The following diagram represents the potential energy for the reaction:

the curves represent two possible reaction paths for the decomposition of ammonia, nh3. one can correctly conclude from the graph that the best explanation for the difference between (i) and (ii) is that:

the concentration of nh3 was increased.
the temperature was increased.
the n2 was removed as soon as it was formed.
a catalyst was added.

A catalyst was added.

Explanation:

Activation energy is the extra energy that must be supplied to reactants in order to cross the energy barrier and thus convert to products.

When a catalyst is added top a reaction, it increases the rate of a reaction by taking the reaction through a different path which involves lower activation energy and thus more molecules can cross the energy barrier and convert to products.

The concentration of reactants and products remain same even after addition of catalyst.

B represents activation energy for uncatalyzed reaction and E represents activation energy for catalyzed reaction.

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