Free Thermochemistry and Energetics of Chemical Reactions MCQs with Answers
22 Thermochemistry and Energetics of Chemical Reactions MCQs from Chemistry, each with the correct answer and a written explanation of why it is correct. Free and unlimited, with no account needed.
22 questions · page 3 of 3
21. Majority of reactions taking place at ordinary temperatures with ssH are
- A. Endothermic
- B. Exothermic
- C. Thermally unstable
- D. Reversible
Explanation: The symbol in the original stem is the enthalpy change, which did not print cleanly in the paper. Reactions that proceed readily at ordinary temperature without a continuous supply of heat are overwhelmingly those that release energy, that is exothermic ones with a negative enthalpy change. Endothermic reactions generally need heating to be maintained, which is why combustion and neutralisation happen spontaneously while thermal decomposition does not.
Correct answer: Exothermic22. If 100 kJ of heat is absorbed by the system and 40 kJ of work is done on the system, what is the change of internal energy?
- A. -60 kJ
- B. +60 kJ
- C. -140 kJ
- D. +140 kJ
Explanation: By the first law, the change in internal energy is the heat added to the system plus the work done on it, so 100 plus 40 gives plus 140 kJ. Both inputs raise the internal energy, which is why they add rather than cancel. Had the system done 40 kJ of work on the surroundings instead, the answer would have been plus 60 kJ, which is the distractor offered.
Correct answer: +140 kJ