Chemistry · Ch 7 — Chemical Kinetics
Effect of Presence of a Catalyst
Effect of Presence of a Catalyst
Catalysts act differently from the other four factors. Concentration, temperature and surface area all speed a reaction up only to a limited extent, and only by changing conditions the chemist directly controls. A catalyst instead is a substance that alters a reaction's rate WITHOUT itself undergoing any permanent chemical change -- it may genuinely take part in an intermediate step of the mechanism (as did in Section 7.4's hydrogen-peroxide example), but it is fully regenerated, unconsumed, by the time the reaction is complete.
The mechanism: lowering the activation energy. A catalyst works by opening up an ALTERNATIVE reaction pathway that has a LOWER activation energy than the original, uncatalysed route. Since the fraction of molecules with enough energy to react is (Section 7.7), even a modest reduction in lets a much larger fraction of collisions succeed at the same temperature -- so more molecules cross the (now lower) energy barrier per second, and the observed rate rises. Crucially, a catalyst changes ONLY the pathway/rate, never the reaction's thermodynamics: the starting reactants and final products, and the overall energy released or absorbed, are exactly the same with or without the catalyst present -- only the height of the energy hill in between is different. …
What this figure shows. A potential-energy-vs-reaction-progress diagram overlaying two curves that share the same flat 'Reactants' starting level and the same flat, lower 'Products' ending level. The uncatalysed path (red curve) rises to a HIGH peak, with a double-headed arrow labelled ' Without catalyst' spanning from the reactants' level up to that peak. The catalysed path (green curve) rises to a noticeably LOWER peak over the same reaction progress, with its own shorter double-headed arrow labelled ' With catalyst' -- visually showing the catalyst opens a lower-energy alternative route between the same reactants an …
Worked out. Two test tubes A and B are each set up with 7 ml of 0.1N oxalic acid, 5 ml of 0.1N and 5 ml of 2N dilute -- both solutions appear pink from the permanganate. A few crystals of manganese sulphate () are added ONLY to tube A. Observation: the pink colour in tube A fades and disappears noticeably faster than in tube B (which has no added ). Here acts as a catalyst, speeding up the oxidation of oxalate ion () by permanganate () -- a case where the catalyst is chemically related to one of the reaction's own products (Mn forms as is reduced), an autocatalytic-flavoured example …