Free Activation Energy and Activated Complex MCQs with Answers

1. 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.

2. 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.

3. 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.

4. Amount of energy needed to weaken the existing bonds to an extent that can be broken through collision is called:

• A. Bond energy
• B. Activation energy
• C. Average energy
• D. Enthalpy

Explanation: Activation energy is the barrier reactant molecules must climb to reach the transition state, and only collisions carrying at least this much energy, with the right orientation, lead to reaction. Bond energy is the energy needed to break a particular bond completely in the gaseous state, a different quantity. A catalyst works by lowering the activation energy, not the bond energy.