S(s) + 3/2 O2(g) ➞ SO3(g) + 2x KJ SO2(g) + 1/2 O2(g) ➞ SO3(g) + y kJFind the heat of formation of SO2(g) .
Correct answer: C. (2x -y)
- A. (y - 2x)
- B. (x + y)
- C. (2x -y)
- D. 2x/y
Explanation
Reverse the second reaction and change the sign of its enthalpy change: SO3(g) + y kJ → SO2(g) + 1/2 O2(g) Multiply the first reaction by 2 to match the stoichiometry of the oxygen molecules in the reversed second reaction: 2(S(s) + 3/2 O2(g) → SO3(g) + 2x kJ) Add the two reactions together to get the desired reaction: 2(S(s) + 3/2 O2(g) → SO3(g) + 2x kJ) + (SO3(g) + y kJ → SO2(g) + 1/2 O2(g)) The intermediate terms cancel out: 2S(s) + 3O2(g) → 2SO3(g) + 2x kJ + y kJ Simplify the equation: 2S(s) + 3O2(g) → 2SO3(g) + (2x + y) kJ Now, we can isolate the desired reaction and the heat of formation of SO2(g): 2SO3(g) → 2S(s) + 3O2(g) + (2x + y)kJ The heat of the formation of SO2(g) from its elements in their standard states is equal to the value of (2x + y) kJ, hence option C is correct.
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Thermochemistry measures energy changes in chemical reactions and distinguishes exothermic from endothermic processes. Work covers systems, surroundings and state functions, internal energy, the first law of thermodynamics, enthalpy and Hess's law, including the sign conventions used when heat enters or leaves a system.
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