Free Le Chatelier's Principle MCQs with Answers

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 temperature. Adding a reactant therefore drives the reaction forward, and removing a product does the same. The principle predicts direction only; it says nothing about how far or how fast the shift occurs.

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 since the forward reaction releases heat a lower temperature also favours it. A catalyst speeds the approach to equilibrium without changing the yield at all, which is the standard trap. In practice a compromise temperature near 450 degrees Celsius is used because the reaction is too slow when cold.

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 side, so neither direction relieves the stress. The rate of reaching equilibrium does rise with pressure, but the position stays put. Temperature changes will still shift this equilibrium, since the two directions differ in enthalpy.

4. Adding a catalyst to a system at equilibrium

• 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 equal at exactly the same position. What changes is the time taken to get there, which is commercially vital in a process such as the Haber synthesis. Only temperature changes the value of Kc itself.

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 energy, which is Le Chatelier's principle applied to temperature. Because the position genuinely moves rather than being restored, the value of Kc increases as well. For an exothermic reaction the same reasoning gives the opposite result.

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

• 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 until the ratio required by Kc is restored. This is why one reactant is often used in excess industrially when it is the cheaper one. The value of Kc is unchanged, since only temperature can alter it.