Problem 96 - Chemistry: The Central Science

Question

Consider two reactions. Reaction (1) has a constant half-life, whereas reaction (2) has a half-life that gets longer as the reaction proceeds. What can you conclude about the rate laws of these reactions from these observations?

Step-by-Step Solution

Answer

In summary, Reaction 1 has a constant half-life, indicating a zero-order rate law with the formula Rate = k. On the other hand, Reaction 2's half-life increases as the reaction proceeds, suggesting a first-order rate law with a possible formula Rate = k [A]. However, the rate law for Reaction 2 may also depend on other factors and reactants.

Step 1: Understanding reaction rates and rate laws

Reaction rates describe how fast the concentration of reactants change as a reaction proceeds, while rate laws represent the relationship between the reaction rate and the concentrations of reactants. Rate laws typically have the form: Rate = k [A]^m [B]^n where k is the rate constant, [A] and [B] are the concentrations of reactants A and B, and m and n are their respective reaction orders.

Step 2: Considering Reaction 1's constant half-life

For Reaction 1, the half-life is constant, which means the rate of Reaction 1 does not depend on the concentration of the reactants. Hence, the rate is directly proportional to the concentration to the power of 0. This gives us the following rate law for Reaction 1: Rate = k Here, k is the rate constant.

Step 3: Considering Reaction 2's increasing half-life

For Reaction 2, the half-life increases as the reaction proceeds, meaning the reaction slows down as it progresses. This indicates that the reaction rate is dependent on the concentration of reactants. Since the half-life increases, this is suggestive of a first-order reaction, where the rate is directly proportional to the concentration of a reactant to the power of 1. Given this information, an example rate law for Reaction 2 could be: Rate = k [A] where k is the rate constant and [A] is the concentration of reactant A. Please note that it is just an example of a possible rate law based on the given information, and it may also be dependent on other factors and reactants.

Step 4: Concluding the rate laws of the reactions

In summary, Reaction 1 has a constant half-life, indicating that it has a zero-order rate law, which can be written as: Rate = k On the other hand, Reaction 2 has a half-life that increases as the reaction proceeds, suggesting it could have a first-order rate law, which can be written as: Rate = k [A] However, for Reaction 2, the rate law provided is an example based on the given information, and it may also be dependent on other factors and reactants.