A-LevelChemistryChemical energetics9701/21 · May/June 2024 · Q46 Marks
Define enthalpy change of formation.
- (a)Define enthalpy change of formation.[2]
- (b)Iron is made when iron(III) oxide is heated with carbon monoxide, as shown by reaction 2. reaction 2 Fe2O3 + 3CO → 2Fe + 3CO2 Table 4.1 shows enthalpy change of formation data measured at 298K and 101 kPa.
- (b)(i)Complete Table 4.1 by adding equations with relevant state symbols to represent: • standard enthalpy change of formation for Fe2O3 • standard enthalpy change of formation for CO. Table 4.1 substance | equation | value for ΔH/kJ mol-1 Fe2O3 | | -824.2 CO | | -110.5 CO2 | C(s) + O2(g) → CO2(g) | -393.5[2]
- (b)(ii)Use the data in Table 4.1 to calculate the enthalpy change of reaction, ΔHᵣ, in kJ mol-1, for reaction 2. Show your working. ΔHᵣ = .............................................. kJ mol-1[2]
Mark scheme
Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.
- [2] M1 (enthalpy change / energy change) when one mole of a compound / substance is formed
- M2 from its elements in their standard states
- [2] Fe2O3: 2Fe (s) + 3/2O2(g) → Fe2O3(s)
- CO: C (s) + 1/2O2 (g) → CO (g)
- [2] ∆Hreaction = -ΔHFe2O3 − (3 × −∆Hco) + (3 × −∆HC02)
- = -(-824.2) – (3 × −110.5) + (3 × −393.5)
- = -24.8 kJ mol-1
- • evidence of use of correct expression and/or any rearrangement of only these bond energy values
- • use correct stoichiometry for these bond energy values only
- • calculate their expression correctly provided the expression uses only addition / subtraction and all 3 values
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