Math Problem Statement

Calculate the internal energy and enthalpy changes that occur when air is changed from an initial state of 277 K and 10 bar where molar volume is 2.28 m^3/kmol to a final state 333 K and 1 atm. Assume air follows PV/T = constant and Cv = 21 and Cp = 29.3 kJ/kmol·K.

Solution

To calculate the internal energy (ΔU\Delta U) and enthalpy (ΔH\Delta H) changes for air, given the conditions in the image, we'll use the following formulas for an ideal gas:

  1. Change in internal energy: ΔU=nCvΔT\Delta U = n C_v \Delta T where:

    • nn is the number of moles (not directly given, but we assume conditions are per kmol),
    • CvC_v is the specific heat at constant volume (21 kJ/kmol·K),
    • ΔT\Delta T is the temperature change, ΔT=T2T1\Delta T = T_2 - T_1.
  2. Change in enthalpy: ΔH=nCpΔT\Delta H = n C_p \Delta T where:

    • CpC_p is the specific heat at constant pressure (29.3 kJ/kmol·K),
    • ΔT=T2T1\Delta T = T_2 - T_1.

Given dataTo calculate the internal energy (ΔU\Delta U) and enthalpy (ΔH\Delta H) changes for air, given the conditions in the image, we'll use the following formulas for an ideal gas:

  1. Change in internal energy: ΔU=nCvΔT\Delta U = n C_v \Delta T where:

    • nn is the number of moles (not directly given, but we assume conditions are per kmol),
    • CvC_v is the specific heat at constant volume (21 kJ/kmol·K),
    • ΔT\Delta T is the temperature change, ΔT=T2T1\Delta T = T_2 - T_1.
  2. Change in enthalpy: ΔH=nCpΔT\Delta H = n C_p \Delta T where:

    • CpC_p is the specific heat at constant pressure (29.3 kJ/kmol·K),
    • ΔT=T2T1\Delta T = T_2 - T_1.

Given

  • Initial temperature T1=277KT_1 = 277 \, \text{K},
  • Final temperature T2=333KT_2 = 333 \, \text{K},
  • Initial pressure P1=10barP_1 = 10 \, \text{bar},
  • Final pressure P2=1atm1.013barP_2 = 1 \, \text{atm} \approx 1.013 \, \text{bar},
  • Initial molar volume Vm=2.28m3/kmolV_m = 2.28 \, \text{m}^3/\text{kmol},
  • Cv=21kJ/kmol.KC_v = 21 \, \text{kJ/kmol.K},
  • Cp=29.3kJ/kmol.KC_p = 29.3 \, \text{kJ/kmol.K}.

Steps:

  1. Calculate temperature change: ΔT=T2T1=333K277K=56K\Delta T = T_2 - T_1 = 333 \, \text{K} - 277 \, \text{K} = 56 \, \text{K}

  2. Change in internal energy (ΔU\Delta U): ΔU=nCvΔT=(1kmol)×21kJ/kmol.K×56K\Delta U = n C_v \Delta T = (1 \, \text{kmol}) \times 21 \, \text{kJ/kmol.K} \times 56 \, \text{K} ΔU=1176kJ\Delta U = 1176 \, \text{kJ}

  3. Change in enthalpy (ΔH\Delta H): ΔH=nCpΔT=(1kmol)×29.3kJ/kmol.K×56K\Delta H = n C_p \Delta T = (1 \, \text{kmol}) \times 29.3 \, \text{kJ/kmol.K} \times 56 \, \text{K} ΔH=1640.8kJ\Delta H = 1640.8 \, \text{kJ}

Thus, the internal energy change is 1176kJ1176 \, \text{kJ}, and the enthalpy change is 1640.8kJ1640.8 \, \text{kJ}.

Would you like more details on any part of the solution? Here are a few related questions to consider:

  1. How would the calculation change if the specific heats were temperature-dependent?
  2. What assumptions are made about the ideal gas behavior in this calculation?
  3. How does the change in pressure affect the internal energy and enthalpy?
  4. What would happen if the process involved heat exchange with the surroundings?
  5. How does the molar volume relate to the ideal gas law in this problem?

Tip: For ideal gases, internal energy depends only on temperature, not pressure.

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Math Problem Analysis

Mathematical Concepts

Thermodynamics
Ideal Gas Law
Heat Capacity

Formulas

ΔU = n * Cv * ΔT
ΔH = n * Cp * ΔT

Theorems

Ideal Gas Law
First Law of Thermodynamics

Suitable Grade Level

Undergraduate