Notice that the equation above also tells us that enthalpy becomes a less important contributor to the equilibrium constant as temperature increases, so the higher the temperature, the more important is the entropy term. This fact means that some reactions favour one side of the equilibrium at low temperature but the other at high temperature. Here is an example: the dimerization of cyclopentadiene. You will meet the mechanism of this reaction in Chapter 34, but for now we can just treat it as a simple dimerization reaction in which two C = C π bonds are replaced by two C–C σ bonds—enthalpically a very favourable process because σ bonds are stronger than π bonds. On standing at low temperature, cyclopentadiene converts to the dimer even though two monomer molecules have more entropy than one molecule of the dimer.

But on heating, the dimer breaks down to give monomeric cyclopentadiene: the equilibrium constant now favours the starting materials. As we predicted, because the reaction is exothermic, heating it makes it less favourable. You can also think of it in terms of our earlier equation ΔG = ΔH – TΔS: at low temperature, the large negative ΔH term dominates, and ΔG is large and negative too. But as T increases, the positive ΔS becomes more important, and eventually TΔS overtakes ΔH and ΔG becomes positive, and the reaction now favours starting materials. If you want to use cyclopentadiene, you have to heat the dimer to ‘crack’ it (‘cracking’ is the term used for getting monomers from dimers or polymers). If you lazily leave the monomer overnight and plan to do your reaction tomorrow, you will return in the morning to find dimer. This idea becomes even more pointed when we look at polymerization. Polyvinyl chloride is the familiar plastic PVC and is made by reaction of large numbers of monomeric vinyl chloride molecules. There is, of course, an enormous decrease in entropy in this reaction any polymerization will not occur above a certain temperature.

Some polymers can be depolymerized at high temperatures and this can be the basis for recycling.

● Summary: Practical points from thermodynamic theory Cl

• The free energy change ΔG in a reaction is proportional to lnK (that is, ΔG = –RTlnK).

• ΔG and K are made up of enthalpy and entropy terms (that is, ΔG = ΔH – TΔS).

• The enthalpy change ΔH is the difference in stability (bond strength) of the reagents and products.

• The entropy change ΔS is the difference between the disorder of the reagents and that of the products.

• The enthalpy term alone determines how K varies with temperature.

• The entropy change come to dominate control of equilibrium as temperature is raised.

مواضيع ذات صلة

Catalysis

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

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