When a gas is heated, it absorbs 167 joules of heat from the surroundings. At th
ID: 567287 • Letter: W
Question
When a gas is heated, it absorbs 167 joules of heat from the surroundings. At the same time, the gas expands, doing pressure-volume work (P · V work) on the surroundings that is equal to 303 joules.
(a) Determine whether the amounts of heat (q) and work (w) exchanged should have positive or negative signs.
heat (q)
positivenegative
work (w)
positivenegative
(b) Calculate the change in internal energy (E) of the gas.
J
(c) Determine whether one or more of the following is a state function: internal energy (E) of a system, change in internal energy (E) of a system, heat (q) absorbed or released by a system, work (w) done on or by a system. (Select all that apply.)
internal energy (E) of a system
change in internal energy (E) of a system
heat (q) absorbed or released by a system
work (w) done on or by a system
Explanation / Answer
(a) The first law of thermodynamics is given as
E = Q – W
where E = change in internal energy of the system; Q = heat change of the system and W = work involved in the change.
Heat is absorbed by the system; hence the sign of Q is positive. The system does work on the surroundings, hence the sign of W is negative (ans).
(b) Plug in the values of Q and W in the first law expression with appropriate sign and obtain
E = (+167 J) – (-303 J) = 470 J (ans).
(c) A state function is a thermodynamic function that depends only on the initial and final states of the system. Internal energy (E) and hence, change in internal energy are state functions.
A path function is a thermodynamic function that depends on the path taken by the system to reach from one state to another state. Heat absorbed or released by a system and work done on or by a system are path functions.
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