3.1.6.1 Chemical equilibria and Le Chatelier's principle Quiz: AQA Chemistry, Unit 1

The 20 questions

1. For the equilibrium N2(g) + 3H2(g) <=> 2NH3(g), delta H = -92 kJ/mol, what happens to the position of equilibrium when the temperature is increased?

   • It shifts to the right, towards the products, raising the equilibrium yield of ammonia
   • It does not move, because the temperature change does not affect a gas reaction
   • It shifts to the right, because a catalyst is always added along with the temperature rise
   • It shifts to the left, towards the reactants, lowering the equilibrium yield of ammonia

2. For N2(g) + 3H2(g) <=> 2NH3(g), what happens to the equilibrium position when the total pressure is increased at constant temperature?

   • It shifts to the left, because the products occupy more moles of gas than the reactants
   • It does not move, because pressure only affects the rate and never the equilibrium position
   • It shifts to the left, because increasing pressure always favours the reactant side of any equilibrium
   • It shifts to the right, because the products occupy fewer moles of gas than the reactants

3. In the equilibrium 2SO2(g) + O2(g) <=> 2SO3(g), delta H is negative. Which change gives the highest equilibrium yield of SO3?

   • Low temperature and low pressure
   • Low temperature and high pressure
   • High temperature and low pressure
   • High temperature and high pressure

4. Adding more reactant to a homogeneous equilibrium mixture at constant temperature and pressure has what effect?

   • The equilibrium constant increases, so the yield of product rises permanently
   • The position of equilibrium shifts to the right, using up some of the added reactant
   • Nothing changes, because reactant concentration has no effect on any equilibrium position
   • The position of equilibrium shifts to the left, to restore the original reactant concentration

5. Removing a product from an equilibrium mixture as it forms will do what to the position of equilibrium?

   • It increases the equilibrium constant Kc, so the yield of every species rises
   • It shifts towards the products, so more product forms to replace the removed amount
   • It shifts towards the reactants, so the product is restored to its original level
   • It stops the reaction entirely, because the product is no longer present to react

6. Why does a catalyst have no effect on the position of an equilibrium?

   • It is consumed in the reaction, so it removes a reactant and shifts the balance to the left
   • It changes the enthalpy change of the reaction, so the equilibrium moves to the product side
   • It raises the activation energy of the reverse reaction, which shifts the equilibrium to the right
   • It speeds up the forward and reverse reactions by the same amount, so the balance is unchanged

7. In the Haber process a compromise temperature of about 400 to 450 C is used. Why is a compromise used rather than the lowest possible temperature?

   • A lower temperature increases the activation energy, so the reaction cannot start
   • A lower temperature favours nitrogen, which is the only product that can be sold
   • A higher temperature lowers the rate, so the lowest temperature gives the fastest production
   • A lower temperature favours ammonia but makes the reaction rate too slow to be economic

8. In the Contact process, 2SO2(g) + O2(g) <=> 2SO3(g) is exothermic. Which statement best explains why a compromise pressure, rather than the highest pressure, is used in practice?

   • Very high pressure makes sulfur dioxide liquid, so the reaction stops before the sulfur trioxide forms
   • Very high pressure raises the cost of plant and equipment, while yield is already high at moderate pressure
   • Very high pressure stops the catalyst working, so the reaction cannot occur at all
   • Very high pressure lowers the yield of SO3, because the forward reaction produces more gas molecules

9. A mixture of Cu(H2O)6^2+ (blue) and Cl- ions is at equilibrium. Adding concentrated hydrochloric acid turns the solution yellow-green. What does this show?

   • The equilibrium constant falls to zero, because the solution has become concentrated
   • The equilibrium shifts towards the chloro complex, as chloride concentration is increased
   • The catalyst has been destroyed by the acid, so the colour change is due to decomposition
   • The equilibrium shifts towards the aqua complex, which is blue, as chloride is added

10. For a homogeneous reaction at equilibrium, which change will shift the position of equilibrium when temperature is held constant?

    • Changing the concentration of one of the reactants or products
    • Mixing the gases with an inert gas in a vessel of fixed volume
    • Letting the reaction run for a longer time at the same conditions
    • Adding a catalyst to the mixture at the same temperature

11. For the equilibrium H2(g) + I2(g) <=> 2HI(g), what happens to the position when the pressure is increased at constant temperature?

    • The equilibrium constant changes, so the yield of hydrogen iodide increases with pressure
    • It does not move, because the number of gas moles is the same on both sides
    • It shifts to the left, because increasing pressure always favours the reactant side of any equilibrium
    • It shifts to the right, because increasing pressure always favours the side with more molecules

12. An exothermic reversible reaction is at equilibrium. The temperature is lowered. Which statement is correct?

    • The equilibrium shifts towards the reactants, because the reaction is endothermic in the forward direction
    • The equilibrium does not move, because only pressure can shift an equilibrium position
    • The equilibrium shifts towards the reactants, because cooling always favours the reactants
    • The equilibrium shifts towards the exothermic products, raising their yield

13. Which pair of conditions gives the highest equilibrium yield for the endothermic reaction N2O4(g) <=> 2NO2(g)?

    • High temperature and high pressure, since the forward reaction is endothermic and produces fewer gas moles
    • Low temperature and low pressure, since the forward reaction is endothermic and produces fewer gas moles
    • High temperature and low pressure, since the forward reaction is endothermic and produces more gas moles
    • Low temperature and high pressure, since the forward reaction is endothermic and produces more gas moles

14. A student predicts the effect of raising the temperature on the equilibrium 2NO2(g) <=> N2O4(g), delta H = -58 kJ/mol. Which prediction is correct?

    • The equilibrium does not move, because the number of gas moles changes only with pressure
    • The equilibrium shifts to the right, because raising temperature favours the exothermic direction
    • The equilibrium shifts to the left, increasing the amount of NO2 present
    • The equilibrium shifts to the right, increasing the amount of N2O4 present

15. For the equilibrium CO(g) + 2H2(g) <=> CH3OH(g), delta H is negative. Which pair of conditions gives the highest equilibrium yield of methanol?

    • Low pressure and low temperature
    • Low pressure and high temperature
    • High pressure and low temperature
    • High pressure and high temperature

16. In a reaction at equilibrium, a compromise temperature is chosen even though a lower temperature would give a higher yield. Which factor is the main reason for the compromise?

    • A lower temperature makes the catalyst inactive, so no reaction could take place at all
    • A lower temperature increases the equilibrium constant to a value that cannot be measured
    • A lower temperature changes the reaction into a different reaction with different products
    • A lower temperature slows the rate of attainment of equilibrium, so production would be too slow

17. Which change will shift the position of the equilibrium N2(g) + O2(g) <=> 2NO(g) (delta H positive) to the right?

    • Adding a catalyst, since a catalyst lowers the enthalpy change of the forward reaction
    • Increasing the temperature, since the forward reaction is endothermic
    • Increasing the pressure, since the forward reaction produces more gas molecules
    • Decreasing the temperature, since the forward reaction is endothermic

18. A mixture of N2, H2 and NH3 is at equilibrium in a closed vessel. The volume is halved at constant temperature. Which statement is correct?

    • The equilibrium is unaffected, because volume changes never affect the position of any gas equilibrium
    • The equilibrium shifts to the left, because halving the volume reduces the pressure on the mixture
    • The equilibrium shifts to the right, because the pressure rises and there are fewer gas molecules on the right
    • The equilibrium constant Kp doubles, so the position of equilibrium moves to the right

19. A reaction has delta H = +40 kJ/mol. Which statement about its equilibrium position with rising temperature is correct?

    • Higher temperature does not affect the position, because enthalpy change only affects the rate
    • Higher temperature favours the forward endothermic direction, so the yield of products increases
    • Higher temperature favours the forward reaction only if a catalyst is also present
    • Higher temperature favours the reverse exothermic direction, so the yield of products falls

20. Which is a correct use of Le Chatelier's principle to explain a shift when a gas-phase system is compressed?

    • Compression has no effect on gas-phase equilibria, because the concentration of a gas never changes
    • The side with more gas molecules is favoured, because it produces more pressure to oppose the compression
    • The side with fewer gas molecules is favoured, because it produces less pressure to oppose the compression
    • The equilibrium constant rises, so the side with more gas molecules gains yield

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