For which of the following reactions the change in enthalpies is equal to the change in internal energy:
Correct answer: A. H2 + I2 ➞ 2HI
- A. H2 + I2 ➞ 2HI
- B. 2H2O2 ➞ 2H2O + O2
- C. PCl5 ➞ PCl3 + Cl2
- D. C + O2 ➞ CO2
Explanation
The correct answer is H2 + I2 ➞ 2HI. In this reaction, the change in the number of moles of gas (∆n) is zero, so the change in enthalpy (∆H) is equal to the change in internal energy (∆U). For reactions where ∆n is not zero, the enthalpy change is not equal to the internal energy change. For instance, in the reaction 2H2O2 ➞ 2H2O + O2, ∆n is not zero due to the production of gas. Similarly, in PCl5 ➞ PCl3 + Cl2, and C + O2 ➞ CO2, the conditions for ∆H = ∆U are not met because of changes in the number of gaseous moles or significant heat of formation values.
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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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