For the following three processes, state whether each of the four thermodynamic

Question

For the following three processes, state whether each of the four thermodynamic quantities q, w, Delta U, and Delta? is greater than, equal to, or less than zero for the system described. Briefly explain your reasoning for each answer. (Adapted from Tenneco, 2.15) Two copper bars, one initially at 80 degree C and the other initially at 20 Degree C are brought into contact with each other in a thermally insulated compartment and then allowed to come to equilibrium. 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. An ideal gas is compressed in a frictionless piston to half its initial volume at constant pressure. An ideal gas is compressed in a frictionless piston to half its initial volume at constant temperature.

Explanation / Answer

a. from 1st law of thermodynamics, deltaU= Q-W

since the system is insulated, Q=0 and there is no work, W= 0 and hence deltaU=0 change in intenral energy =0 and

and deltaH= change in enthalpy = 0

b. since the container is insulated, q=0 and delaU= W change in internal energy >0 and hence W>0.

change in enthalpy = mcp*(T2-T1), Where T2= temperature before stirring and T1= temperature after stirring. T2>T1. deltaH>0.

3. This is isobaric process. work done = -PdV = -P*(V2-V1). V2=1/2V2

Work done = +ve. Q= deltaH=n*Cp*(T2-T1), since the gas is compressed T2>T1. Q= +ve and hence deltaH is +ve.

deltaU= nCv*dT. deltaT is +ve. deltaU is also +ve.

4. for ideral gas at constant temperature, deltaU= 0 and deltaH=0,

deltaU= 0 = Q-W or Q=W

W= -integral pdV or = nRT* ln (V1/V2) or nRT ln(P2/P1)

since P2>P1, W is +ve and Q is +ve. Anything added to the system is +ve. and leaving is -ve.

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