First Law Of Thermodynamics
Easychemistry
A gas absorbs 120 J of heat and expands against a constant external pressure, performing 50 J of work on the surroundings. The change in internal energy (ΔU) of the gas is:
Select the correct option:
Solution
Incorrect! Answer:
+70 J
- State the First Law: Δcup=q+w. By IUPAC sign convention, heat absorbed by the system is positive and work done by the system is negative.
- Assign Values: q=+120 J (heat absorbed); w=−50 J (expansion work done by the gas).
- Calculate: Δcup=120+(−50)=+70 J.
- Interpret: The internal energy rises by 70 J — the system retained the portion of absorbed heat that was not used for expansion work.
- Why Not the Others?
- +170 J would result from incorrectly adding q and |w|.
- −70 J reverses the sign of ΔU.
- −170 J has no physical basis for this scenario.
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About This Question
- Subject
- chemistry
- Chapter
- chemical thermodynamics
- Topic
- first law of thermodynamics
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
+70 J
- State the First Law: Δcup=q+w. By IUPAC sign convention, heat absorbed by the system is positive and work done by the system is negative.
- Assign Values: q=+120 J (heat absorbed); w=−50 J (expansion work done by the gas).
- Calculate: Δcup=120+(−50)=+70 J.
- Interpret: The internal energy rises by 70 J — the system retained the portion of absorbed heat that was not used for expansion work.
- Why Not the Others?
- +170 J would result from incorrectly adding q and |w|.
- −70 J reverses the sign of ΔU.
- −170 J has no physical basis for this scenario.
This easy difficulty chemistry question is from the chapter chemical thermodynamics, covering the topic of first law of thermodynamics. It appeared in the 2025 exam.
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