
(i)
Interpretation: The values of q, w, ΔU and ΔH have to be calculated at constant pressure when the temperature is raised from 25 οC to 100 οC.
Concept Introduction: The system is said to have gone through a process when the initial state is changed to the final state. The properties of the system such as temperature, pressure, enthalpy, entropy are changes in thermodynamic process.
(i)

Answer to Problem 2B.3AE
The value of value of q and ΔH at constant pressure is 10.73kJ_.
The value of w at constant pressure is −0.62355kJ_.
The value of ΔU at constant pressure is 10.10kJ_.
Explanation of Solution
It is given that the constant-pressure heat capacity of a sample of a perfect gas varies with temperature according to the expression shown below.
Cp/(J K−1)=20.17+0.3665(T/K) (1)
The temperature is raised from 25 οC to 100 οC.
The conversion of Celsius to Kelvin is done as,
0 οC=273 K
Therefore, the conversion of 25 οC to Kelvin is done as,
25 οC=25+273 K=298 K
Similarly, the conversion of 100 οC to Kelvin is done as,
100 οC=100+273 K=373 K
At constant pressure,
q=ΔH=T2∫T1CpdT (2)
Substitute the value of heat capacity at constant pressure from equation (1) to equation (2).
ΔH=T2∫T1CpdT=T2∫T120.17dT+T2∫T10.3665 TdT=20.17(T2−T1)+0.3665(T222−T212)
Now, substitute the values of initial and final temperature to calculate value of q and ΔH.
ΔH=20.17(373−298)+0.3665((373)22−(298)22)=20.17×75+0.18325(139129−88804)=10734.80 J=10.73kJ_
Therefore, the value of value of q and ΔH at constant pressure is 10.73kJ_ respectively.
The work done for a perfect gas is calculated by the formula,
w=−pdV=−nRdT=−(1)RdT=−RdT (3)
Substitute the values of universal gas constant and temperature change in the above equation to calculate the work done.
w=−8.314 J mol−1 K−1×(373 K−298 K)=−8.314 J mol−1 K−1×75 K=−623.55 J=−0.62355kJ_
Therefore, the work done at constant pressure is −0.62355kJ_.
The first law of thermodynamics is given by the expression,
ΔU=q+w (4)
Substitute the values of q and w in the above formula to calculate the internal energy.
ΔU=10.73 kJ+(−0.62355 kJ)=10.73 kJ−0.62355 kJ=10.10kJ_
Hence, the value of ΔU is 10.10kJ_.
(ii)
Interpretation: The values of q, w, ΔU and ΔH have to be calculated at constant volume when the temperature is raised from 25 οC to 100 οC.
Concept Introduction: The system is said to have gone through a process when the initial state is changed to the final state. The properties of the system such as temperature, pressure, enthalpy, entropy are changes in thermodynamic process.
Thermodynamics address temperature and heat and their relationship to work, energy and matter. Work is defined as the form of mechanical energy that is transferred from system to another.
(ii)

Answer to Problem 2B.3AE
The value of ΔU at constant volume is 10.1112kJ_.
The value of value of ΔH at constant volume is 10.73475kJ_.
At constant volume, w=0.
The value of q at constant volume is 10.1112kJ_.
Explanation of Solution
At constant volume,
Cp−Cv=nR (5)
For a perfect gas,
n=1
Therefore,
Cp−Cv=RCv=Cp−R
Substitute the value of heat capacity at constant pressure in the above formula to calculate the heat capacity at constant volume.
Cv=Cp−RCv=20.17+0.3665×T−8.314Cv=11.856+0.3665×T
Now,
Cv=(∂U∂T)v∂U=∫CvdT (6)
Substitute the values of heat capacity at constant volume and integrate it.
∂U=T2∫T1(11.856+0.3665×T)dT=T2∫T111.856dT+T2∫T10.3665×TdT=11.856(T2−T1)+0.3665(T222−T212)=11.856(373 K−298 K)+0.3665((373)22−(298)22)
Simplify the above equation,
ΔU=11.856×75+0.18325(139129−88804)=889.2+9222.0=10111.2 J=10.1112kJ_
Hence, the value of ΔU is 10.1112kJ_.
The relation between change in enthalpy, change in internal energy, and change in volume is,
ΔH=ΔU+Δ(pV) (7)
Where,
- Ø ΔH is the change in enthalpy.
- Ø ΔU is the change in internal energy.
- Ø ΔV is the change in volume.
- Ø p is the pressure.
The equation (7) is also written as,
ΔH=ΔU+Δ(pV)ΔH=ΔU+ΔpV+pΔV
At constant volume, pΔV=0. Therefore,
ΔH=ΔU+ΔpVΔH=ΔU+V(nRT2V−nRT1V)ΔH=ΔU+nR(T2−T1)
Substitute the values of ΔU, universal gas constant and temperatures in the above formula to calculate the change in enthalpy.
ΔH=10111.2 J+1×8.314 J mol−1 K−1(373 K−298 K)=10111.2 J+8.314 J mol−1 K−1×75 K=10734.75 J=10.73475kJ_
Hence, the value of value of ΔH is 10.73475kJ_.
At constant volume, w=0. Now, the value of q is calculated as,
ΔU=q+0ΔU=qq=10.1112kJ_
Hence, the value of q is 10.1112kJ_.
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Chapter 2 Solutions
Atkins' Physical Chemistry
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