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Catalysts are substances that speed up the rate of a chemical reaction. For a particular reaction which can proceed via both catalyzed as well as uncatalyzed pathways, the `Delta H^o` or `DeltaG^o` remain unaltered in both catalyzed and uncatalyzed pathways. But, the free energy of activation, i.e. the energy hill that reactants must cross before being able to be converted into products, is modified (refer to the attached image) in the catalyzed pathway. Free energy of activation of the catalyzed reaction is much less than the uncatalyzed reaction. Result is that, under comparable external conditions, reactions are often speeded up by a factor of `10^2` to `10^4` by the involvement of a catalyst.
There are sometimes a common misconception about a 'negative catalyst' often 'retarding' a particular reaction, but in strict chemical sense there is nothing like a 'negative catalyst'. Substances that seem to retard a reaction are actually inhinitors or blockers, which stops certain reaction sites altogether. Modifying activation energy of a reaction upto the higher end would have no effect on the original rate as the reactants would have followed the uncatalyzed path then.
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