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MCAT Le Chatelier's Principle and Equilibrium Constant Practice Questions

By StudyStore · Published 2026-11-04 · 8 original questions · about 12 min

Eight free MCAT-style chemistry practice questions on Le Chatelier's principle, K and Q, writing equilibrium expressions, temperature, pressure and catalyst effects, with explanations.

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Quick concept primer

At equilibrium forward and reverse rates are equal and concentrations are constant. For aA + bB <-> cC + dD, K = [C]^c [D]^d / ([A]^a [B]^b). Pure solids and pure liquids are left out of K. The reaction quotient Q has the same form using current concentrations: if Q < K the reaction shifts toward products, if Q > K it shifts toward reactants, and if Q = K it is at equilibrium.

Le Chatelier's principle says a system disturbed from equilibrium shifts to partly oppose the change. Adding reactant shifts toward products; removing product does the same. Raising pressure favors the side with fewer moles of gas. Raising temperature favors the endothermic direction (and changes K). A catalyst speeds both directions and does not shift the equilibrium or change K. Adding an inert gas at constant volume does not change partial pressures, so no shift occurs.

8 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

For N2(g) + 3 H2(g) <-> 2 NH3(g), what is the effect of increasing the pressure by decreasing the container volume?

  1. The equilibrium shifts toward reactants
  2. The equilibrium shifts toward products
  3. No shift occurs
  4. K increases
Show answer and explanation

Answer: B. The equilibrium shifts toward products

The reactant side has 4 moles of gas and the product side has 2. Higher pressure favors the side with fewer gas moles, so the system shifts toward ammonia. Pressure changes alter the position of equilibrium but not the value of K.

Question 2

The forward reaction in N2 + 3 H2 <-> 2 NH3 is exothermic. What is the effect of raising the temperature?

  1. The equilibrium shifts toward products and K increases
  2. The equilibrium shifts toward products and K decreases
  3. The equilibrium shifts toward reactants and K decreases
  4. No shift occurs
Show answer and explanation

Answer: C. The equilibrium shifts toward reactants and K decreases

Treat heat as a product of an exothermic reaction. Adding heat shifts the system toward reactants to absorb it, and K for the forward reaction decreases. Temperature is the only common disturbance that changes the value of K.

Question 3

A catalyst is added to a system at equilibrium. Which statement is correct?

  1. The forward and reverse rates both increase and equilibrium is reached sooner
  2. K increases
  3. The equilibrium shifts toward products
  4. The reaction quotient Q is halved
Show answer and explanation

Answer: A. The forward and reverse rates both increase and equilibrium is reached sooner

A catalyst lowers the activation energy of both directions equally, so it speeds up the approach to equilibrium but does not change the position of equilibrium or the equilibrium constant. Thermodynamic quantities such as delta G are unaffected.

Question 4

For 2 NO2(g) <-> N2O4(g), which expression gives the equilibrium constant?

  1. K = [NO2]^2 / [N2O4]
  2. K = [N2O4] / [NO2]^2
  3. K = [N2O4] / [NO2]
  4. K = [N2O4] x [NO2]^2
Show answer and explanation

Answer: B. K = [N2O4] / [NO2]^2

Products go in the numerator and reactants in the denominator, with each concentration raised to its stoichiometric coefficient. The coefficient 2 for NO2 becomes the exponent. Choice A is the reciprocal, which corresponds to the reverse reaction.

Question 5

For CaCO3(s) <-> CaO(s) + CO2(g), what is the correct form of K?

  1. K = [CaO][CO2] / [CaCO3]
  2. K = [CaCO3] / [CO2]
  3. K = [CaO] / [CaCO3]
  4. K = [CO2]
Show answer and explanation

Answer: D. K = [CO2]

Pure solids have constant activity and are not included in K. Only the gaseous product appears, so K = [CO2] (or P_CO2 in terms of pressure). At a fixed temperature, adding more solid does not change the CO2 level.

Question 6

For a reaction with K = 50, the current mixture has Q = 20. Which statement is correct?

  1. The system is at equilibrium
  2. The reaction proceeds in the reverse direction
  3. The reaction proceeds in the forward direction
  4. K will change to 20
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Answer: C. The reaction proceeds in the forward direction

Because Q is less than K, there are too many reactants and too few products relative to equilibrium, so the net reaction proceeds forward until Q rises to 50. K itself remains constant at a given temperature.

Question 7

Helium gas is added at constant volume to an equilibrium mixture of gases. What happens?

  1. The reaction moves toward whichever side has the larger number of gas moles
  2. Nothing shifts, since each reactant and product keeps its original partial pressure
  3. The reaction moves toward whichever side has the smaller number of gas moles
  4. K decreases
Show answer and explanation

Answer: B. Nothing shifts, since each reactant and product keeps its original partial pressure

Helium is inert here. In a rigid container it raises the total pressure, yet the amount of each species per liter stays fixed, so every concentration and partial pressure in Q is unchanged and Q remains equal to K. Only if the container expanded would those partial pressures drop and push the system toward whichever side holds more gas particles.

Question 8

For a reaction at 25 degrees C, K is much greater than 1. What does this indicate about the standard Gibbs free energy change?

  1. Delta G degrees is positive
  2. Delta G degrees is zero
  3. Delta G degrees is negative
  4. It cannot be related to K
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Answer: C. Delta G degrees is negative

Delta G degrees = -RT ln K, so a large K makes ln K positive and delta G degrees negative. Products are favored at equilibrium. A K below 1 gives a positive delta G degrees, and K = 1 gives zero.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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