Le Chatelier's Principle: Quiz

Le Chatelier's Principle: Quiz

7 questions

1. Question 1EasyFor $\ce{N2 + 3H2 <=> 2NH3}$, what is the effect of removing $\ce{NH3}$ as it forms?

   AThe equilibrium shifts leftBThe equilibrium shifts right, making more NH₃CK increasesDNo effect
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   Answer: The equilibrium shifts right, making more NH₃

   Removing a product makes Q smaller than K, so the reaction goes forward to replace some of it. K is unchanged.

2. Question 2MediumWhich equilibrium shifts to the right when the pressure is increased?

   A$\ce{2NO2(g) <=> N2O4(g)}$B$\ce{H2(g) + I2(g) <=> 2HI(g)}$C$\ce{PCl5(g) <=> PCl3(g) + Cl2(g)}$D$\ce{CaCO3(s) <=> CaO(s) + CO2(g)}$
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   Answer: $\ce{2NO2(g) <=> N2O4(g)}$

   Higher pressure favours fewer moles of gas: 2 mol → 1 mol. HI: no change (2 = 2). The other two have more gas on the right, so they shift left.

3. Question 3MediumFor an exothermic reaction at equilibrium, what does raising the temperature do?

   AShifts right and increases KBShifts left and decreases KCShifts left but K is unchangedDHas no effect
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   Answer: Shifts left and decreases K

   Heat is a "product" of an exothermic reaction. Adding heat shifts the equilibrium left, and K itself becomes smaller.

4. Question 4EasyWhat is the effect of adding a catalyst to an equilibrium mixture?

   AMore products formBMore reactants formCEquilibrium is reached faster, with the same compositionDK increases
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   Answer: Equilibrium is reached faster, with the same composition

   A catalyst speeds up the forward and reverse reactions equally, so the position of equilibrium and K are unchanged.

5. Question 5EasyWhich change alters the value of $K_\text{c}$?

   AAdding more reactantBIncreasing the pressureCChanging the temperatureDAdding a catalyst
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   Answer: Changing the temperature

   Concentration and pressure changes shift the position but leave K unchanged. Only temperature changes K.

6. Question 6Medium$\ce{N2O4(g) <=> 2NO2(g)}$, ΔH = +57 kJ. $\ce{N2O4}$ is colourless and $\ce{NO2}$ is brown. What happens when the mixture is cooled?

   AIt becomes darker brownBIt becomes palerCNo colour changeDIt turns colourless instantly
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   Answer: It becomes paler

   The forward reaction is endothermic. Cooling favours the exothermic reverse reaction, forming more colourless N₂O₄, so the brown colour fades.

7. Question 7HardAn equilibrium mixture of $\ce{H2 + I2 <=> 2HI}$ ($K_\text{c} = 49.0$) has 0.0222 M H₂, 0.0222 M I₂ and 0.156 M HI. 0.100 M H₂ is added. What is Q just after the addition?

   A49.0B8.99C0.111D7.03
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   Answer: 8.99

   Q = (0.156 M)² / ((0.122 M)(0.0222 M)) = 8.99. Q < K, so the reaction goes forward, using up H₂ and I₂.