Outline two differences between heterogeneous and homogeneous catalysts.

Suggest, giving a reason, how elastomers used for the tyre tread can increase the traction between the tyre and the road.

Tyre fires emit trace quantities of polychlorinated dibenzofurans and polychlorinated dibenzo-p-dioxin.

Outline, using section 31 of the data booklet, why polychlorinated dibenzofuran is not classed chemically as a dioxin but considered “dioxin-like”.

The trace quantities of dioxins from tyre fires are rarely inhaled and instead settle on the ground.

Describe why this is a health concern.

Any two of:
heterogeneous catalyst is in different phase than reactants AND homogeneous catalyst in same phase  [✔]

homogeneous catalysts chemically change/react and are reformed at end of reaction
OR
reactants adsorb onto heterogenous catalyst and products desorb  [✔]

heterogeneous catalysts are more easily removed than homogenous catalysts [✔]

heterogeneous catalysts can function at higher temperatures [✔]

homogeneous catalysts are «generally» more selective [✔]

homogeneous catalysts offer a broader range of reactions [✔]

Note: Accept “state” for “phase”.

Accept “heterogeneous catalyst provides a surface to activate reaction”.

elastomers bend under force «and return to original form when force is released»
OR
elastomers make tyre more flexible  [✔]

allows greater contact with road  [✔]

does not contain heterocyclic ring with 2 oxygen atoms
OR
middle ring has only 1 oxygen atom  [✔]

produces similar toxic effects to dioxins  [✔]

Note: Accept “does not contain dioxin ring” for M1.

taken up by plants, which are eaten by animals «and then further dispersed»
OR
passed on in food chain  [✔]

Note: Accept “do not break down and can be remobilised as dust”.

Many students were aware of the fundamental difference between homogeneous and heterogeneous catalysts, but only a few could identify another difference to obtain full marks.

Whilst some students realised that elastomers would increase the area of road in contact, hardly any linked this to their fundamental property of deforming under stress.

This was a question that highlighted a perennial problem; students appearing to know the answer but not having sufficient command of the English language to express it accurately. A number did identify similar toxic effects as the reason for being “dioxin like”.

Only a few students linked this to the toxin entering the food chain.