D.I. Mendeleev A.M.Butlerov

The velocity of chemical reactions
Table of content

Catalysis and inhibition

There are substances that influence the velocity of chemical reaction, while not being consumed in the process. Such substances are referred to as catalysts if they accelerate reaction, and inhibitors if they slow it down.

75 % of processes used in chemical industry are catalytic. Examples are: ammonia synthesis and oxidation, sulfuric acid production, and fine organic synthesis, for example, medicines.

All biochemical reactions in living organisms require natural catalysts - enzymes and co-enzymes. Enzymes are large protein molecules with the so-called active center, which is usually a molecule of not protein nature or an ion of metal chemically connected to protein. Enzymes have an outstanding activity, accelerating reactions billions and trillions times at room temperature. At high temperatures they lose activity as protein molecule denaturates. Besides, enzymes are characterized by extreme selectivity, catalyzing only specified reactions and only in a very narrow range of conditions (i.e. pH or temperature). It is due to their shape which reagents should match - it's like a key and a lock.

Scheme of enzyme action (flash file)

The catalysts work by reducing the activation energy of reaction. Do you want to know about it more in detail?

Many catalysts does not simply accelerate reactions, but lead them in the another way. So, the reaction of oxidation of ammonia without the catalyst proceeds as:

4NH3 + 3O2 = 2N2 + 6H2O

which is useless for the industry, but with a catalyst of platinum, the reaction goes the following way

4NH3 + 5O2 = 4NO + 6H2O

and a valuable product - nitrogen oxide is formed.

Inhibitors, on the contrary, increase energy of activation of reaction. In the food-processing industry inhibitors which prevent hydrolysis of fats, reactions of oxidation and fermentation are widely used.

There is special case of catalysis - autocatalysis, or acceleration of reaction by one of its products. In this case the rate of reaction does not decrease with the consumption of reagents, but even increase. I.e. the reaction

2KMnO4 + 5K2SO3 + 3H2SO4 = 2MnSO4 + 6K2SO4 + 3H2O

accelerates with accumulation of ions Mn2+, formed at the process of reduction of permanganate.


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