Free Reversible Reactions MCQs with Answers

1. A chemical system is said to be in dynamic equilibrium when

• A. both reactions have stopped completely
• B. the forward and reverse reactions continue at equal rates
• C. the amounts of reactants and products are equal
• D. all the reactants have been converted to products

Explanation: The word dynamic means both reactions are still running; they simply cancel out, so the concentrations stay constant while molecules continue to convert in both directions. Equal concentrations of reactant and product are not required and are usually not the case, which is the standard misconception. Equilibrium can only be reached in a closed system.

2. For the reaction N2 + 3H2 gives 2NH3, the equilibrium constant expression Kc is

• A. [N2][H2]^3 divided by [NH3]^2
• B. [NH3] divided by [N2][H2]
• C. [NH3]^2 divided by [N2][H2]^3
• D. 2[NH3] divided by [N2] + 3[H2]

Explanation: Kc is written as the product concentrations over the reactant concentrations, each raised to the power of its coefficient in the balanced equation. Inverting the expression gives the equilibrium constant for the reverse reaction, which is why the first option is offered. Coefficients become exponents, never multipliers, which rules out the last option.

3. A very large value of the equilibrium constant Kc indicates that

• A. the reaction reaches equilibrium very quickly
• B. the reaction is exothermic
• C. the reactants are favoured at equilibrium
• D. the products are strongly favoured at equilibrium

Explanation: Kc compares product to reactant concentrations, so a large value means the equilibrium position lies far to the right and the reaction goes almost to completion. Kc says nothing at all about how fast equilibrium is reached, which is a matter of kinetics, and this is the confusion the question targets. The Haber process is the classic case of a favourable equilibrium that is nevertheless slow without a catalyst.

4. In a reversible reaction, the equilibrium constant Kc changes only when there is a change in

• A. concentration of the reactants
• B. pressure of the system
• C. temperature
• D. the catalyst used

Explanation: Concentration and pressure changes shift the position of equilibrium so that the same value of Kc is restored, and a catalyst affects only the rate, so none of them alters the constant. Temperature is different because it changes the relative rates of the forward and reverse reactions unequally, giving a genuinely new value of Kc. For an exothermic reaction Kc falls as the temperature rises.