Free Factors Affecting Rate of Reaction MCQs with Answers

1. Which of the following will NOT increase the rate of a reaction between a solid and a solution?

• A. Raising the temperature
• B. Powdering the solid
• C. Increasing the volume of solvent without adding more solute
• D. Adding a suitable catalyst

Explanation: Adding solvent alone dilutes the solution, so the concentration of the reacting species falls and collisions become less frequent, which slows the reaction rather than speeding it. Heating, increasing surface area and adding a catalyst all raise the frequency or the effectiveness of collisions. Note that a larger volume gives more total reactant but a lower concentration, and rate depends on concentration.

2. A rise of 10 degrees Celsius roughly doubles the rate of many reactions mainly because

• A. the activation energy of the reaction falls
• B. the concentration of the reactants increases
• C. the molecules become larger
• D. a much larger fraction of molecules acquire energy equal to or above the activation energy

Explanation: The Maxwell Boltzmann distribution shifts to the right on heating, and because the activation energy lies out in the tail of the curve, even a small temperature rise multiplies the number of molecules beyond it. The increase in collision frequency is real but accounts for only a few per cent of the effect. Activation energy itself is a property of the reaction and is not changed by heating.

3. A catalyst increases the rate of a reaction by

• A. increasing the kinetic energy of the reacting molecules
• B. raising the temperature of the reaction mixture
• C. providing an alternative pathway with a lower activation energy
• D. increasing the concentration of the reactants

Explanation: By offering a route with a smaller energy barrier, a catalyst allows a far larger proportion of collisions to be effective at the same temperature. It is recovered chemically unchanged at the end and it speeds the forward and reverse reactions equally, so the equilibrium position is untouched. A catalyst neither supplies energy nor alters concentration.

4. Powdering a solid reactant increases the rate of reaction because it increases the

• A. concentration of the solid
• B. temperature of the mixture
• C. activation energy
• D. surface area available for collisions

Explanation: A reaction between a solid and a fluid can only occur at the exposed surface, so grinding the solid exposes far more particles and the collision frequency rises in proportion. This is why a fine dust of flour or coal can explode while a solid lump merely burns. The concentration of a pure solid has no meaning, and neither the temperature nor the activation energy is affected.

5. Which statement about a catalyst is INCORRECT?

• A. It remains chemically unchanged at the end of the reaction
• B. It is needed only in a small amount
• C. It shifts the equilibrium towards the products
• D. It lowers the activation energy of both the forward and reverse reactions

Explanation: Because a catalyst lowers the barrier equally in both directions, the two rates rise by the same factor and the equilibrium position and the value of Kc are completely unaffected. What changes is how quickly equilibrium is reached. The other three statements are all true, and the small quantity needed follows from the catalyst being regenerated in each cycle.