Reactions occurring at high rate at a temperature of 25 °C are characterized by:

• A. Low activating energy
• B. High activating energy
• C. High heat of reaction
• D. Small heat of reaction

Answer: (A)

Reaction rate at a given temperature is governed by the activation energy—the energy barrier that must be overcome for reactants to form products. If this barrier is low, more molecules have enough energy to reach the transition state at 25 °C, so the reaction proceeds quickly. That’s why a high-rate reaction at room temperature is associated with low activation energy. The heat of reaction tells you how much energy is released or absorbed during the process, not how fast it goes; a reaction can be very exothermic or endothermic yet slow if the activation energy is large. Similarly, a small or large heat of reaction doesn’t determine the rate.