USA Biology Olympiad (USABO) Practice Exam

What does the half-life of a reaction indicate?

1. The time it takes for the concentration to double.

2. The time it takes for the concentration to decrease by half.

3. The total duration of a reaction.

4. The average lifetime of a molecule in a reaction.

The correct answer is: The time it takes for the concentration to decrease by half.

The half-life of a reaction is defined as the amount of time required for the concentration of a reactant to reduce to half of its initial value. This concept is particularly important in kinetics, as it provides insight into the speed or rate at which a reaction occurs. It is relevant for both first-order and second-order reactions, although the specific mathematical expression may differ between the two. Understanding half-life is critical in various fields such as pharmacology, where it helps in determining how quickly a drug will be eliminated from the body, or in radiochemistry, where it indicates the decay rate of radioactive substances. The focus is specifically on the change in concentration over a specified period, making it a useful metric for understanding reaction dynamics. The other choices do not accurately capture the essence of half-life: the time it takes for the concentration to double pertains to a different concept known as doubling time, the total duration of a reaction does not specify a change in concentration, and the average lifetime of a molecule does not clearly indicate the specific focus of half-life on concentration reduction.