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Difficulty: HardFactors Affecting Rates of Reaction

Match each modification of reaction conditions on the left with its corresponding microscopic mechanism according to collision theory on the right.

  • Increasing the pressure of a gaseous reaction mixture at constant temperatureIncreases particle concentration per unit volume, raising collision frequency without changing the kinetic energy distribution.
  • Adding a positive catalyst to the reaction systemLowers the activation energy threshold (EaE_a), thereby increasing the fraction of collisions that result in reaction without altering molecular energy.
  • Grinding a solid reactant into a fine powderIncreases the exposed surface area, leading to a greater number of collision contact sites per unit time.
  • Increasing the temperature of the reaction mixtureShifts the Maxwell-Boltzmann energy distribution curve toward higher kinetic energies, significantly increasing the fraction of collisions with E≥EaE \ge E_a.

Answer

Increasing pressure matches increasing particle concentration per unit volume without altering kinetic energy distribution; Adding a catalyst matches lowering activation energy to increase successful collisions; Grinding solid into powder matches increasing exposed surface area and contact sites; Increasing temperature matches shifting molecular energy distribution to increase the fraction of particles possessing E≥EaE \ge E_a.
Each factor alters reaction rate through a distinct collision theory mechanism: pressure increases gas concentration and collision frequency; catalysts lower the activation energy barrier; surface area increases exposed contact points; and temperature increases average kinetic energy and the fraction of energetic collisions exceeding activation energy.

Step-by-Step Solution

1
Analyze the microscopic effect of increasing pressure on gases.
Gaseous molecules are forced into a smaller volume, increasing concentration and therefore the total frequency of collisions, while temperature-dependent kinetic energy stays constant.
Pressure directly alters volumetric density of gas particles.
2
Analyze the action of a catalyst.
A catalyst lowers the activation energy (EaE_a) barrier by offering an alternative route, allowing a larger percentage of existing collisions to be successful.
Catalysts change reaction energetics without altering particle kinetic energy.
3
Determine the physical result of reducing solid particle size.
Finely dividing a solid exposes interior atoms/molecules to the surrounding fluid phase, increasing collision contact opportunities per second.
Heterogeneous rates depend on exposed interfacial area.
4
Evaluate the kinetic effect of a temperature rise.
Higher temperatures broaden and flatten the Maxwell-Boltzmann distribution curve, exponentially increasing the population of molecules with energy E≥EaE \ge E_a.
Temperature measures average molecular kinetic energy.

Key Concept

Collision Theory Mechanisms for Reaction Rate Factors
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Topics

Factors Affecting Rates of Reaction