10 questions🇬🇧English
⚖️ Equilibria & Kc – A-level
Chemistry · A-level — Le Chatelier's principle, writing Kc expressions, units and equilibrium calculations
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Questions
Which statement is true for a reaction at dynamic equilibrium in a closed system?
The forward and reverse rates are equal
The reactant and product concentrations are equal
The forward rate is greater than the reverse rate
Both the forward and reverse reactions have stopped
20s
What effect does adding a catalyst have on a reversible reaction that has not yet reached equilibrium?
Position shifts right; equilibrium reached sooner
Position shifts left; equilibrium reached sooner
Position unchanged; equilibrium reached sooner
Position unchanged; value of Kc increases
20s
For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), what happens to the equilibrium position if the pressure is increased?
It shifts right, as the reaction is endothermic
It does not move, as all species are gases
It shifts left, towards more gas moles
It shifts right, towards fewer gas moles
20s
What is the Kc expression for N₂(g) + 3H₂(g) ⇌ 2NH₃(g)?
[NH₃] / ([N₂][H₂])
2[NH₃] / ([N₂] + 3[H₂])
[N₂][H₂]³ / [NH₃]²
[NH₃]² / ([N₂][H₂]³)
30s
What are the units of Kc for N₂(g) + 3H₂(g) ⇌ 2NH₃(g)?
mol² dm⁻⁶
mol⁻² dm⁶
mol⁻¹ dm³
No units
30s
A reaction has a negative ΔH for the forward direction. What happens to Kc when the temperature is increased?
Kc stays the same
Kc decreases
Kc increases
Kc increases, then returns to its old value
30s
For CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O at equilibrium, extra ethanol is added. What happens?
Position shifts right; Kc increases
Position shifts right; Kc unchanged
Position shifts left; Kc unchanged
Position unchanged; Kc increases
30s
For H₂(g) + I₂(g) ⇌ 2HI(g), equilibrium moles are 0.20 mol H₂, 0.20 mol I₂ and 1.6 mol HI. What is Kc?
64
6.4
40
0.016
60s
1.00 mol PCl₅ is put in a 2.00 dm³ flask. At equilibrium 0.40 mol PCl₅ remains. For PCl₅ ⇌ PCl₃ + Cl₂, what is Kc?
0.45 mol dm⁻³
0.90 mol dm⁻³
0.45 mol⁻¹ dm³
1.5 mol dm⁻³
60s
The Haber process forward reaction is exothermic. Why is a temperature of about 450 °C used rather than a much lower one?
A lower temperature would decrease the value of Kc
A higher temperature increases the ammonia yield
The forward reaction is actually endothermic
A lower temperature would give too slow a rate
30s