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Free Le Chatelier's Principle MCQs with Answers

26 Le Chatelier's Principle MCQs from Chemistry, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.

Le Chatelier's principle predicts how an equilibrium responds when concentration, pressure or temperature changes. Questions apply it to reversible reactions, identify the direction of shift and distinguish genuine equilibrium changes from the effect of a catalyst, which changes the rate of reaching equilibrium but not its position.

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26 questions · page 1 of 2

1. Le Chatelier's principle states that when a stress is applied to a system at equilibrium, the system

  • A. shifts in the direction that relieves the stress
  • B. shifts in the direction that increases the stress
  • C. stops reacting altogether
  • D. always shifts towards the products

Explanation: The position of equilibrium moves so as to partly oppose the change imposed, whether that change is in concentration, pressure or…

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Correct answer: shifts in the direction that relieves the stress

2. For the equilibrium N2 + 3H2 gives 2NH3, which is exothermic, the yield of ammonia is increased by

  • A. raising the temperature and lowering the pressure
  • B. raising the pressure and lowering the temperature
  • C. adding a catalyst
  • D. removing nitrogen from the mixture

Explanation: There are four moles of gas on the left and two on the right, so high pressure pushes the equilibrium towards the smaller volume, and…

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Correct answer: raising the pressure and lowering the temperature

3. Increasing the pressure has NO effect on the position of the equilibrium H2 + I2 gives 2HI because

  • A. the reaction is exothermic
  • B. hydrogen iodide is a gas
  • C. the number of moles of gas is the same on both sides
  • D. the reaction has no catalyst

Explanation: Pressure changes shift an equilibrium only when the two sides differ in the number of gas molecules, and here there are two moles on each…

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Correct answer: the number of moles of gas is the same on both sides

4. Adding a catalyst to a system at equilibrium

Fairly easy
  • A. shifts the equilibrium towards the products
  • B. shifts the equilibrium towards the reactants
  • C. increases the value of Kc
  • D. speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but is unchanged

Explanation: A catalyst lowers the activation energy of both directions by the same amount, so both rates rise in proportion and they still become…

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Correct answer: speeds up the forward and reverse reactions equally, so equilibrium is reached sooner but is unchanged

5. For an endothermic reaction at equilibrium, raising the temperature will

  • A. shift the equilibrium towards the products and increase Kc
  • B. shift the equilibrium towards the reactants
  • C. have no effect
  • D. decrease the rate of both reactions

Explanation: Heat can be treated as a reactant in an endothermic reaction, so supplying more of it drives the system forward to absorb the extra…

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Correct answer: shift the equilibrium towards the products and increase Kc

6. In the reaction A + B gives C + D at equilibrium, adding more of substance A will

Moderate
  • A. decrease the amount of C formed
  • B. have no effect on the position of equilibrium
  • C. shift the equilibrium to the right, forming more C and D
  • D. change the value of Kc

Explanation: The system responds to the extra A by consuming some of it, which means running the forward reaction further and producing more products…

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Correct answer: shift the equilibrium to the right, forming more C and D

7. By decreasing the pressure the reaction will go to that direction where_________________?

  • A. volume is decreased
  • B. volume increased
  • C. heat absorbed
  • D. no. of moles of specie decreased

Explanation: When pressure is decreased, a gaseous equilibrium shifts toward the side with more gas molecules because that side occupies a greater…

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Correct answer: volume increased

8. For a reaction involving only gases at 25?C the equilibrium constant can be expressed in terms of molarity Ke or partial pressure Kp. Which is true about the numerical value of Kp ?

  • A. Kc is generally greaer than Kp
  • B. Kc is generally less than Kp
  • C. Kc is generally equal to Kp
  • D. Kc is equal to Kp if the total moles of reactants and products are equal

Explanation: The relationship is Kp = Kc(RT)^Δn, where Δn is the difference between gaseous product and reactant coefficients.

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Correct answer: Kc is equal to Kp if the total moles of reactants and products are equal

9. Which of the following will not change the concentration of ammonia at the equilibrium ?

  • A. Increase of pressure
  • B. Increase of volume
  • C. Addition of catalyst
  • D. Decrease of temperature

Explanation: A catalyst speeds up both the forward and reverse reactions equally, so it does not shift equilibrium or change the equilibrium…

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Correct answer: Addition of catalyst

10. A chemical engineer wants to use the Haber process in order to produce ammonia as raw material for fertilizers. Which of the following parameters must the chemical engineer AVOID in order to obtain the maximum yield of ammonia?N2(g) + 3H2(g) -> 2NH3(g) ΔH0 = -92.6 kJ/mol

  • A. Increase the temperature
  • B. Use a catalyst
  • C. Adding more nitrogen gas
  • D. Increase the pressure

Explanation: According to Le Chatelier's Principle, increasing the temperature will shift the equilibrium of the exothermic Haber process to the left…

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Correct answer: Increase the temperature

11. With different number of moles of reactants and product, the volume of system:

  • A. Remains unchanged
  • B. Changes
  • C. Decreases
  • D. Increases

Explanation: Option B is correct since Avogadro's law states that under the same conditions of temperature and pressure, equal volumes of different…

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Correct answer: Changes

12. The following changes to the system at equilibrium have no effect on Kc except:

  • A. Increasing reactant concentrations.
  • B. Equal number of reactant and product gas molecules.
  • C. Increasing temperature.
  • D. Using a catalyst.

Explanation: Increasing the temperature will either increase or decrease the concentration of the reactants or products depending on whether the…

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Correct answer: Increasing temperature.

13. An exothermic reaction is allowed to reach equilibrium; if heat energy is then removed, the equilibrium will shift

  • A. Towards the product side
  • B. toward the reactant side.
  • C. Towards the middle.
  • D. None of these

Explanation: When heat is removed, the temperature of the system is lowered, and that favors the exothermic reaction, which, according to Le…

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Correct answer: Towards the product side

14. Le Chatelier's Principle is about?

  • A. Reaction Mixture.
  • B. Reactants.
  • C. Equilibrium Mixture.
  • D. Products.

Explanation: Le Chatelier's principle is a fundamental principle of chemical equilibrium that predicts the effect of a change in conditions on a system…

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Correct answer: Equilibrium Mixture.

15. What is Le Chatelier Principle about?

  • A. Reaction Mixture
  • B. Reactants
  • C. Equilibrium Mixture
  • D. Products

Explanation: Option C is correct since Le Chatelier's principle is an observation about the chemical equilibria of reactions.

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Correct answer: Equilibrium Mixture

16. With different number of moles of reactants and product, what happens to the volume of the system?

  • A. It remains unchanged
  • B. It changes
  • C. It decreases
  • D. It increases

Explanation: According to the ideal gas law, PV = nRT, the volume of a gas is directly proportional to the number of moles of gas present (n), assuming…

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Correct answer: It changes

17. Change in pressure will only affect the substance which are in:

  • A. Liquid state
  • B. Solid State
  • C. Plasma state
  • D. Gaseous state

Explanation: A change in pressure primarily affects substances in the gaseous state.

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Correct answer: Gaseous state

18. Consider the following reversible reaction:Initial concentrations:CH3-CH2-OH) = 1 mol.dm-3(CH3-COOH)= 1 mol.dm-3(CH3-CH2-O-CO-CH3)= 0 mol.dm-3(H2O)= 0 mol.dm-3Equilibrium concentrations:(CH3-CH2-OH) = 0.33mol.dm-3(CH3-COOH)= 0.33 mol.dm-3(CH3-CH2-O-CO-CH3)= 0.66 mol.dm-3(H2O)= 0.66 mol.dm-3Kc= 4 at temperature 100 CWhat are the new equilibrium concentrations of all species if 1 mol.dm-3 of CH3-CH2-OH and CH3-COOH are added to this equilibrium mixture? (Apply Le-Chatelier's principle)(Temperature and Kc remain constant)

  • A. (CH3-COOH)= 0.333 mol.dm-3(CH3-CH2-OH) = 1.333mol.dm-3(CH3-CH2-O-CO-CH3)= 1.666 mol.dm-3(H2O)= 1.666 mol.dm-3
  • B. (CH3-COOH)= 1.333 mol.dm-3(CH3-CH2-OH) = 0.333mol.dm-3(CH3-CH2-O-CO-CH3)= 0.666 mol.dm-3(H2O)= 0.666 mol.dm-3
  • C. (CH3-COOH)= 0.666 mol.dm-3(CH3-CH2-OH) = 0.666 mol.dm-3(CH3-CH2-O-CO-CH3)= 1.333 mol.dm-3(H2O)= 1.333 mol.dm-3
  • D. (CH3-COOH)= 0.333 mol.dm-3(CH3-CH2-OH) = 0.333mol.dm-3(CH3-CH2-O-CO-CH3)= 1.333 mol.dm-3(H2O)= 1.333 mol.dm-3

Explanation: Change in concentration of of CH3COOH is 1-0.33 = 0.66 mol.dm-3Change in concentration of of CH3CH2OH is also 1-0.33 = 0.66 mol.dm-3Where…

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Correct answer: (CH3-COOH)= 0.666 mol.dm-3(CH3-CH2-OH) = 0.666 mol.dm-3(CH3-CH2-O-CO-CH3)= 1.333 mol.dm-3(H2O)= 1.333 mol.dm-3

19. Consider the reaction 3H2 (g} + N2 (g) ➔ 2NH3 (g). All of the following will lead to this reaction to the production. of more NH3 except:

  • A. A decrease in the volume of the container .
  • B. An increase pressure by addition of hydrogen
  • C. Removal of NH3
  • D. An increase in pressure by addition.of nitrogen

Explanation: To do such questions, Your concepts about the Le Chatelier principle must be clear and you must know how to apply that.

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Correct answer: An increase in pressure by addition.of nitrogen

20. In the given Exothermic reaction PCl3 + Cl2 (g) ➔PCl5(g) an increase in yield will be favoured by:

  • A. Decrease of temperature
  • B. Increase of temperature
  • C. Keeping temperature constant
  • D. None of these

Explanation: Since the forward reaction is exothermic, decreasing the temperature will cause the reaction to shift in the forward direction by Le…

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Correct answer: Decrease of temperature
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