Ethyne, like ethene, undergoes hydrogenation to form ethane. State the conditions required.

Outline the formation of polyethene from ethene by drawing three repeating units of the polymer.

Under certain conditions, ethyne can be converted to benzene.

Determine the standard enthalpy change, ΔH^ϴ, for the reaction stated, using section 11 of the data booklet.

                                        3C₂H₂(g) → C₆H₆(g)

Determine the standard enthalpy change, ΔH^Θ, for the following similar reaction, using ΔH_f values in section 12 of the data booklet.

3C₂H₂(g) → C₆H₆(l)

Explain, giving two reasons, the difference in the values for (b)(i) and (ii). If you did not obtain answers, use −475 kJ for (i) and −600 kJ for (ii).

One possible Lewis structure for benzene is shown.

State one piece of physical evidence that this structure is incorrect.

State the characteristic reaction mechanism of benzene.

nickel/Ni «catalyst»

high pressure

OR

heat

Accept these other catalysts: Pt, Pd, Ir, Rh, Co, Ti.

Accept “high temperature” or a stated temperature such as “150 °C”.

[2 marks]

Ignore square brackets and “n”.

Connecting line at end of carbons must be shown.

[1 mark]

ΔH^ϴ = bonds broken – bonds formed

«ΔH^ϴ = 3(C≡C) – 6(C=C)_benzene/3 × 839 – 6 × 507 / 2517 – 3042 =»

–525 «kJ»

Award [2] for correct final answer.

Award [1 max] for +525 «kJ»

Award [1 max] for:

«ΔH^ϴ = 3(C≡C) – 3(C–C) – 3(C=C) / 3 × 839 – 3 × 346 – 3 × 614 / 2517 – 2880 =» –363 «kJ».

[2 marks]

ΔH^Θ = ΣΔH_f(products) – ΣΔH_f(reactants)

«ΔH^Θ = 49 kJ – 3 × 228 kJ =» –635 «kJ»

Award [2] for correct final answer.

Award [1 max] for “+635 «kJ»”.

[2 marks]

ΔH_f values are specific to the compound

OR

bond enthalpy values are averages «from many different compounds»

condensation from gas to liquid is exothermic

Accept “benzene is in two different states «one liquid the other gas»“ for M2.

[2 marks]

equal C–C bond «lengths/strengths»

OR

regular hexagon

OR

«all» C–C have» bond order of 1.5

OR

«all» C–C intermediate between single and double bonds

Accept “all C–C–C bond angles are equal”.

[1 mark]

electrophilic substitution

OR

S_E

[1 mark]