Chemistry - Thermochemistry Question with Solution | TestHub
Study the following thermochemical equations:
(a) C2H6(g) → C2H4(g) + H2(g); ΔH = +32.7 kcal
(b) C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l); ΔH = -337.2 kcal
(c) H2(g) + 1/2 O2(g) → H2O(l); ΔH = -68.3 kcal
(d) C(graphite) + O2(g) → CO2(g); ΔH = -94.0 kcal
Ethylene can be obtained from ethane by reaction (a) (dehydrogenation) or by the oxidative route C2H6(g) + 1/2 O2(g) → C2H4(g) + H2O(l), which is obtained by adding (a) and (c). Which of the following is correct regarding these routes, for the same mass of ethylene formed?
Options:
Answer:
Solution:
Both routes yield 1 mol (28 g) of C₂H₄.
For dehydrogenation, the reactants require 30 g of ethane. For the oxidative route, the reactants require 30 + 16 = 46 g. Therefore, dehydrogenation requires less reactant mass (option b is true, option a is false).
Regarding heat: dehydrogenation has a value of +32.7 kcal (heat is absorbed, so option d is false). The oxidative route has a value of 32.7 + (-68.3) = -35.6 kcal (heat is released), so it does not absorb more heat (option c is false).
The correct answer is (b).
