O-LevelChemistryRates of Reaction5070/22 · Oct/Nov 2017 · Q610 Marks

At high temperatures, hydrogen reacts with iodine to form hydrogen iodide. H2(g) + I2(g) ⇌ 2HI(g) ΔH = + 53.0 kJ/mol

(a)(i)Describe and explain the effect, if any, on the position of equilibrium when the pressure is increased,[2]
(a)(ii)the temperature is decreased.[2]
(b)(i)Hydrogen iodide reacts with water to form a strong acid, hydriodic acid, HI(aq). What is meant by the term strong acid?[1]
(b)(ii)Construct the equation for the dissociation of hydrogen iodide molecules into ions.[1]
(c)Hydrogen iodide reacts with ethene to form iodoethane. CH2=CH2 + HI → CH3 CH2I What is the name of this type of reaction?[1]
(d)(i)The table shows some properties of five alkenes. alkene formula melting point /°C boiling point /°C ethene C2H4 -168.9 -103.6 propene C3H6 -47.3 butene C4H8 -185.2 -6.2 pentene C5H10 -165.0 30.0 hexene C6H12 -139.7 63.4 How does the boiling point change as the number of carbon atoms in the formula of the alkenes increases?[1]
(d)(ii)What is the physical state of butene at −7°C? Explain your answer.[1]
(d)(iii)Why is it difficult to predict the melting point of propene using only the information from the table?[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[1] No effect
[1] Equal number of moles (of gases) on each side of the equation / each volumes (of gases) on each side of the equation
[1] Equilibrium moves to the left
[1] The (forward) reaction is endothermic / backward reaction exothermic / goes in the direction of the exothermic reaction
[1] Substance which completely dissociates to form H+ ions / substance which completely ionises to form H+ ions
[1] HI → H⁺ + I¯
[1] Addition
[1] Increases (as the number of C atoms increases)
[1] Liquid because –7 °C / it is above its melting point and below its boiling point / liquid because –7 °C / it is between the melting point and boiling point
[1] There is no clear trend / the values go up and down / the values go down and up

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