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?

Short answer

(a) A successful chemical reaction between two colliding molecules depends on two factors: 1) having enough activation energy (Ea) to overcome the energy barrier and break necessary bonds in reactant molecules, and 2) correct orientation during the collision, which allows the reactive parts of the molecules to come into direct contact (steric factor). (b) According to the collision model, temperature affects the rate constant by increasing both the kinetic energy of molecules and the collision frequency. This results in more molecules having the required activation energy and higher chances of collisions with the correct orientation, leading to an increased rate constant.

Step-by-step solution

Factors determining a successful chemical reaction from molecular collision

To have a successful chemical reaction following a collision between two molecules, the following two conditions must be met: 1. The molecules must have enough energy, called the activation energy (Ea), to overcome the energy barrier and react. Activation energy is the minimum amount of energy needed to break the required bonds in the reactant molecules. 2. The molecules must be oriented correctly when they collide, such that the reactive parts of the molecules come into direct contact. This is called the steric factor.

Collision Model

The collision model is a theory which states that a chemical reaction can occur only when reactant molecules come into direct contact with sufficient energy and the correct orientation. This theory primarily depends on two factors: collision frequency (how often the molecules collide) and the fraction of collisions possessing the minimum activation energy (Ea) and correct orientation.

Temperature effect on the value of the rate constant

According to the collision model, increasing the temperature leads to an increase in the rate constant because temperature has two main effects on the reaction: 1. Increased temperature leads to an increase in the kinetic energy of the molecules. This means that more molecules will possess the required activation energy (Ea) to overcome the energy barrier and successfully react. The higher the temperature, the greater the fraction of molecules with enough energy to react. 2. Increased temperature also increases the collision frequency between molecules. When molecules have higher energy, they move faster and collide more frequently. Hence, the chances of having successful collisions with the correct orientation increase. These two factors combine to make the overall rate of the reaction much faster, increasing the value of the rate constant at high temperatures.