	3.6	13/17	26%	1
9	NaOH and NH₃ precipitates, amphoteric behaviour	60	3.5	13/17	27%	2
10	Grignard reagents and chain lengthening	55	3.2	14/17	55%	2

Hard marks means the proportion of that topic's marks flagged as poorly answered in Pearson's own examiner reports — not my personal read of the chemistry. 6-mark essays counts the starred extended-response questions across the whole series.

Read the table twice. The first read tells you what to revise. The second read tells you something less obvious: the biggest topic is not the hardest one, and the hardest topic is nowhere near the top of the list.

How I worked this out

This ranking came out of building my own IAL Chemistry solved past-paper package and notes, not from a spreadsheet exercise done for its own sake. While working through the papers question by question, the same topics kept reappearing so insistently that I started counting them properly.

Every mark-carrying part in all 17 sittings was recorded with its marks, its command word, and its mark-scheme points, then classified by content. That gives 831 parts and 1,530 marks — 17 papers at exactly 90 each, which is a useful check that nothing was dropped.

Three things you should know about the limits of this, because a ranking without its caveats is worth less than no ranking at all:

• Each part was assigned one dominant topic. Section C questions are deliberately synoptic and spread across several, so these figures slightly understate the reach of the big topics rather than overstating it.
• About 8% of marks resisted confident classification — mostly small synoptic calculation steps. Those were excluded rather than force-fitted into a bucket to make the numbers tidier.
• Difficulty comes from examiner-report language only. If Pearson's principal examiner flagged a part as poorly answered, it counts as hard. If I personally think something is hard, it does not.

The specification itself explains why so many marks cluster: this paper may contain synoptic questions requiring knowledge from Units 1, 2 and 4, so mole calculations and organic reactions you met a year earlier come back wearing Unit 5 clothes. If your Units 1 and 2 foundations are shaky, Unit 5 will expose them — which is why some students rebuild that base alongside A2 rather than after it, and why our AS Chemistry course is taken by A2 students more often than you would expect.

What surprised me most, after 16 years teaching to Cambridge and Edexcel mark schemes, was not which topics came top. It was how often the same question came back — same format, same context, same number of marks — with only the compound or the metal changed.

The five topics that appear in every single paper

Complex ions, redox titrations, electrophilic substitution, electrode potentials and vanadium/chromium chemistry appeared in all 17 sittings from October 2020 to January 2026, and together they carry 35% of every mark in the unit.

Not "usually". Not "most years". All 17.

That changes what a sensible revision plan looks like. If you have a fortnight and you are choosing between finishing your amino-acid notes and drilling redox titrations, the arithmetic answers the question for you: one of those has never failed to appear, and the other missed two sittings out of seventeen.

There is a second implication that students miss. Because these five appear every time, they are also where the paper does its discrimination — they carry a wide spread of demand, from a single recall mark up to a full 6-mark essay. Getting the easy marks in a topic you have seen every year is not the same as owning it.

I keep the condensed version of all five in the IAL Chemistry and Biology notes eBooks, written specifically against the wording Edexcel mark schemes accept, because the gap between knowing the chemistry and writing the accepted sentence is where most of these marks disappear.

Which Unit 5 topics carry the 6-mark essays?

Redox titrations carry more starred extended-response questions than any other topic — six of the 32 across the series — followed by complex ions with four. Between them, two topics own nearly a third of the unit's essay marks.

You need to understand how those six marks are built, because they are not built the way most students assume. Pearson's mark scheme splits them into indicative content and structure: six indicative points earns four marks, and the remaining two marks come from whether the answer shows a coherent and logical structure with linkages and fully sustained lines of reasoning. An unstructured answer with every chemical point present scores four out of six. There is also a sting in the tail: incorrect chemistry is deducted from the reasoning marks.

This is where I see the most avoidable loss. Students read the question quickly, then start writing their argument as it occurs to them. A 6-mark question needs planning before the pen moves — sketch the structure in pencil first, decide the order of your points, check they link, then write. Two minutes of planning buys you the two structure marks that four minutes of extra writing never will.

The examiners say the same thing in plainer words. In the October 2022 report, the principal examiner noted that many candidates lost marks because they did not answer the question that was set, and that the best answers were carefully structured around the guidance given in the question stem.

The hardest topics aren't the biggest ones

Grignard reagents rank only 10th on marks but first on difficulty — 55% of their marks were flagged in examiner reports — with spectra second at 51%. Volume and demand are two different axes, and confusing them wrecks revision plans.

Look at what that means practically. Electrophilic substitution is the third-biggest topic in the unit and only 16% of its marks are flagged as hard. It is high-volume, low-demand: reliable marks, quickly secured. Grignard chemistry is the reverse — barely three marks a paper, but it anchors the "devise a synthesis" questions where A grades turn into A*s.

The examiner reports show exactly where it breaks down. In October 2022, candidates knew the Grignard involved dry ether and could draw the structure, but the reactions of the Grignard reagent with the carbonyl compounds seemed unfamiliar, and those who did know it often drew the intermediate with a trivalent carbon. In January 2022, the pattern repeated: candidates formed the Grignard correctly, reacted it with a suitable carbonyl compound, then omitted the hydrolysis step that releases the product.

So the failure is not the concept. It is the last step of a three-step chain, every time.

If you are short on time, secure the low-demand volume first. Then spend your best hours — not your leftover hours — on Grignard synthesis and spectra.

Why amines lose more marks than any other topic

Amines rank sixth on marks but attract more examiner criticism than any topic in the unit, and almost all of it traces to one incomplete explanation: students describe the delocalisation and stop, without linking it to the ability to accept a proton.

Here is the shape of the accepted answer. When you compare the basicity of an aromatic amine with an aliphatic one, the mark scheme wants two effects, not one — the electron-donating alkyl groups on one side, and the lone pair on nitrogen being delocalised into the aromatic π-system on the other. If only one effect is given, the response scores one mark instead of two.

The three-mark version of this chain, in the order the examiner wants it:

1. The lone pair on the nitrogen is delocalised into the benzene ring.
2. So the lone pair is less available.
3. So the amine is less able to accept a proton, and therefore less basic.

Most students write step 1, imply step 2, and never write step 3. That is the whole loss.

There is a related weakness worth flagging. In January 2022, only 40% of the cohort correctly classified an amine as primary, secondary or tertiary — the examiner put it down to unfamiliarity with skeletal formulae, where you have to count the carbon atoms attached to the nitrogen rather than read them off. Fixing your skeletal-formula fluency is a one-evening job that pays back across the entire organic half of the paper.

Complex ions: the biggest topic, and its classic trap

Complex ions carry more marks than any other Unit 5 topic — 140 marks, 8.2 per paper, present in all 17 sittings — and their most reliable trap is explaining a complex's shape without ever mentioning electron pairs.

The January 2022 report is unusually blunt about it. Asked to explain a linear shape, most candidates could name the shape but few could explain it — many discussed there being two ligands, without relating that to the number of bonding electron pairs, which is the idea being tested. "Minimising repulsion" appeared as a standalone phrase rather than linked to the bond pairs.

The same report flags a second precision problem that costs marks every series: candidates write "a full d orbital" when they mean a full d sub-shell, and give a generic 3d answer where the question concerns 4d. In colour and shape questions, the vocabulary is the mark.

Two habits fix most of this:

• Explain every shape in terms of bonding electron pairs and repulsion, in that order, even when the shape seems obvious.
• Say sub-shell unless you specifically mean one orbital, and name the right one.

Redox titrations: where one early slip costs the whole chain

Redox titrations are the second-biggest topic and the biggest source of essay marks, and they punish errors differently from everything else in the paper: one wrong mole ratio near the start propagates through every subsequent line.

Error carried forward saves some of this — mark schemes credit later steps that correctly use an earlier wrong value — but only if your working is visible and your route is clear. A correct answer with no working is fragile; a wrong answer with clean working often keeps most of the marks.

The back-titration variant is where examiners see the most damage. In October 2022, candidates found dealing with the back-titration aspect quite demanding and often omitted it altogether. Most knew a scaling factor was needed to account for using a 25 cm³ sample from a volumetric flask, but introduced it at the wrong point in the calculation. In the same question, using the formula mass of the whole ion instead of the atomic mass of the metal was described as very common.

Three checks that recover most of these marks:

• Write the balanced equation and ring the mole ratio before you calculate anything.
• Decide where the scaling factor belongs before you use it, not after your answer looks wrong.
• When the question asks for a mass of the element , use the atomic mass of the element.

And when a calculation looks unstructured and you cannot see a way in, Pearson's own advice in the January 2022 report is worth memorising: if in doubt, mole it out. Find any amount in moles you can calculate from the data given, and the route usually opens up.

How to use this ranking in your revision

Split the ten topics into three bands and give them different kinds of attention rather than equal time.

Band 1 — the five every-paper topics. These get drilled by question count, not by reading. Work through them across multiple sittings until the standard formats feel routine, because the formats repeat.

Band 2 — the difficulty spikes. Grignard synthesis and spectra. Fewer marks, higher demand, so give them your sharpest hours. For synthesis, build your own flow chart linking functional groups across the whole specification — the January 2022 report recommends exactly this, and it works because synthesis questions test navigation, not recall.

Band 3 — the reliable volume. Electrophilic substitution, formula calculations, precipitate reactions. Low difficulty, decent marks. Secure these early so they are not competing for time in your final week.

In the last week before the exam, narrow to this list and nothing else. Students who used my solved past-paper package alongside the notes and concentrated their final week on the highest-frequency topics improved their grades considerably, both in mock exams and in the real thing — not because the material was new to them, but because the formats stopped being surprising.

If you want a second opinion on where your own gaps sit, send me a photo of a marked script on WhatsApp at +965 5137 5709 or email admin@chem-bio.info, and I will tell you which band to work on first.

Ranks 10 to 13 are a near-tie — read this before you drop anything

Grignard chemistry sits 10th on 55 marks, but catalysis (54), benzene structure and stability (54) and polymers (53) are all within two marks of it. Treat those four as one band, not as a cut-off.

I placed Grignard 10th because of its difficulty profile and its essay weight, not because it beat the others on volume — on marks alone, the four are indistinguishable. If your revision plan involves dropping something at rank 11, you are making a decision the data does not support.

Catalysis deserves a specific mention. One student on The Student Room described sitting the January 2021 paper and finding a 6-mark catalyst essay after assuming it could not appear two sittings running. It can. Nothing in this ranking should be read as a topic being "used up".

FAQ

What is the hardest topic in Edexcel IAL Chemistry Unit 5? Grignard reagents and chain lengthening, with 55% of their marks flagged as poorly answered in Pearson's examiner reports, followed by spectral identification at 51%. Both carry relatively few marks but appear in the synthesis and structure-determination questions that separate A from A*.

How many marks is Edexcel IAL Chemistry Unit 5 worth? WCH15 is a 90-mark paper sat in 1 hour 45 minutes, worth 40% of your IA2 and 20% of the full International A Level. It has three sections: multiple choice, a mixture of short and extended questions, and a contemporary context question.

Do the same questions repeat in IAL Chemistry Unit 5? The formats repeat far more than the contexts do. Across 17 sittings the same question shapes recur with the compound, metal or context changed, which is why working through past papers by topic is more efficient than working through them chronologically.

Which Unit 5 topics come up in every paper? Five topics appeared in all 17 sittings from October 2020 to January 2026: complex ions, redox titrations, electrophilic substitution mechanisms, electrode potentials and cells, and vanadium and chromium redox chemistry. Together they carry about 35% of all marks in the unit.