A-LevelChemistryEquilibria9701/22 · May/June 2023 · Q415 Marks

In industry, ethanol is made by reacting ethene with steam in the presence of H3PO4. reaction 1 C2H4(g) + H2O(g) ⇌ C2H5OH(g)

(a)Use the bond energy values in Table 4.1 to calculate the enthalpy change, ∆H₁, for reaction 1. Table 4.1 bond | bond energy/kJmol⁻¹ ---|--- C-C | 350 C=C | 610 C≡C | 840 C-H | 410 C-O | 360 C=O | 740 O-H | 460 ∆H₁ =[2]
(b)Reaction 1 reaches equilibrium at constant temperature and pressure. Deduce what effect increasing the pressure will have on the amount of ethanol in the new equilibrium mixture. Use Le Chatelier's principle to explain your answer. effect of increasing pressure explanation[2]
(c)(i)The mechanism for reaction 1 can be described in three steps. Steps 1 and 2 for reaction 1 are shown in Fig. 4.1. [Figure 4.1] Describe the behaviour of H3PO4 in step 1 in Fig. 4.1. Explain your answer.[1]
(c)(ii)Identify the species that behaves as an electrophile in step 2 in Fig. 4.1. Explain your answer.[1]
(c)(iii)Complete Fig. 4.2 to show the mechanism for step 3 of reaction 1. Include charges, dipoles, lone pairs of electrons and curly arrows, as appropriate. [Figure 4.2][2]
(c)(iv)Describe how a catalyst affects a reaction. Explain your answer.[2]
(c)(v)Use Fig. 4.1 and Fig. 4.2 to justify why H3PO4 is described as a catalyst in reaction 1.[1]
(c)(vi)Propene also reacts with steam. A mixture of organic products is produced. Explain why propan-2-ol is produced in the higher yield.[2]
(d)Describe the covalent bonds present between the carbon atoms in an ethene molecule by completing Table 4.2. Table 4.2 | sigma (σ) | pi (π) ---|---|--- type of orbitals involved in bond | | how the orbitals overlap | |[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2023). The mark or mark type is shown in brackets.

[2] Method 1 • identification of number and type of bonds when all are broken 2O-H + 4C-H + C=C OR (2 × 460) + (4 × 410) + 610 • Identification of number and type of bonds when all are made O-H + 5C-H + C-C + C-O OR (460 + (5 × 410) + 350 + 360) • calculation of energy to break bonds – energy to make bonds 3170 – 3220 = – 50 (kJ mol⁻¹) Two correct for one mark, three correct for two marks. OR Method 2 • identification of number and type of relevant bonds broken (including the new C-H only) 2O-H + C=C OR (2 × 460) + 610 • identification of number and type of relevant bonds made (including the new C-H only) O-H + C-H + C-C + C-O OR (460 + 410 + 350 + 360) • calculation using energy to break bonds – energy to make bonds 1530 – 1580 = –50 (kJ mol⁻¹) Two correct for one mark, three correct for two marks.
[2] M1 amount ethanol increases M2 equilibrium moves to right AND to produce fewer moles gas OR smaller amount of gas (to reduce the pressure)
[1] acid AND it donates a proton / H⁺
[1] (CH3CH2)⁺ OR CH3C⁺H2 AND accepts a pair of electrons
[2] M1 arrow from bond of one of the O-H to O of the cation M2 arrow from lone pair on O of the anion to between H of H-O
[2] M1 a catalyst increases rate of reaction M2 by producing an alternative reaction pathway with lower activation energy
[1] it is regenerated OR it is reformed
[2] M1 (propan-2-ol is produced via a) more stable carbocation OR more stable intermediate OR (CH3)2C⁺H is more stable M2 (positive) inductive effect of more alkyl/methyl groups OR more electron donating alkyl / methyl groups OR greater electron donating effect of two alkyl / two methyl groups
[2] | sigma | pi ---|---|--- type of orbitals involved in bond | sp²(-sp²) | (2)p(-(2)p) how the orbitals overlap | direct | sideways Two correct for one mark, four correct for two marks.