S.T.A.R.T: Chain Reaction Mechanism

KNOWLEDGE CHECK: SHOW WHAT YOU KNOW!

Which of the following best describes a radical?

• A) A stable molecule with no unpaired electrons

• B) A highly reactive species with unpaired electrons

• C) A product of a complete reaction

• D) A catalyst that speeds up reactions

Answer explanation

Explanation:
Radicals are characterized by their unpaired electrons, which make them highly reactive. Understanding the nature of radicals is essential for studying chain reaction mechanisms

KNOWLEDGE CHECK: SHOW WHAT YOU KNOW!

Which of the following statements best describes the steady-state approximation in the context of chemical reaction mechanisms?

• A) The concentration of reactants decreases continuously until they are fully consumed.

• B) The concentration of products increases steadily over time.

• C) The concentration of reactive intermediates remains relatively constant during the reaction.

• D) The reaction reaches equilibrium, and the rates of forward and reverse reactions are equal.

Answer explanation

Explanation:
The steady-state approximation is a simplification used in the analysis of complex reaction mechanisms, particularly where intermediates are formed and consumed rapidly. It assumes that, after an initial build-up phase, the concentration of reactive intermediates (such as radicals) remains approximately constant throughout the reaction. This occurs because the rate of formation of the intermediates is roughly equal to the rate at which they are consumed, allowing for easier mathematical modeling of the reaction kinetics.

• A) is incorrect because the focus of the steady-state approximation is on intermediates, not on the continuous decrease of reactants.

• B) is incorrect because while products do increase, the steady-state approximation is concerned with intermediates.

• D) is incorrect because equilibrium involves the rates of forward and reverse reactions being equal, which is a different concept from the steady-state approximation.

KNOWLEDGE CHECK: SHOW WHAT YOU KNOW!

What is a chain reaction?

• A) A reaction that occurs in a closed system with constant temperature

• B) A reaction in which the products of one step initiate further reactions

• C) A reaction that always results in equilibrium

• D) A reaction that requires a catalyst to proceed

Answer explanation

Explanation:
A chain reaction involves a series of steps where intermediates (often radicals) generated in one step initiate further steps. This leads to a self-sustaining process. It's characteristic of radical-driven reactions like combustion or polymerization.

BRAIN TEASERS: WHAT'S YOUR BEST GUESS?

Why do continuous reactions occur in a chain reaction mechanism?

• A) Because the reactants are always in excess

• B) Because reactive intermediates (radicals) are continuously generated and consumed

• C) Because the reaction reaches equilibrium quickly

• D) Because the reaction can only occur at high temperatures

Answer explanation

Explanation:
In a chain reaction mechanism, the reaction proceeds continuously due to the rapid formation and consumption of reactive intermediates, such as radicals. These intermediates are produced in the initiation step and then react with stable molecules during the propagation steps to generate new radicals, allowing the reaction to sustain itself. This cycle of generation and consumption is what characterizes a continuous reaction in chain mechanisms.

• A) is incorrect because while having excess reactants can help sustain a reaction, it is not the primary reason for continuity in chain reactions.

• C) is incorrect because reaching equilibrium is not characteristic of chain reactions; they often proceed without reaching equilibrium due to the continuous production of radicals.

• D) is incorrect because chain reactions can occur at various temperatures, and their continuity is not solely dependent on high temperatures.

BRAIN TEASERS: WHAT'S YOUR BEST GUESS?

Which of the following reactions are examples of chain reaction mechanisms? (Select all that apply)

• A) Combustion of hydrogen gas in oxygen

• B) Polymerization of alkenes to form plastics

• C) Fermentation of sugars to produce ethanol

• D) Decomposition of hydrogen peroxide into water and oxygen

Answer explanation

Explanation:

• A) Combustion of hydrogen gas in oxygen:
  This reaction is a classic example of a chain reaction. The combustion process involves the initiation of hydrogen and oxygen, producing radicals that continue to react and generate more radicals, sustaining the reaction until all reactants are consumed.

• B) Polymerization of alkenes to form plastics:
  Many polymerization processes (like those used to create plastics) can be chain reactions. These involve the initiation of alkene monomers through radicals, which then propagate to form long-chain polymers, continuously generating new radicals as the reaction progresses.

• C) Fermentation of sugars to produce ethanol:
  This reaction is not a chain reaction; it is a biochemical process that involves a series of enzymatic reactions, not the radical mechanism characteristic of chain reactions.

• D) Decomposition of hydrogen peroxide into water and oxygen:
  This reaction is also not a chain reaction. It typically proceeds through a single step or series of steps that do not involve the generation of radicals in a manner consistent with chain mechanisms.