WACE Chemistry markers see the same mistakes every year. They are not the kinds of mistakes you fix by understanding the topic better; they are the kind you fix by changing how you write your answer. Below are the ten that come up most consistently in SCSA marking keys, ranked by how many marks they typically cost.
1. Forgetting that K only changes with temperature
"Adding a catalyst increases K." No, it does not. A catalyst speeds up both forward and reverse reactions equally, so it changes the rate of approach to equilibrium, not the position. Only temperature alters K. This appears as a multi-mark Le Chatelier question every year.
2. Quoting only one mark of a two-mark explanation
"Predict the effect on equilibrium and explain." The answer needs both the direction of shift and the reason ("to reduce the stress" / "to counteract the change"). Half the cohort writes only the direction. Marks lost: 1 to 2 per question, multiplied across the paper.
3. Sign convention errors in standard electrode potentials
When you reverse a half-equation (the oxidation half), you reverse the sign of the standard electrode potential. Then you add to the reduction half potential. Many students subtract instead, or forget to reverse the sign. Practice 10 cell potential calculations in a row to embed the rule.
4. Treating weak acid pH like strong acid pH
For a weak acid, pH is not -log[acid]. You must use Ka with an ICE table, or apply the approximation [H+] = sqrt(Ka * c) (only valid when Ka is small relative to c). Skipping this step and using the strong acid formula is a 3-mark error.
5. Wrong organic IUPAC names on long carbon chains
For 2-methylpentan-3-ol, the numbering must give the principal functional group (the alcohol) the lowest locant. Many students number from the wrong end and lose 1 to 2 marks. The rule is: the principal functional group always wins, then the substituent.
6. Forgetting to balance redox half-equations in basic conditions
In acidic conditions you balance with H+ and H2O. In basic conditions you balance with OH- and H2O. Use the wrong one, lose 2 marks. WACE often asks both in the same paper. Read the conditions carefully.
7. Spectroscopy: ignoring the molecular ion in mass spectra
Mass spectra questions almost always require you to identify the molecular ion (M+) first, then deduce molecular formula. Students often jump to fragmentation analysis and miss the easier 1-mark identification of M+.
8. Not naming the functional group in IR analysis
IR spectroscopy questions ask "what functional group does this peak indicate?" The answer must name the functional group (carbonyl, hydroxyl, alkyl, amine), not just the bond (C=O, O-H). The mark scheme is specific.
9. Forgetting limiting reagent in yield calculations
Percentage yield = actual yield / theoretical yield, all multiplied by 100. The theoretical yield is calculated from the limiting reagent, not from the reagent in stoichiometric excess. Identifying the limiting reagent first is a 1-mark step that most students skip.
10. Significant figures and units
WACE Chemistry awards marks for data presentation. The mark scheme typically requires 3 significant figures unless the data given has fewer. It also expects units on every quantitative answer. A correct answer presented as "0.0234" instead of "0.0234 mol L-1" can lose half a mark per question, which adds up across an exam.
The 5-minute self-audit
Before each Chemistry test or mock, do a 5-minute self-audit: read your most recent test paper. For each lost mark, mark which of the 10 errors above it represents. By Term 3 you will know your personal top 3 error types. Drill those, not the topics.
What to actually do this week
- Pull your last Chemistry test. Tag every wrong answer with one of the 10 errors above. If a mistake does not fit, label it "concept gap" and study that concept.
- Set a recurring 30-minute slot weekly to practice mark-scheme phrasing. Pick three answer types: Le Chatelier, weak acid pH, and IUPAC. Drill them as a set.
- Pin this list inside your Chemistry folder. Read it before every test for the rest of the year.
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