The enthalpy of atomization of C is q₁, the bond energy of H₂ is q₂, and the bond energy of a C−H bond is q₃. The ∆H for the reaction C + 2H₂ → CH₄ is:
Correct answer: C. q1 + 2q2 − 4q3
- A. 4q3 − q1 − 2q2
- B. q1 + q2 − q3
- C. q1 + 2q2 − 4q3
- D. q1 + 4q2 − 4q3
Explanation
The enthalpy of formation is calculated as the energy required to break bonds minus the energy released from forming bonds. Energy in = (atomization of 1 C) + (breaking 2 H-H bonds) = q₁ + 2q₂. Energy out = (forming 4 C-H bonds) = 4q₃. The net enthalpy change is ΔH = q₁ + 2q₂ - 4q₃.
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About Thermochemistry and Energetics of Chemical Reactions
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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