Why does the rate of any reaction generally decreases during the course of the reaction?
The rate of a reaction depends on the concentration of the reactants. As the reaction proceeds in forward direction, concentration of reactant decreases and that of products increases. So, the rate of reaction generally decreases during the course of reaction.
Thermodynamic feasibility of the reaction alone cannot decide the rate of the reaction. Explain with the help of one example.
Thermodynamically the conversion or diamond to graphite is highly feasible but this reaction is very slow because its activation energy is high.
Hence, thermodynamic feasibility of the reaction alone cannot decide the rate of reaction.
Why in the redox titration of $\mathrm{KMnO}_4$ vs oxalic acid, we heat oxalic acid solution before starting the titration?
As we know with increase in temperature rate of reaction increases, Hence, we heat oxalic acid solution before starting of titration to increase the rate of decolourisation.
Why can't molecularity of any reaction be equal to zero?
Molecularity of the reaction is the number of molecules taking part in an elementary step. For this we require at least a single molecule leading to the value of minimum molecularity of one. Hence, molecularity of any reaction can never be equal to zero.
Why molecularity is applicable only for elementary reactions and order is applicable for elementary as well as complex reactions?
A complex reaction occurs through a number of steps i.e., elementary reactions. Number of molecules involved in each elementary reaction may be different, i.e., the molecularity of each step may be different. Therefore, it is meaningless to talk of molecularity of the overall complex reaction.
On the other hand, order of complex reaction depends upon the molecularity of the slowest step. Hence, it is not meaningless to talk of the order of a complex reaction.