Exams › JEE Advanced › Chemistry
During an electrolysis experiment, water is electrolysed using 40 F of electricity at 75% current efficiency. The oxygen gas released is used in a bomb calorimeter to completely burn 3.5 g of ethene (C2H4) at 300 K. The calorimeter temperature rises from 300 K to 300.50 K. Given that the heat capacity of the calorimeter is 3 kJ/K and R = 8 J/(mol K), find the magnitude of the heat of combustion of ethene at constant pressure for 5 moles of ethene (in kJ).
- 30
- 36
- 45
- 54
Correct answer: 36
Solution
Effective electricity = 40 * 0.75 = 30 F. O2 released: 30/4 = 7.5 mol (since 4F gives 1 mol O2). Moles of C2H4 = 3.5/28 = 0.125 mol. O2 is in large excess so C2H4 is limiting. Heat at constant volume: qv = C * delta_T = 3 * 0.5 = 1.5 kJ for 0.125 mol. qv per mol = 1.5/0.125 = 12 kJ/mol. For C2H4 + 3O2 -> 2CO2 + 2H2O(g): delta_ng = 2 - (1+3) = -2. qp = qv + delta_ng * R * T = 12 + (-2) * 0.008 * 300 = 12 - 4.8 = 7.2 kJ/mol. For 5 moles: 5 * 7.2 = 36 kJ.
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