Problem 51 - Chemistry: The Central Science

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?

Step-by-Step Solution

Answer

(a) The two main factors determining whether a collision between two molecules will lead to a chemical reaction are: 1. Energy - the molecules must have enough energy to overcome the activation energy barrier, and 2. Orientation - the molecules must be aligned properly for the reaction to occur. (b) According to the collision model, the rate constant increases with the frequency of successful collisions. As temperature increases, the kinetic energy of the particles also increases, leading to more frequent successful collisions and a higher rate constant. (c) The rate constant for a reaction generally increases with an increase in reaction temperature, as molecules have more kinetic energy and are more likely to have enough energy to overcome the activation energy barrier, leading to increased successful collisions.

Step 1: (a) Factors that determine a chemical reaction

In a collision between two molecules, there are two main factors that determine whether they will react chemically or not: 1. Energy: The molecules must have enough energy to overcome the activation energy barrier for the reaction. Molecules with less energy than the activation energy barrier will not react. 2. Orientation: The molecules must come together in the correct orientation to allow the reaction to occur. If the molecules are not aligned properly, they may collide, but they won't react chemically.

Step 2: (b) Collision model and rate constant

According to the collision model, the rate of a reaction is directly proportional to the number of successful (reactive) collisions between the reactant molecules per unit time. In other words, the rate constant increases with the frequency of successful collisions. Temperature is the measure of the average kinetic energy of the molecules. When temperature is increased, the kinetic energy of the particles also increases. This causes the molecules to move faster, which increases their chances of collisions. As a result, the number of successful collisions per unit time is likely to increase, leading to an increase in the value of the rate constant.

Step 3: (c) Effect of temperature on rate constant

The rate constant for a reaction generally increases with an increase in reaction temperature. With a higher temperature, molecules have more kinetic energy and are more likely to have enough energy to overcome the activation energy barrier. This leads to an increased number of successful collisions between the reactant molecules, which in turn results in a higher rate constant. Conversely, if the reaction temperature decreases, the rate constant will generally decrease as well.