A-LevelChemistryChemical energetics9701/23 · May/June 2024 · Q37 Marks

Write an equation to show the reaction for the standard enthalpy change of formation of H2O. Include state symbols.

(a)Write an equation to show the reaction for the standard enthalpy change of formation of H2O. Include state symbols.[2]
(b)Water is one of the products in the reaction of B2O3 and NH3, as shown in reaction 2. reaction 2 B2O3 + 2NH3 → 2BN + 3H2O Table 3.1 shows information about the standard enthalpy change of formation, ∆H, of some substances. Table 3.1 substance ∆H /kJ mol⁻¹ B2O3 -1264 NH3 -46 BN -134 H2O -286 Calculate the enthalpy change, ∆H, for reaction 2 using the data from Table 3.1. ΔH =[2]
(c)(i)Boron carbide is a hard crystalline solid that has a melting point greater than 2000 °C. Suggest the structure and bonding in boron carbide.[1]
(c)(ii)100 g of pure boron carbide contains 78.26 g of boron. Calculate the empirical formula of boron carbide. Show your working. empirical formula of boron carbide[2]

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

[2] H2(g) + 1/2O2 (g) → H2O(l) M1 correct equation only M2 correct state symbols based on this equation
[2] ΔΗ = −(−1264) – (2 × −46) + (2 ×−134) + (3 × −286) = (+)230 kJ mol-1 M1 correct expression in terms of all 4 enthalpy changes of formation AND correct stoichiometry (as described in given equation) M2 correct calculation based on figures given
[1] giant (molecular and) covalent
[2] M1 give correct expression or calculate mol B compared to mol C 78.26 / 10.8 (= 7.246 / 7.25 mol B) AND 21.74 / 12 (=1.81(2) mol C) M2 Find B: C ratio AND use this to state empirical formula 7.246 to 7.25/1.817 to 1.82 = 3.99 to 3.98 mol B: 1 mol C (so) B4C OR CB4

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