All Free Chemistry MCQs with Answers

131. 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.

132. 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.

133. 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.

134. 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.

135. On an energy profile diagram for an exothermic reaction

• A. the products lie at a lower energy level than the reactants
• B. the products lie above the reactants
• C. there is no activation energy barrier
• D. the activation energy is always zero

Explanation: An exothermic reaction releases energy to the surroundings, so the products sit lower and the enthalpy change is negative, but the curve still rises to a peak first because the activation barrier must be crossed. Confusing the barrier height with the overall enthalpy change is the classic error when reading these diagrams. An endothermic profile has the products higher than the reactants.

136. 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.

137. 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.

138. 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.

139. 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.

140. The rate determining step of a multi step reaction is

• A. the fastest step
• B. the slowest step, which controls the overall rate
• C. always the first step
• D. the step with the largest number of molecules

Explanation: An overall reaction can proceed no faster than its slowest stage, in the same way that a road is only as clear as its narrowest point, so the rate equation reflects the species involved up to and including that step. This is why the experimentally measured order often differs from the coefficients in the overall equation. The slow step is the one with the highest activation energy.