Free Reaction Kinetics MCQs with Answers

1. The rate of a chemical reaction is defined as the change in

• A. concentration of a reactant or product per unit time
• B. temperature of the mixture per unit time
• C. mass of the catalyst per unit time
• D. total pressure of the system

Explanation: Rate measures how quickly reactants are consumed or products formed, so its units are usually moles per cubic decimetre per second. It is always positive, which is why a minus sign is written in front of the reactant term. Rate is greatest at the start, when reactant concentration is highest, and falls as the reaction proceeds.

2. 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.

3. According to collision theory, a reaction occurs only when colliding molecules

• A. are of the same size
• B. collide with energy equal to or greater than the activation energy and with the correct orientation
• C. collide at low temperature
• D. collide exactly head on, regardless of energy

Explanation: Most collisions are fruitless because the molecules either rebound with too little energy to break existing bonds or meet at the wrong angle for new bonds to form. Only collisions that satisfy both conditions are effective, and the fraction that do so is what determines the rate. Raising the temperature increases both the collision frequency and, far more importantly, the fraction with sufficient energy.

4. 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.

5. 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.

6. The order of a reaction with respect to a particular reactant

• A. is always equal to its coefficient in the balanced equation
• B. is the power to which its concentration is raised in the experimentally determined rate equation
• C. can never be zero
• D. is always a whole number

Explanation: Order is an experimental quantity and must be measured, because the balanced equation describes the overall stoichiometry while the rate depends only on the slow, rate determining step. This is why orders can be zero, fractional or even negative, none of which a coefficient can be. Only for a genuinely single step reaction do order and coefficient happen to agree.

7. For a reaction with the rate equation rate = k[A][B]^2, the overall order of the reaction is

• A. 1
• B. 2
• C. 3
• D. 4

Explanation: Overall order is the sum of the individual orders, so 1 for A plus 2 for B gives 3. The reaction is first order in A and second order in B, and doubling the concentration of B alone would therefore quadruple the rate while doubling A alone would only double it. Adding rather than multiplying the exponents is the step candidates most often get wrong.

8. In a zero order reaction, the rate

• A. doubles when the concentration doubles
• B. is independent of the concentration of the reactant
• C. falls to zero immediately
• D. is proportional to the square of the concentration

Explanation: A zero order reaction proceeds at a constant rate until the reactant is nearly exhausted, because something other than concentration limits it, typically a saturated catalyst surface or a fixed light intensity. The graph of concentration against time is therefore a straight line with a constant slope. Enzyme catalysed reactions become zero order once every active site is occupied.

9. Activation energy is the minimum energy required to

• A. melt the reactants
• B. break all the bonds in the products
• C. form the activated complex from the reactants
• D. keep the products stable

Explanation: Reactant molecules must climb an energy barrier to reach the transient, unstable arrangement called the activated complex or transition state, from which they can fall through to products. The height of that barrier controls the rate, while the difference in energy between reactants and products controls the enthalpy change. The two quantities are independent, which is why a very exothermic reaction can still be extremely slow.

10. The activated complex in a reaction is

• A. a stable intermediate that can be isolated
• B. the same as the catalyst
• C. the product of the reaction
• D. a short lived arrangement at the top of the energy barrier in which bonds are partly broken and partly formed

Explanation: At the transition state old bonds are stretched and new ones are only beginning to form, so the species exists for the duration of a molecular vibration and cannot be isolated. A genuine reaction intermediate, by contrast, sits in an energy well and may sometimes be detected. The activated complex may collapse either forwards to products or backwards to reactants.