Question 1

What is a consecutive reaction and how does it differ from a single-step reaction?

Accepted Answer

A consecutive reaction is a sequence where the product of one reaction becomes the reactant for the next. In the scheme A → B → C, reactant A transforms into product B, which then forms C. Unlike single-step reactions, consecutive reactions involve multiple rate constants (k1 and k2) and produce intermediate products that accumulate or deplete depending on relative reaction rates.

Question 2

How does the rate of change of intermediate B depend on the overall reaction sequence?

Accepted Answer

The rate of change for B is influenced by two opposing effects: an increase from A's transformation into B and a decrease from B's conversion to C. The net rate depends on both rate constants k1 and k2. When k1 is much larger than k2, B accumulates initially before slowly converting to C.

Question 3

What role does the conservation of matter play in consecutive reactions?

Accepted Answer

Conservation of matter states that the sum of concentrations of A, B, and C at any time equals the initial concentration [A]0. This principle generates three coupled differential equations that determine the concentrations of all species over time, ensuring mass is neither created nor destroyed throughout the reaction sequence.

Question 4

How do rate constants k1 and k2 affect the accumulation of intermediate B?

Accepted Answer

When k1 is much greater than k2 (k1 ≫ k2), the second reaction is slower, causing an initial buildup of B that gradually transforms into C. Conversely, when k1 is much less than k2 (k1 ≪ k2), B rapidly converts to C with minimal accumulation. This behavior reflects kinetic control governed by the relative magnitudes of the rate constants.

Question 5

Why are the mathematical expressions for B and C concentrations more complex than for A?

Accepted Answer

The expressions for B and C depend on differences between rate constants k1 and k2, making them more complex than A's first-order decay. B's concentration reflects both formation from A and loss to C, while C's concentration depends on the accumulated product from B. These coupled dependencies require integration of differential equations.

Question 6

How is the concentration of A determined in a consecutive reaction?

Accepted Answer

The concentration of A at time t is determined by integrating the first-order rate law with initial concentration [A]0. Since A only decays into B and does not form from any other species, its concentration follows simple exponential decay: [A]t = [A]0 exp(-k1t), independent of the second reaction rate.

Question 7

What is an example of a consecutive reaction in nature?

Accepted Answer

A radioactive decay series exemplifies consecutive reactions, where an unstable nucleus decays into an intermediate product, which then decays into a final stable product. Each decay step follows first-order kinetics with distinct rate constants, demonstrating how intermediate products accumulate and deplete based on the relative decay rates.

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