You’ve met the idea in several places in this book (and probably elsewhere) that a catalyst increases the rate of a reaction. From what you have just read, it must therefore be the case that a catalyst lowers the activation energy for a reaction. It can do this in one or both of two ways: it can lower the energy of the transition state (as shown in the diagram on the left, or it can raise the energy of the starting materials.

● Catalysts work by lowering the activation energy for a reaction.

To illustrate the point with one simple example, let’s take a reaction which simply does not work without a catalyst: the isomerization of butene. As you saw on p. 105, cis but-2-ene is about 2 kJ mol−1 higher in energy than trans but-2-ene. This small energy difference would correspond to a 2.2:1, 70:30, trans: cis ratio of alkenes if they were in equilibrium. But it’s a big if: the activation energy needed to get from one to the other is of the order of 260 kJ mol−1, which is practically unattainable. A quick calculation predicts that the half-life for the reaction would be approximately 1025 years at room temperature, a time many orders of magnitude longer than the age of the universe. At 500 °C, however, the half-life is a more reasonable 4 hours, but unfortunately, when most alkenes are heated to these sorts of temperatures other unwanted reactions occur. In order to interconvert the cis and trans isomers we must use a different strategy: catalysis. You will meet several ways of doing this in Chapter 27, but for the moment we will use just one: catalytic acid. As you saw in Chapter 5, alkenes are nucleophiles, and either isomer of but-2-ene can react with H+ from acid to form a transient species known as a carbocation. The activation energy for formation of the carbocation is much less than that for rotation about the C=C bond. The carbocation can now easily lose a proton again, to reform either cis- or trans-but-2 ene, regenerating the catalyst and allowing the interconversion to take place. Overall, the activation energy is much lower than in the uncatalysed reaction. We will come back to other examples of catalysis later in the chapter.

مواضيع ذات صلة

Reaction intermediates

Why some reactions are done at low temperature

The route from reactants to products: the transition state

How fast do reactions go? Activation energies

Some reactions are reversible on heating : cracking

Equilibrium constants vary with temperature

Enthalpy versus entropy—some examples

Energy, enthalpy, and entropy: ΔG, ΔH, and ΔS

Typical method for making an ester

How to make the equilibrium favour the product you want The direct formation of esters

A small change in ΔG makes a big difference in K

The sign of ΔG tells us whether products or reactants are favoured at equilibrium