Common Mistakes and How to Avoid Them

Every year, examiner reports highlight the same recurring mistakes. Knowing what these are — and checking for them — can add several marks to your total. This lesson catalogues the most common and costly mistakes across all topic areas, with worked examples showing how to avoid each one.

Energetics Mistakes

Sign Errors

The most common energetics mistake is getting the sign of ΔH wrong. Remember:

Exothermic reactions release energy: ΔH is negative
Endothermic reactions absorb energy: ΔH is positive

When using q = mcΔT, remember that ΔH = -q/n (the negative sign accounts for the convention that energy released by the system is negative).

Worked Example: A calorimetry experiment measures a temperature rise of 8.5°C when 50.0 cm³ of 1.00 mol dm⁻³ acid reacts with excess alkali.

q = 50.0 × 4.18 × 8.5 = 1776.5 J n = 1.00 × 0.0500 = 0.0500 mol ΔH = **-**1776.5 / 0.0500 = -35530 J mol⁻¹ = -35.5 kJ mol⁻¹

The negative sign is essential — this is an exothermic reaction. Many students write +35.5, which would indicate an endothermic reaction and is incorrect.

Hess's Law Direction Errors

When constructing enthalpy cycles, pay attention to the direction of arrows. If you reverse a reaction, you reverse the sign of ΔH. Students frequently forget to reverse the sign when going "against the arrow" in a cycle.

Worked Example: Given: (1) C(s) + O₂(g) → CO₂(g) ΔH₁ = -393 kJ mol⁻¹ (2) H₂(g) + ½O₂(g) → H₂O(l) ΔH₂ = -286 kJ mol⁻¹ (3) CH₃OH(l) + 1½O₂(g) → CO₂(g) + 2H₂O(l) ΔH₃ = -726 kJ mol⁻¹

Calculate the enthalpy of formation of methanol: C(s) + 2H₂(g) + ½O₂(g) → CH₃OH(l)

ΔH_f = ΔH₁ + 2×ΔH₂ - ΔH₃ (reversing reaction 3 reverses its sign) ΔH_f = (-393) + 2×(-286) - (-726) ΔH_f = -393 - 572 + 726 = -239 kJ mol⁻¹

Common error: Forgetting to reverse the sign of ΔH₃ when going backwards through reaction 3. This gives -393 - 572 - 726 = -1691 kJ mol⁻¹, which is completely wrong.

Born-Haber Cycle Errors

The most frequent Born-Haber mistake is applying electron affinity in the wrong direction. Remember:

First electron affinity of oxygen is exothermic (negative): O(g) + e⁻ → O⁻(g)
Second electron affinity is endothermic (positive): O⁻(g) + e⁻ → O²⁻(g)

Many students get these signs reversed, which cascades through the entire calculation.

Equation Mistakes

Not Balancing Equations

Always check that your equation balances for atoms and charge (for ionic equations). An unbalanced equation receives zero marks.

Checklist for balancing:

Count atoms of each element on both sides
Adjust coefficients (not subscripts)
For ionic equations: check total charge on each side is equal
Add state symbols if required

Forgetting State Symbols

State symbols (s, l, g, aq) are required in thermochemistry, equilibrium, and redox equations. Missing state symbols can cost 1 mark per equation.

When state symbols are essential:

Any equation involving ΔH values (the enthalpy depends on state)
Equilibrium expressions (Kc calculations require correct identification of phases)
Electrode half-equations
Questions that specifically ask for a "full equation with state symbols"

Half-Equation Errors

When writing half-equations for redox:

Balance atoms other than O and H
Balance O by adding H₂O
Balance H by adding H⁺
Balance charge by adding electrons

Worked Example: Write the half-equation for the reduction of dichromate ions in acid solution. Cr₂O₇²⁻ → 2Cr³⁺ (balance Cr) Cr₂O₇²⁻ → 2Cr³⁺ + 7H₂O (balance O with H₂O) Cr₂O₇²⁻ + 14H⁺ → 2Cr³⁺ + 7H₂O (balance H with H⁺) Charge: left = -2 + 14 = +12; right = +6. Need +6 electrons on left. Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O ✓

Organic Chemistry Mistakes

Mechanism Arrows

Use curly arrows starting from an electron pair (lone pair or bond), not from an atom
Arrows point to where the electrons go
For free radical mechanisms, use fish-hook (half) arrows

graph LR
    A["Correct: Arrow starts from<br/>lone pair or bond"] --> B["Shows electron<br/>pair movement"]
    C["Incorrect: Arrow starts<br/>from an atom"] --> D["Meaningless — atoms<br/>don't move as electrons"]
    E["Correct: Fish-hook arrow<br/>for radicals"] --> F["Shows single<br/>electron movement"]

Common error 1: Drawing an arrow from the Br atom to the OH group. Correct: Draw the arrow from the lone pair on OH⁻ to the δ+ carbon.

Common error 2: Drawing the arrow from the C-Br bond to the Br atom. Correct: The arrow goes from the C-Br bond to the Br atom (showing the bonding electrons moving onto bromine as it leaves).

Common error 3: In electrophilic addition to alkenes, the first arrow must come from the C=C π bond, not from the electrophile to the alkene.

Confusing Reagents and Conditions

Students often give the reagent but forget the conditions:

"NaOH" is not complete — specify "NaOH(aq), reflux" or "NaOH in ethanol, reflux"
"K₂Cr₂O₇" is not complete — specify "K₂Cr₂O₇/H₂SO₄"
"Heat" is not complete — specify "reflux" or "distil"

Aqueous vs Ethanolic: The Critical Distinction

Reagent	Solvent	Product	Reaction type
NaOH	Water (aqueous)	Alcohol	Nucleophilic substitution
NaOH	Ethanol	Alkene	Elimination
NH₃	Water	No useful reaction with halogenoalkane	—
NH₃	Ethanol (sealed tube)	Amine	Nucleophilic substitution

This is one of the most commonly tested distinctions. If you write "NaOH" without specifying the solvent, the examiner cannot tell which reaction you mean and cannot award full marks.

Equilibrium vs Rate Confusion

This is one of the most common conceptual errors in the entire specification. Learn this table:

Change	Effect on rate	Shifts equilibrium?	Changes Kc/Kp?
Increase temperature	Increases (both directions)	Yes — favours endothermic	Yes
Increase concentration of reactant	Increases forward rate	Yes — shifts right	No
Increase pressure (gases)	Increases rate	Yes — favours fewer moles	No
Add a catalyst	Increases rate (both equally)	No	No

Why Students Get This Wrong

The confusion arises because students think "if a catalyst increases the rate, it must make more product." But a catalyst increases both the forward and reverse rates equally, so the equilibrium position is unchanged. The system simply reaches equilibrium faster.

Worked Example: "Explain the effect of a catalyst on the equilibrium yield of ammonia in the Haber process."

Wrong answer: "A catalyst increases the rate, so more ammonia is produced, shifting the equilibrium to the right."

Correct answer: "A catalyst increases the rate of both the forward and reverse reactions equally. It does not change the position of equilibrium or the equilibrium yield of ammonia. It only allows the system to reach equilibrium more quickly."

Temperature and Kc

When temperature increases for an exothermic forward reaction:

The endothermic (reverse) reaction is favoured
The equilibrium shifts left (towards reactants)
The value of Kc decreases (because the equilibrium now favours reactants more)

Many students correctly state the shift direction but incorrectly say "Kc does not change." Kc only stays constant when temperature is constant.

Significant Figures

Never give more significant figures in your answer than in the data
Do not round intermediate calculations — round only the final answer
"3 significant figures" means 3 significant figures, not 3 decimal places

Example: A student is given data to 3 s.f. and writes their answer as 0.04821974 mol dm⁻³. This is 7 significant figures. The correct answer is 0.0482 mol dm⁻³ (3 s.f.). Note: the leading zeros are not significant — 0.0482 has 3 significant figures.

Quick Reference: Counting Significant Figures

Value	Sig figs	Why
4320	3 (or 4 if trailing zero is intentional)	Ambiguous — use scientific notation
0.00340	3	Leading zeros not significant, trailing zero is
1.00	3	Trailing zeros after decimal point are significant
100	1 (or 2 or 3)	Ambiguous — write 1.00 × 10² for clarity

Not Reading the Question

The most frustrating mistake of all. Check:

How many marks is the question worth? (This tells you how many points to make)
Does it say "explain" or "describe"? (These require fundamentally different answers)
Does it specify a particular reagent, condition, or context?
Does it say "using the data in the table"? If so, you must reference specific numbers.
Does it say "give one reason"? Then give one, not three.
Does it ask about a specific substance, or the general trend?

Costly example: A question asks "Give one reason why the boiling point of water is higher than that of hydrogen sulfide." A student writes three reasons. The examiner marks only the first. If the first reason is wrong (e.g., "water has a higher molar mass" — incorrect), the student scores zero even though the second reason was correct.

Pre-Submission Checklist

Before moving to the next question, run through this mental checklist:

graph TD
    A[Finished writing answer] --> B{Equations balanced?}
    B -->|No| C[Balance atoms and charge]
    B -->|Yes| D{State symbols included?}
    D -->|No| E[Add s, l, g, aq]
    D -->|Yes| F{Units on all numerical answers?}
    F -->|No| G[Add units]
    F -->|Yes| H{Correct sig figs?}
    H -->|No| I[Round appropriately]
    H -->|Yes| J{Answered the actual question asked?}
    J -->|No| K[Re-read question, adjust answer]
    J -->|Yes| L[Move to next question]
    C --> D
    E --> F
    G --> H
    I --> J
    K --> L

This takes 15-20 seconds per question and can prevent the loss of 1-2 marks per question — potentially 10+ marks across a paper.