Multiple Choice
Which concept is described by the first law of thermodynamics?
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8. Thermochemistry
First Law of Thermodynamics
Multiple Choice
In which of the following scenarios would the change in internal energy (ΔU) of a system be negative according to the First Law of Thermodynamics?
A
A gas expands against a constant external pressure and does work on the surroundings.
B
No heat is exchanged, and the system does no work.
C
A system absorbs heat from the surroundings without doing any work.
D
A gas is compressed by the surroundings, and heat is absorbed by the system.
Verified step by step guidance1
Recall the First Law of Thermodynamics, which states that the change in internal energy of a system (\( \Delta U \)) is given by the equation:
\[ \Delta U = q + w \]
where \( q \) is the heat exchanged with the surroundings and \( w \) is the work done on the system.
Understand the sign conventions:
- Heat absorbed by the system is positive (\( q > 0 \)), heat released is negative (\( q < 0 \)).
- Work done on the system is positive (\( w > 0 \)), work done by the system on the surroundings is negative (\( w < 0 \)).
Analyze each scenario by identifying the signs of \( q \) and \( w \):
- For a gas expanding against constant external pressure and doing work on the surroundings, \( w < 0 \) (work done by system), and if no heat is exchanged, \( q = 0 \). Thus, \( \Delta U = 0 + (negative) = negative \).
- For no heat exchange and no work done, both \( q = 0 \) and \( w = 0 \), so \( \Delta U = 0 \).
- For heat absorbed without work, \( q > 0 \) and \( w = 0 \), so \( \Delta U > 0 \).
- For gas compressed by surroundings (work done on system, \( w > 0 \)) and heat absorbed (\( q > 0 \)), both terms are positive, so \( \Delta U > 0 \).
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