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Difficulty: MediumRate of Reaction and Collision Theory

In an experiment investigating reaction kinetics, a strip of magnesium ribbon is reacted with excess dilute hydrochloric acid. If the ribbon is ground into a fine powder while keeping the total mass of magnesium, temperature, and acid concentration constant, which of the following best explains why the initial rate of reaction increases based on collision theory?

  1. A
    The activation energy of the reaction is lowered because smaller particles react faster.
  2. B
    The average kinetic energy of the reacting particles increases, causing more energetic collisions.
  3. The greater exposed surface area increases the frequency of collisions between reactant particles per unit time.Answer
  4. D
    A greater proportion of colliding particles possess energy equal to or greater than the activation energy.

Answer

The greater exposed surface area increases the frequency of collisions between reactant particles per unit time.
Increasing the surface area of a solid reactant by powdering it exposes a larger number of atoms to reactant ions in solution. This increases the total frequency of collisions occurring per unit time, resulting in a higher rate of effective collisions and thus a faster overall reaction rate.

Step-by-Step Solution

1
Analyze the physical change made to the solid reactant.
Grinding the magnesium ribbon into a powder increases its total surface area exposed to the aqueous hydrochloric acid solution.
Smaller particle size provides a greater number of exposed surface atoms per unit mass.
2
Apply collision theory principles to evaluate collision mechanics.
A higher number of exposed surface atoms increases the number of collisions per second (collision frequency) between magnesium atoms and hydrogen ions.
Collision theory states that reaction rate is directly proportional to collision frequency when kinetic energy and orientation parameters are constant.
3
Distinguish surface area effects from temperature and catalyst effects.
Neither activation energy nor particle kinetic energy changes, as temperature and reaction pathway remain identical.
Activation energy is lowered only by catalysts, and average kinetic energy is increased only by raising temperature.

Key Concept

Effect of Surface Area on Collision Frequency in Collision Theory
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Topics

Rate of Reaction and Collision Theory