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Difficulty: MediumFactors Affecting Rates of Reaction

Match each chemical system modification on the left with the primary kinetic mechanism on the right that accounts for the observed increase in reaction rate.

  • Irradiating a gas mixture of methane and chlorine with ultraviolet radiationSupplies photon energy to cleave covalent bonds and generate reactive free-radical intermediates
  • Pulverizing calcium carbonate lumps into fine powder prior to reacting with hydrochloric acidMaximizes contact area between reactants, raising total collision frequency per unit time
  • Raising the temperature of a gaseous reaction mixture by 10 K10\text{ K}Elevates average particle kinetic energy, dramatically expanding the proportion of collisions exceeding the activation energy threshold (EaE_a)
  • Introducing a finely divided catalyst into a reversible gas-phase systemEstablishes a lower activation energy pathway, accelerating both forward and reverse reactions equally

Answer

Ultraviolet irradiation corresponds to supplying photon energy for bond cleavage and free-radical generation; pulverizing calcium carbonate corresponds to maximizing reactant contact area and collision frequency; raising temperature by 10 K corresponds to elevating kinetic energy to expand the fraction of collisions exceeding activation energy; introducing a catalyst corresponds to establishing a lower activation energy pathway for both forward and reverse processes.
Each factor influences the rate through a distinct physical or chemical mechanism: light supplies photochemical activation energy; surface area controls collision site availability; temperature dictates the population of molecules with E≥EaE \ge E_a; and catalysts lower the energy barrier for both reaction directions.

Step-by-Step Solution

1
Analyze the photochemical process (UV light application)
Light acts as an energy source for specific photochemical reactions, breaking bonds to yield radicals.
Light intensity and wavelength directly influence rates of photochemical reactions like halogenation of alkanes.
2
Analyze the effect of particle size (pulverization of solid)
Breaking solid chunks into powder exposes more surface atoms to acid molecules.
Greater exposed surface area increases the total frequency of particle collisions per second.
3
Analyze thermal energy and kinetic distribution
A temperature rise increases average kinetic energy and Maxwell-Boltzmann tail population.
The rate increases exponentially with temperature because a much larger percentage of collisions meet the activation energy threshold.
4
Analyze catalytic action in reversible systems
Catalysts lower activation energy (EaE_a) for both forward and reverse reactions without altering equilibrium position.
Catalysts alter the reaction mechanism to provide an alternative pathway with lower energy requirements.

Key Concept

Collision theory principles underlying reaction rate factors (light intensity, surface area, thermal kinetic energy distribution, and catalytic pathways)
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Factors Affecting Rates of Reaction