15) For the following processes, state whether each of the four thermodynamic qu
ID: 885869 • Letter: 1
Question
15) For the following processes, state whether each of the four thermodynamic quantities q, w, U, H is greater than, equal to or less than zero for the system described. Consider all gases to behave ideally. State explicitly any reasonable assumptions that you may need to make.
a)Two copper bars, one initially at 80 degrees celsius and the other initially at 20 degrees celsius, are brought into contact with each other in a thermally insulated compartment and then allowed to come to equilibrium.
b) A sample of liquid in a thermally insulated container (a calorimeter) is stirred for 1 hour by a mechanical linkage to a motor in the surroundings
c) A sample of H2 gas is mixed with an equimolar amount of N2 gas at the same temperature and pressure under conditions where no chemical reaction occurs between them.
Explanation / Answer
15) thermodynamic quantities q (heat), w (work), dH (enthalpy) and dU (internal energy)
a) two copper bars one at 80 oC and another at 20 oC brought in contant with each other and allowed to equilibriate in a thermally insulated container.
since no heat eachange between system (two bars) and the surrounding occurs (thermally insulated)
q = 0
first law states,
dU = q + w
so,
dU = w
as dV (change in volume) = 0 for the system [solids do not have appreciable volume change, only gases does]
w = PdV = 0
therefore,
dU = 0
and also dH = 0 since no change in internal energy dU occurs here.
b) A sample of liquid in thermally insulated container is stirred for 1 h by a mechanical linkage attached to a motor in the surrounding.
as the system is thermally insulated,
q = 0
work is done on the system by mechanical linkage, so,
w = greater than zero
dU = greater than zero [internal energy also increases for such system]
dH = greater than zero [enthalpy increase with interna energy here]
c) A sample of H2 gas mixed with N2 gas at same temoerature and pressure, no reaction occured
no heat change
q = 0
w = greater than zero [overall volume increases]
dU = increases [as w is increased]
dH = increases [with increase in volume]
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