Hi
Before the reactants can be converted into products, the free energy of the system must overcome the activation energy for the reaction, as shown in the figure below. The vertical axis in this diagram represents the free energy of a pair of molecules as a chlorine atom is transferred from one to the other. The horizontal axis represents the the sequence of infinitesimally small changes that must occur to convert the reactants into the products of this reaction.

To understand why reactions have an activation energy, consider what has to happen in order for ClNO2 to react with NO. First, and foremost, these two molecules have to collide, thereby organizing the system. Not only do they have to be brought together, they have to be held in exactly the right orientation relative to each other to ensure that reaction can occur. Both of these factors raise the free energy of the system by lowering the entropy. Some energy also must be invested to begin breaking the Cl-NO2 bond so that the Cl-NO bond can form.
NO and ClNO2 molecules that collide in the correct orientation, with enough kinetic energy to climb the activation energy barrier, can react to form NO2 and ClNO. As the temperature of the system increases, the number of molecules that carry enough energy to react when they collide also increases. The rate of reaction therefore increases with temperature. As a rule, the rate of a reaction doubles for every 10oC increase in the temperature of the system.
 
This explains why backward reaction wud be favored if activation energy of backward rxn is lower as the probability of the products colliding and forming back the reactants wud be more , as they can overcome the threshold activation energy easily.