Free Rate Constant MCQs with Answers

1. The units of the rate constant k for a first order reaction are

• A. mol dm-3 s-1
• B. s-1
• C. mol-1 dm3 s-1
• D. mol-2 dm6 s-1

Explanation: For a first order reaction rate equals k times concentration, so k must have units of rate divided by concentration, which reduces to reciprocal seconds. The units of k therefore change with the order, being mol dm-3 s-1 for zero order and mol-1 dm3 s-1 for second order. Checking the units is a quick way to confirm that an order has been deduced correctly.

2. The value of the rate constant k for a given reaction changes when

• A. the concentration of the reactants is increased
• B. the volume of the vessel is changed
• C. the temperature is changed
• D. more product is removed

Explanation: The rate constant is constant only at a fixed temperature; the Arrhenius equation shows it rising sharply as temperature increases or as activation energy falls, which is also why a catalyst increases k. Changing concentration changes the rate but leaves k untouched, since the concentration term is already written separately in the rate equation. This distinction between rate and rate constant is examined frequently.

3. The half life of a first order reaction

• A. doubles as the reaction proceeds
• B. depends on the initial concentration
• C. is constant and independent of the initial concentration
• D. is always one second

Explanation: For first order kinetics the time taken for the concentration to fall by half is fixed, so successive halvings take equal times, and this is precisely why radioactive decay is described by a half life. The relationship is half life equals 0.693 divided by k. For zero and second order reactions the half life does depend on the starting concentration.

4. The Arrhenius equation shows that the rate constant increases when

• A. the activation energy increases
• B. the temperature falls
• C. the activation energy decreases or the temperature rises
• D. the concentration of reactants rises

Explanation: The equation contains an exponential term in which activation energy appears with a negative sign and temperature in the denominator, so lowering the barrier or raising the temperature both increase k, and the dependence is exponential rather than linear. This is why a catalyst and a temperature rise produce such large changes in rate. Concentration does not appear in the equation at all.

5. Final equation for the representation of rate of reaction in term of concentration is called

• A. Rate law
• B. Rate constant
• C. Reaction rate
• D. Reaction order

Explanation: The rate law, or rate equation, states the rate as the rate constant multiplied by concentration terms raised to their experimentally determined powers. The rate constant is only the proportionality factor within it and the order is the exponent, so both are components of the rate law rather than the equation itself. Orders must be found by experiment, since they need not match the coefficients in the balanced equation.