What is the difference between thermodynamic stability and the kinetic stability?
                       The thermodynamic stability is linked with the free energy change (∆G) where as kinetic stability is linked with energy of activation. A chemical reaction accompanied by the decrease in energy are spontaneous and thus are thermodynamically unstable. But the reactions which have high activation energy are kinetically stable because the collisions in the reactive species will be effective in case they reach the energy barrier. Consider the reaction
H2(g)+1/2O2(g)žH2O(g)   ∆H=-ve i.e. exothermic
This reaction is thermodynamically unstable as free energy change is negative. Still it does not take place at room temperature as it is kinetically stable because the reaction molecule does not get sufficient energy to collide affectively.
When is the rate of reaction equal to specific reaction rate?
                      Rate is equal to k in case of zero order reactions or when the conc. of each reactant molecule is equal to unity.
Why are ionic reactions instantaneous?
                      Oppositely charged ions exert strong attractive forces among each other and thus combine instantaneously.
What is the main difference between a catalyst and a photosensitiser?
                      A catalyst increases the rate of a reaction but does not initiate a reaction. However, a photosensitiser only initiates a reaction.
Can a catalyst alter the pathway of a reaction?
                       Yes, a catalyst provides an alternative pathway having lower activation energy.
Comment on the statement that reactions with low activation energy are fast as compared to reactions with higher activation energy.
                       Reacton having higher rate constant are fast and rate constant is given by : k= Ae-Ea/RT.
Thus, k will be higher for reactions having low activation energy and thus the reaction with lower activation energy will have higher rate.
If the similar reactions having the same rate constant at 250C, do the two reactions will have the same rate constant at 350C also?
                       No, it is just accidental that both the reactions have same rate constant at 250C. At 350C, the reaction with high activation energy will have higher rate constant.
A catalyst speeds up the reaction but it does not change the head of reaction. Explain.
                        A catalyst speeds up the reaction by providing an alternative path having low activation energy. However, it does not affect the enthalpies of the reactants and products. Therefore, ∆H=Hp-Hr. Does not change.
What is meant by an accepted or probable mechanism? Is there any such thing as a proved mechanism?
                         A mechanism which fits all the data available is called an accepted mechanism.  No mechanism can be proved. It is only a tentative proposal which can be modified in the light of further data.




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