The following reaction occurs during the cracking of tetradecane, \({{\text{C}}_{{\text{14}}}}{{\text{H}}_{{\text{30}}}}\).

\[{{\text{C}}_{14}}{{\text{H}}_{30}}{\text{(g)}} \to {{\text{C}}_{10}}{{\text{H}}_{22}}{\text{(g)}} + {\text{2}}{{\text{C}}_2}{{\text{H}}_4}{\text{(g)}}\]

Suggest a use for each of the products formed in the reaction.

\({{\text{C}}_{{\text{10}}}}{{\text{H}}_{{\text{22}}}}\):

\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}\):

State the main type of product obtained from steam cracking.

Catalytic cracking uses silica as a heterogeneous catalyst. Explain the mode of action of a heterogeneous catalyst.

State one advantage of using a heterogeneous catalyst rather than a homogeneous catalyst.

Discuss two factors that need to be considered when choosing a catalyst for a process.

C₁₀H₂₂: gasoline/petrol / fuel / kerosene;

Do not allow just combustion or cars.

Allow gas for cars/automobiles instead of gasoline but not gas alone.

C₂H₄: chemical feedstock / OWTTE;

Accept suitable example such as manufacturing plastics/polymers but not just plastics.

alkenes;

solid surface has active sites / reactants adsorb on solid surface;

Do not accept absorb instead of adsorb.

brings reactants close together in correct orientation;

weakens reactant bonds / reactants bonds are easier to break;

can be easily removed/filtered from reaction mixture / large amount of reactant molecules pass over catalyst that is in a fixed position / can be used at high temperatures;

selectivity to produce (a high yield of) the desired product / OWTTE;

extent to which rate of reaction is increased/Ea is lowered;

amount of reactant converted to product per amount of catalyst;

Accept efficiency / conversion rate.

ability to work under different/a range of conditions;

environmental/health impact;

catalytic poisoning / active sites become blocked;

cost in relation to life expectancy / OWTTE;

ease of removal from reaction mixture;

Less than half of the candidates knew the uses of the products of cracking in (a) and very few candidates knew the product of steam cracking in (b).

Less than half of the candidates knew the uses of the products of cracking in (a) and very few candidates knew the product of steam cracking in (b).

The mode of action of heterogeneous catalysts was also not well answered. The majority of candidates wrote about catalysts in general gaining no marks on part (c).

Parts (d) and (e) about the advantage of heterogeneous catalysts over homogeneous catalysts, and factors to be considered when selecting a catalyst were well answered by the majority of candidates.

Parts (d) and (e) about the advantage of heterogeneous catalysts over homogeneous catalysts, and factors to be considered when selecting a catalyst were well answered by the majority of candidates.