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Use Figure 3 and the data shown in Table 1 to calculate ΔH<sub>f</sub> - AQA - A-Level Chemistry - Question 4 - 2021 - Paper 2
Question 4
Use Figure 3 and the data shown in Table 1 to calculate ΔH<sub>f</sub>. Figure 3 shows the relative stability of benzene compared to cyclohexane. Table 1 provides ... show full transcript
Worked Solution & Example Answer:Use Figure 3 and the data shown in Table 1 to calculate ΔH<sub>f</sub> - AQA - A-Level Chemistry - Question 4 - 2021 - Paper 2
Step 1
Calculate ΔH<sub>f</sub> for cyclohexane (C<sub>6</sub>H<sub>12</sub>)
Answer
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For cyclohexane, the enthalpy change when forming from its elements can be calculated using the bond enthalpies discussed.
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The reaction is as follows:
C (s) + H<sub>2</sub> (g) → C<sub>6</sub>H<sub>12</sub> (g)
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Total bond enthalpy = 6 × (C–C bond) + 12 × (C–H bond) = 6 × 348 + 12 × 412.
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Therefore, Total bond energy = 6 × 348 + 12 × 412 = 5328 kJ mol<sup>-1</sup>.
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For benzene (C<sub>6</sub>H<sub>6</sub>): ΔH<sub>f</sub> = 3 × 612 + 2 × 412 - ΔH<sub>reaction</sub>.
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Calculate ΔH<sub>f</sub> = 3 × 348 + 6 × 412 = 5508 kJ mol<sup>-1</sup>.
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Finally, ΔH<sub>reaction</sub> = ΔH<sub>f</sub> (benzene) - ΔH<sub>f</sub> (cyclohexane).
ΔH<sub>f</sub> = -83 kJ mol<sup>-1</sup>.