Question

(a) What factors determine whether a collision between two molecules will lead to a chemical reaction? (b) According to the collision model, why does temperature affect the value of the rate constant? (c) Does the rate constant for a reaction generally increase or decrease with an increase in reaction temperature?

Short answer

(a) A successful collision between two molecules leading to a chemical reaction depends on 1) the energy of the collision being equal to or greater than the activation energy (Ea), and 2) proper orientation of the molecules during the collision. (b) Temperature affects the rate constant in the collision model as it increases the average kinetic energy of molecules, resulting in a higher fraction of molecules with enough energy to overcome Ea, and causing them to move faster, leading to a higher frequency of collisions. (c) The rate constant for a reaction generally increases with an increase in reaction temperature due to increased kinetic energy of the molecules and a higher rate of successful collisions.

Step-by-step solution

(a) Factors determining a successful collision)

To have a successful collision, two factors must be met: (1) the energy of the collision should be equal to or greater than the activation energy (Ea) of the reaction and (2) the molecules should have proper orientation when they collide. 1. Activation Energy (Ea): The minimum energy required for a reaction to occur. If the colliding molecules have kinetic energies equal to or greater than Ea, their collisions can overcome the energy barrier, break the reactant's bonds and form new bonds. 2. Orientation: Even if the molecules have enough energy, they must also collide with the correct orientation that allows the necessary bonds to break and form new ones. If the molecules don't have the proper orientation, they will not react, and no products will be formed.

(b) Collision model and the effect of temperature on the rate constant)

According to the collision model, the rate constant of a reaction is directly related to the frequency of successful collisions between the reactant molecules. Temperature plays a significant role in determining the rate constant for the following reasons: 1. As temperature increases, the average kinetic energy of the molecules increases. This results in an increased fraction of molecules having enough energy to overcome the activation energy barrier and thereby react. 2. Increased temperature also causes the molecules to move faster. This increased speed results in a higher frequency of collisions between the molecules. Both these factors contribute to an increase in the rate of successful collisions and, therefore, the rate constant with an increase in temperature.

(c) Increase or decrease of the rate constant with an increase in reaction temperature)

In general, the rate constant for a reaction increases with an increase in reaction temperature. This is because, as the temperature increases, the kinetic energy of the molecules increases, leading to more collisions between the reactant molecules. Moreover, a higher fraction of the molecules have enough energy to overcome the activation energy barrier, leading to a higher rate of successful collisions and an increased rate constant for the reaction.