Question

The rate of gas phase chemical reactions generally increases rapidly with rise in temperature. This is mainly because:
A) The collision frequency increases with temperature
B) The fraction of molecules having energy in excess of the activation energy increases with temperature
C) The activation energy decreases with temperature
D) The average kinetic energy of molecules increases with temperature

Answer 1

Hint:As we know that rate of reaction temperature of the system has a marked effect on the rate of a chemical reaction and generally the rate of a reaction increases with rise in temperature and we are also aware that all molecules of the reactant do not have the same amount of energy and they must possess a certain amount of minimum energy to change into product.

Complete answer
As we know that during a molecular collision, molecules must possess a minimum amount of energy for effective collision to occur. All molecules of the reactant do not have the same amount of energy and they must possess a certain amount of minimum energy to change into a product. That energy is commonly known as the threshold energy.

And we also know that the activation energy is the minimum energy which should be supplied to an inactive molecule in order to make it an active molecule. However, most of the molecules of the reactant have much less kinetic energy than the threshold energy at room temperature.

So when the temperature is increased, the average kinetic energy of reactant molecules increases and becomes equal to or greater than the threshold energy. Therefore, we can say that the rate of gas phase chemical reaction generally increases with rise in temperature because the fraction of molecules having energy in excess of the activation energy increases with temperature.

Therefore the correct answer is option (B).

Note:At room temperature the rate of reaction almost doubles for every ${10^\circ }C$increase in temperature and in some cases triples the rate of a reaction. So, if a substance is heated, the molecules move faster and collide more frequently increasing the activation energy as well as the rate of a reaction.