State a balanced equation for the thermal cracking of \({{\text{C}}_{{\text{20}}}}{{\text{H}}_{{\text{42}}}}\) in which octane and ethene are products.

(i)     Other than density, state two differences in the physical properties of HDPE and LDPE.

(ii)     Outline how the differences in (b)(i) relate to differences in their chemical structure.

It has been said that bitumen and heavy fuel oils are too valuable a resource to use for road surfacing and electricity generation. Comment on this statement.

\({{\text{C}}_{20}}{{\text{H}}_{42}} \to {{\text{C}}_8}{{\text{H}}_{18}} + {{\text{C}}_2}{{\text{H}}_4} + {{\text{C}}_{10}}{{\text{H}}_{20}}/{{\text{C}}_{20}}{{\text{H}}_{42}} \to {{\text{C}}_{\text{8}}}{{\text{H}}_{{\text{18}}}} + {\text{6}}{{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}\)

Accept any correctly balanced equation that includes octane and at least one ethene molecule as products.

correct reactants and products;

balanced equation;

M2 can only be scored if M1 is correct.

(i)     Award [1] for any two.

HDPE has higher mp;

HDPE is more rigid / less flexible;

HDPE is stronger;

Accept opposite statements for LDPE.

(ii)     HDPE has straight chain and LDPE has branched chain / LDPE has more branched chains;

more valuable for (cracking to provide) chemical precursors/petrochemicals / may be cracked to produce same substances now obtained from lighter fractions / OWTTE;

Only about a third of the candidates were able to score both marks by giving a balanced equation producing octane and ethene. Others scored one mark as they failed to balance the equation.

(i)     Less than half the candidates gave two physical properties that differed between LDPE and HDPE.

(ii)     A small number of candidates attributed the difference to the branching in the chains.

The answers were mostly unsatisfactory as they failed to recognize the value of cracking products for the petrochemical industry.