WASSCE Chemistry · Paper 1 style

Chemical equilibrium

One sample question with the full working. 7 more in the app, with instant feedback and repair sessions for the traps below.

For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ mol⁻¹, which change increases the equilibrium yield of ammonia?

AIncreasing the temperature
BIncreasing the pressure
CRemoving nitrogen
DAdding an iron catalyst

Show the answer and working

Answer: B

The forward reaction reduces 4 moles of gas to 2, so higher pressure shifts equilibrium right.

Trap A Reverses a trend, direction or effect: State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.

Trap C Misapplies Le Chatelier's principle: An equilibrium shifts to OPPOSE the change: added pressure → side with fewer gas moles; added heat → endothermic direction. A catalyst speeds both directions equally and does NOT change the position.

Trap D Thinks a catalyst shifts equilibrium, adds energy or is used up: A catalyst gives an alternative route with lower activation energy for BOTH directions. It is unchanged at the end, so it changes the rate only: never the yield, ΔH or Kc.

Traps in Chemical equilibrium

Reverses a trend, direction or effect
State the rule with its direction before looking at the options (down group 1 reactivity INCREASES; exothermic forward reaction: heating shifts LEFT). An option that is the mirror image of the rule is the trap.
Misapplies Le Chatelier's principle
An equilibrium shifts to OPPOSE the change: added pressure → side with fewer gas moles; added heat → endothermic direction. A catalyst speeds both directions equally and does NOT change the position.
Thinks a catalyst shifts equilibrium, adds energy or is used up
A catalyst gives an alternative route with lower activation energy for BOTH directions. It is unchanged at the end, so it changes the rate only: never the yield, ΔH or Kc.
Picks a related substance or process with a different role
Learn reagents and processes as one-line 'name → job' cards and contrast neighbours in a table (process | product | catalyst | conditions). Match the job in the stem, not just the topic.
Over-generalises a rule beyond where it holds
Every rule in chemistry has conditions. Before applying one, check the conditions in the stem (electrode type, concentration, moles of gas on each side, metal position).
Uses everyday or macroscopic thinking instead of particles
Ask what the particles, ions or electrons are doing. Properties of a substance come from its particles and bonds, not from how it looks or feels.
Takes one factor as the whole answer
Ask whether the option is necessary or sufficient. If the stem involves two changes or two conditions, both must appear in the answer.
Chooses a true but irrelevant statement
Re-read the question word (why, which, what causes). A true fact that answers a different question is still wrong.

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