
(a)
Interpretation:
An expression for the difference between the change in enthalpy and the change in internal energy for a gas phase process has to be determined assuming all the species behave as perfect gas.
Concept introduction:
Kirchhoff’s law:
This law states that the variation of change of enthalpy of a reaction with temperature at constant pressure is equal to the change in specific heat capacity at constant temperature of the system.
Internal energy:
Internal energy of a system is the total energy contained in the system. It keeps an account for the loss and gain of energy of the system due to changes in internal state. It is dependent on temperature and pressure.
It is denoted as U.
From 1st law of
ΔU = W +Q
Where,
ΔU = Change in the internal energy of system.
W = Energy transferred as the form of work to system
Q = Energy transferred as the form of heat to system
Enthalpy:
Enthalpy is a property of a thermodynamic system that is equal to the sum of the internal energy of the system and the product of pressure and volume. For a closed system where transfer of matter between system and surroundings is prohibited, for the processes that occur at constant pressure, the heat absorbed or released equals to the change in enthalpy.
It is denoted as H. The molar enthalpy is defined as the enthalpy per mole. It is denoted as Hm .
From thermodynamics,
H = U + PV
Where,
H = enthalpy of system
U = internal energy of system
P = pressure of system
V = volume of system
Specific heat capacity at constant pressure:
Specific heat capacity at constant pressure can be defined as the amount of energy required to increase the temperature of a substance by 1οC.
It is denoted as Cp. The molar specific heat capacity is defined as the specific heat capacity per mole and it is denoted by Cp,m.
Cp = (∂H∂T)p
Heat capacity at constant volume:
Specific heat capacity at constant volume is defined as the amount of heat required to increase the temperature by 1οC keeping the system at constant volume.
It is denoted as Cv
Cv = (∂U∂T)v
U = internal energy of system
T = temperature
(a)

Explanation of Solution
Given that for perfect gas Cp = 52R.
Applying the Kirchhoff’s law,
d(ΔH)P = ΔCPdT
d(ΔU)v = ΔCvdT
Thus by integration of both,
ΔH = ΔCpT ΔU = ΔCvT
Thus the difference is,
ΔH - ΔU = (ΔCp - ΔCv)TΔH - ΔU = (52 - 32)RT [Cp - Cv = R]ΔH - ΔU = RT
The difference between the change in enthalpy and the change in internal energy for a gas phase process is RT.
(b)
Interpretation:
For ionization ΔHion - ΔUion = 52RT that has to be established.
Concept introduction:
Ionization enthalpy:
Ionization enthalpy is the energy required to remove an electron from a gaseous atom or ion
Ionization energy:
Ionization energy is defined as the minimum amount of energy required to remove the most loosely bound valence electron of an isolated neutral gas molecule.
Kirchhoff’s law:
This law states that the variation of change of enthalpy of a reaction with temperature at constant pressure is equal to the change in specific heat capacity at constant temperature of the system.
(b)

Explanation of Solution
According to Kirchhoff’s law,
d(ΔH)P = ΔCPdT
Applying integration,
H(T)∫H(0)(ΔH)P = ΔCPT∫0dT ΔH(T) - ΔH(0) = ΔCPTΔHion - ΔUion = 52RT [ΔCP = 52RΔUion = ionisation energy]
Thus it has been established that, ΔHion - ΔUion = 52RT.
(c)
Interpretation:
Difference between the standard enthalpy of ionization of Ca(g) to Ca2+(g) and the accompanying change in internal energy at 25οC has to be calculated.
Concept introduction:
Ionization enthalpy:
Ionization enthalpy is the energy required to remove an electron from a gaseous atom or ion
(c)

Answer to Problem 2.10PR
The difference between the standard enthalpy of ionization of Ca(g) to Ca2+(g) and the accompanying change in internal energy at 25οC is 6193.93 J mol−1.
Explanation of Solution
From part (b) it can be concluded that,
ΔHion - ΔUion = 52RT
According to the question, difference between the standard enthalpy of ionization of Ca(g) to Ca2+(g) and the accompanying change in internal energy at 25οC has to be calculated.
Given that,
T = (25+273) KT = 298 K
Hence,
ΔHion - ΔUion = 52RTΔHion - ΔUion = 52×8.314 J mol−1 K−1×298 KΔHion - ΔUion = 6193.93 J mol−1
Hence the difference between the standard enthalpy of ionization of Ca(g) to Ca2+(g) and the accompanying change in internal energy at 25οC is 6193.93 J mol−1.
(d)
Interpretation:
For electron gain ΔHeg - ΔUeg = −52RT has to be established.
Concept introduction:
Electron gain enthalpy:
Electron gain enthalpy can be defined as the amount of energy released when an electron is added to an isolated gaseous atom for electron gain enthalpy the sign convention is negative.
Electron affinity can be defined as the change in energy of a neutral atom in the gaseous phase when an electron is added to the atom to form the negative ion.
Kirchhoff’s law:
This law states that the variation of change of enthalpy of a reaction with temperature at constant pressure is equal to the change in specific heat capacity at constant temperature of the system.
(d)

Explanation of Solution
According to Kirchhoff’s law,
d(ΔH)P = ΔCPdT
Applying integration,
H(T)∫H(0)(ΔH)P = ΔCPT∫0dT ΔH(T) - ΔH(0) = ΔCPTΔHeg - ΔUeg = −52RT [ΔCP = −52R due to electron gainΔUeg = electron affinity]
Thus it has been established that, ΔHeg - ΔUeg = −52RT.
(e)
Interpretation:
Difference between the standard electron gain enthalpy of Br(g) and the corresponding change in the internal energy at 25οC has to be calculated.
Concept introduction:
Electron gain enthalpy:
Electron gain enthalpy can be defined as the amount of energy released when an electron is added to an isolated gaseous atom for electron gain enthalpy the sign convention is negative.
(e)

Answer to Problem 2.10PR
The difference between the standard electron gain enthalpy of Br(g) and the corresponding change in the internal energy at 25οC is −6193.93 J mol-1.
Explanation of Solution
From the part (d) it can be concluded that,
ΔHeg - ΔUeg = −52RT
Now according to the question the difference between the standard electron gain enthalpy of Br(g) and the corresponding change in the internal energy at 25οC has to be calculated.
Given that,
T = (25+273) KT = 298 K
Hence,
ΔHeg - ΔUeg = −52RTΔHeg - ΔUeg = −52×8.314 J mol-1 K-1×298 KΔHeg - ΔUeg = −6193.93 J mol-1
Hence the difference between the standard electron gain enthalpy of Br(g) and the corresponding change in the internal energy at 25οC is −6193.93 J mol-1.
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Chapter 2 Solutions
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