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CAIE A-Level Chemistry 5.2.2 Hess-Cycle and Energy Calculations

Practise calculating inaccessible enthalpy changes from formation, combustion or bond-energy data with correct signs.

Syllabus
2028–2030
Course
Chemistry 9701
Level
AS

Exam points

  • for formation data, calculate sum of product values minus sum of reactant values with coefficients
  • for combustion cycles, compare routes to the same fully combusted products and preserve arrow direction
  • use bond energies as broken minus formed and recognize the result is approximate when averages are used

5.2.2—Calculations: cycles and relevant energy terms question 1

[Maximum number: 2]

The Group 17 elements are oxidising agents.

Cl2\mathrm{Cl}_{2} and Br2\mathrm{Br}_{2} can each react with NH3\mathrm{NH}_{3} to give N2\mathrm{N}_{2} and a hydrogen halide, HX .

3X2+2NH3→ N2+6HX3 \mathrm{X}_{2}+2 \mathrm{NH}_{3} \rightarrow \mathrm{~N}_{2}+6 \mathrm{HX}

X = Cl or Br

The relative bond strengths of X-X and H-X determine the difference in enthalpy change of the two reactions.

Table 1.1 gives data relevant to the reaction of Cl2( g)\mathrm{Cl}_{2}(\mathrm{~g}) with NH3( g)\mathrm{NH}_{3}(\mathrm{~g}).

Use the data in Table 1.1 to calculate the enthalpy change of the reaction of Cl2( g)\mathrm{Cl}_{2}(\mathrm{~g}) with NH3( g)\mathrm{NH}_{3}(\mathrm{~g}).

Table 1.1

Table 1.1

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