A-LevelChemistryChemical bonding9701/23 · May/June 2021 · Q214 Marks

Methanol, CH3OH, is soluble in water because it forms hydrogen bonds with water molecules.

(a)Draw a fully labelled diagram to show how a hydrogen bond forms between a water molecule and a methanol molecule.[3]
(b)(i)Methanol has a melting point of –97.6°C and a boiling point of 64.7°C. A sample of pure liquid methanol is added to a flask and then sealed. The sealed flask is left for several days at constant temperature. The vapour pressure is then measured as 17kPa. Describe what is meant by the term vapour pressure of methanol.[2]
(b)(ii)Explain why some of the liquid becomes a vapour.[1]
(b)(iii)Suggest and explain why the vapour pressure of water at room temperature is lower than the vapour pressure of methanol at room temperature. Refer to the correct intermolecular forces in your answer.[2]
(c)(i)Methanol is made by reacting carbon monoxide with hydrogen. CO(g) + 2H2(g) <=> CH3OH(g) Carbon monoxide and hydrogen react at 1.0 × 107 Pa and 200°C. Eventually the reaction mixture reaches dynamic equilibrium. The table shows the amounts of each species present in the mixture. initial amount/mol CO(g) 1.0 H2(g) 2.0 CH3OH(g) 0 equilibrium amount/mol CO(g) 0.030 H2(g) 0.060 CH3OH(g) 0.97 Explain what is meant by dynamic equilibrium.[2]
(c)(ii)Calculate the partial pressure of methanol vapour at equilibrium under these conditions. Show your working. Pa[2]
(c)(iii)Write an expression for the equilibrium constant, Kp, for this reaction. State the units in your answer. Kp = units =[2]

Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] 2(a) M1 H-bond between an O and an H in OH groups in water and methanol molecules
[1] M2 minimum three partial charges (in sequence) over one water and one methanol molecule, i.e.:
[1] M3 either δ-О—Нδ+- – – – –O or δ+- - - - -О-Нδ+ lone pair of electrons on O of H-bond, in line with H-bond
[1] 2(b)(i) M1 (methanol) gas (particles / molecules) in equilibrium with liquid
[1] M2 (methanol) gas (particles) exert a pressure (on the walls of a container / on the surface of the liquid)
[1] 2(b)(ii) (liquid) particles (at the surface) have enough energy (to overcome attractive forces / evaporate / to become a vapour)
[1] 2(b)(iii) M1 (liquid) H2O molecules are held by stronger hydrogen bonding OR it takes more energy to break the hydrogen bonds between water molecules (in the liquid state) OR each water molecule forms two hydrogen bonds (whereas methanol can (only) form one per molecule
[1] M2 fewer H2O liquid molecules (able to) escape / become gaseous ora
[1] 2(c)(i) M1 rates of forward and reverse / backward reactions are equal
[1] M2 closed / sealed system / container OR no change in measurable properties / no change in macroscopic properties
[1] 2(c)(ii) M1 mol fraction = 0.97 / (0.030 + 0.060 + 0.97) = 0.97 / 1.06 = (0.9151)
[1] M2 M1 × 1.0 × 107 = 9.2 × 106 (Pa) √
[1] 2(c)(iii) M1 Kp = p(CH3OH) / p(CO) × p(H2)2
[1] M2 units = Pa-2

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