What are Catalysts?
Certain substances, called catalysts, can increase the rate of a reaction without being consumed by it.
An example is the decomposition of H2O2:
2H2O2 ----> 2H2O + O2 - this reaction is ordinarily very slow, however, add NaI(aq) to the H2O2 and the reaction becomes very fast.Catalysts can be described as being homogeneous or heterogeneous.
- Homogeneous: reaction takes place within a single phase, as in the example above with the H2O2.
- Heterogeneous: reaction involves two different phases, as in the example that follows.
2N2O ----> 2N2 + O2 - this reaction is usually slow, but if the reactants are bought into contact with a metal such as gold, the reaction speeds up considerably.
A catalyst lowers the activation energy required for the reaction.
Consider the reaction 2N2O ----> 2N2 + O2 again. The Ea = 250 kJ, however, using the Au catalyst, the Ea is lowered to 120 kJ.
The reduction in activation energy occurs because the catalyst provides an alternate pathway of lower energy for the reaction.
The N2O is chemically adsorbed on the metal surface. A bond is formed between the O of the N2O and an Au atom. This weakens the bond joining the O to the N, thus making it easier for the molecule to break apart.
Now, consider again the decomposition of the hydrogen peroxide. It appears to take place in a 2-step process:
- H2O2 + I-(aq) ----> H2O + IO-(aq)
- H2O2 + IO-(aq) ----> H2O + O2 + I-(aq)
The sum of these two reactions is then 2H2O2 ---->H2O
The I- are not consumed in the reaction, for every I- usesd in the first step, an I- is produced in the second.
A catalyst has no effecct on the relative energies of the reactants and products, nor on the equilibrium constant, Kc. It merely speeds up the reaction, thus allowing it to reach equilibrium more quickly.
So, What Are Enzymes?
Enzymes are organic catalysts (proteins) of high molecular mass. They allow reactions which occur slowly under ordinary conditions to occur readily in the body.
An important example is that of the metabolism of sugar. The reaction is difficult to bring about directly. It will usually turn into a charred, black mess if you try to burn it in a flame, however, in the body, sugar is metabolized at 37 oC in a series of biochemical reactions. The end products are CO2 and H2O.
Each step is catalyzed by a particular enzyme adapted for that purpose. The reactant, called the substrate fits into a specific site on the enzyme surface and is held in place by intermolecular forces. There, it can react with another species.
Inhibitors are substances which diminish the activity of an enzyme. One way they do this is to occupy the space on the enzyme where the substrate is supposed to fit. These usually have a structure similar to that of the substrate they replace.
The structures of enzymes have been studied with x-ray crystallography.
Send questions, comments or suggestions to
Gwen Sibert, at the
Roanoke Valley Governor's School
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