Quick questions on Collision theory and reaction rates explained: HSC Chemistry Module 5

Collision theory states that a chemical reaction between two particles can only occur if their collision satisfies all three conditions:

Increasing the concentration of a solution, or the pressure of a gas mixture, packs more particles into the same volume. This increases the collision frequency because particles are closer together and encounter each other more often. The fraction of collisions that are successful is unchanged, since the average kinetic energy of the particles has not changed. The rate increase comes entirely from more collisions happening per second.

Increasing temperature increases the average kinetic energy of the particles. This raises the rate through two compounding effects, not just one:

For a reaction involving a solid, breaking the solid into smaller pieces (for example, crushing a lump into powder) increases its total exposed surface area without changing its mass, its concentration, or the concentration of the other reactant. Only particles at an exposed surface can collide with the other reactant, so a larger surface area increases the collision frequency between the solid and the other reactant. As with concentration, the fraction of successful collisions is unchanged, only how often a collision can happen at all.

A catalyst provides an alternative reaction pathway with a lower activation energy, $E_a$. At the same temperature, this means a larger fraction of collisions now have enough energy to react, since fewer particles need to clear a lower energy barrier. A catalyst:

Using collision theory, explain why crushing a solid reactant into a powder increases reaction rate but does not increase the total mass of product formed. [3 marks]

Explain, with reference to the Maxwell-Boltzmann distribution, why a 10 degree C rise in temperature typically has a much larger effect on reaction rate than a 10 percent increase in concentration. [4 marks]

A catalyst is added to a reaction that has already reached equilibrium. State and justify the effect, if any, on (a) the forward and reverse reaction rates, (b) the position of equilibrium, (c) the value of $K_{eq}$. [1+1+1 marks]