Question

Activation energy of a chemical reaction can be determined by:
A. evaluating rate constants at two different temperature
B. evaluating velocities of reaction at standard temperature
C. evaluating rate constant at standard temperature
D. changing concentration of reactants

Answer 1

In physical chemistry, activation energy is often studied and a very important concept related to chemical kinetics. To go forward, nearly all chemical reactions require some activation energy.Heat is typically the source of the activation energy. A chemical reaction 's energy of activation is closely related to its rate.
Formula used: The Arrhenius equations relates the rate of a chemical reaction to the magnitude of the activation energy:
$\;k = {\text{ }}A{e^{Ea/RT}}$
Or, $ln({k_1}/{k_2}) = - Ea/R(1/{T_1}-1/{T_2})$
Where,
$k$ is the rate constant of the reaction
$A$ is the pre-exponential factor which is the frequency of correctly oriented collisions between the reacting species, in terms of the collision theory.
$e$ is the base of the natural logarithm (Euler’s number)
$Ea$ denotes the activation energy of the chemical reaction (in terms of energy per mole)
$R$ denotes the universal gas constant
$T$ denotes the absolute temperature associated with the reaction (in Kelvin)

Complete step by step solution:
As we can see from the equation given by Arrhenius that activation energy depends upon temperature and in order to find out the rate one needs to examine and study experimentally the results obtained at two different temperatures. This is because the rate constant and also the activation energy of a reaction increases with increase in temperature. Let me explain you how:
Consider two billiard balls, when they collide they simply just bounce off of each other the same might happen when molecules, say A and B, come together. They may bounce off without undergoing any change at all. In order for that collision to convert into a meaningful reaction chemically, the collision should happen with sufficient energy that will help in breaking the chemical bonds of A and B molecules so that when these bonds will break, other new bonds will be formed. Thus for effective collision we need that the molecules move with a certain minimum
We know from kinetic theory of gases that kinetic energy depends directly upon the temperature of the gas. As the temperature is increased the molecules get more energy and move even faster and collision increases. Thus by calculating rate constants at two different temperatures will give accurate results since if we will calculate at a standard temperature the rate might change at other temperatures leading to anomalies in the results.
Hence the correct answer is Option A.

Note:
Many reactions have such high energies of activation that they simply do not proceed at all without an energy supply. Activation energy required for proceeding a chemical reaction can be decreased by adding a catalyst to that reaction which leads to more molecules colliding with enough energy to surmount the smaller energy barrier.