A-LevelChemistryChemical bonding9701/22 · Oct/Nov 2021 · Q126 Marks

Hydrogen iodide, HI, is a colourless gas at room temperature.

(a)(i)Explain why HI has a higher boiling point than HCl and HBr.[2]
(a)(ii)The bar chart shows the boiling points of HCl, HBr and HI. The boiling point of HF is not shown. [Figure 1.1] Hydrogen bonds form between HF molecules. Draw a bar on the bar chart to predict the boiling point of HF. Explain your answer.[2]
(b)The standard enthalpy change of formation, ∆H, of HI(g) is +26.5kJmol⁻¹. Define the term standard enthalpy change of formation.[2]
(c)(i)Construct an expression for the equilibrium constant, Kₚ, for the reaction of H₂(g) and I₂(g) to form HI(g). Kₚ =[1]
(c)(ii)The equilibrium partial pressures of the gases at 200°C are as follows. P(H₂(g)) = 895 Pa, P(I₂(g)) = 895 Pa, P(HI(g)) = 4800 Pa. Calculate Kₚ for this reaction. Kₚ =[1]
(c)(iii)State how the value of Kₚ would change, if at all, if the reaction were carried out at 100°C rather than 200°C. Explain your answer.[2]
(d)(i)Construct an equation for the reaction of HI with oxygen.[1]
(d)(ii)Explain, with reference to oxidation numbers, why this reaction is a redox reaction.[2]
(e)HI(g) can also be formed by the reaction of I₂(g) with hydrazine, N₂H₄(g). 2I₂(g) + N₂H₄(g) → 4HI(g) + N₂(g). State the change in pressure that would occur when 2 mol I₂(g) fully reacts with 1 mol N₂H₄(g) in a sealed container at constant temperature. Explain your answer.[2]
(f)(i)Draw a 'dot-and-cross' diagram of a PI₃ molecule.[2]
(f)(ii)Name the type of reaction in step 2.[1]
(f)(iii)H₃PO₃(aq) and HI(aq) are both strong Brønsted-Lowry acids. Give the meaning of the term strong Brønsted-Lowry acid.[2]
(f)(iv)Give the formula of the conjugate base of H₃PO₃.[1]
(g)(i)Identify which of 1-iodopropane and 2-iodopropane is the major product of this reaction. Explain your answer.[2]
(g)(ii)Complete the diagram to show the mechanism of the reaction between HI and CH₃CH=CH₂ that forms the major product identified in (g)(i). Include curly arrows, lone pairs of electrons and charges as necessary. [Figure 1.2][3]

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 (Oct/Nov 2021). The mark or mark type is shown in brackets.

[2] M1 (HI / I / iodine / hydrogen iodide has a) greater number of electrons
[2] M2 greater induced dipoles (between molecules)
[2] M1 bar at HF shows any boiling point above HI on graph
[2] M2 explanation of difference in boiling point of a sample of HF in terms of strength (sum of) hydrogen bonds (and induced dipoles in HF) are stronger than (sum of) induced dipoles (and permanent dipoles in HCl/HBr/HI)
[2] (enthalpy/energy change) when one mole of a compound is formed
[2] from its elements in their standard states
[1] Kp = PHI² / (PH2 P I2)
[1] 28.76 OR 28.8 OR 29
[2] EITHER option 1 which assumes ∆Ηreaction is (still) endothermic (using the value shown in (b)).
[2] M1 (Kp) decreases AND endothermic / ∆H(f) = + / positive
[2] M2 reaction favours formation of reactants / hydrogen and iodine OR (product) yield / partial pressure of HI decreases / equilibrium shifts to the left
[2] OR option 2 which realises that ∆Hreaction is in fact exothermic (using bond energy data in Data Booklet)
[2] M1 (Kp) increases AND exothermic / ∆H(f) = + / negative
[2] M2 reaction favours formation of products / hydrogen iodide yield increases / partial pressure of HI increases / equilibrium shifts to the right
[1] 4HI + O2 → 2I2 + 2H2O
[2] M1 I/iodine (increases) oxidation number -1 → 0 so oxidation / is oxidised OR HI/is oxidised as I (increases) oxidation number -1 → 0
[2] M2 O (decreases) oxidation number 0 → -2 so reduction / is reduced
[2] M1 pressure increases
[2] M2 (pressure goes up as) number of moles/molecules increases in ratio 3 (gas) reactants to 5 (gas) products OR pressure is (directly) proportional to number of moles/molecules
[2] M1 correct bonding pairs
[2] M2 correct number of remaining outer electrons on each atom
[1] hydrolysis
[2] proton donor / H+ donor
[2] fully dissociates / fully ionises
[1] H2PO3-
[2] M1 2-iodopropane – formed from a (more) stable (secondary) (carbo)cation/intermediate
[2] M2 (because of) greater (positive) inductive effect / (+)l of two alkyl groups OR (because of positive) inductive effect / (+)l of more R/ more methyl / more alkyl groups
[3] M1 curly arrow from = of C=C to H AND curly arrow from bond of H—I to I
[3] M2 curly arrow from lone pair of I– to C(+) of their intermediate
[3] M3 correct carbocation AND product for 2-iodopropane