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    Home > Chemicals Industry > Chemical Technology > Basic Concepts of Thermodynamics (2)

    Basic Concepts of Thermodynamics (2)

    • Last Update: 2021-06-18
    • Source: Internet
    • Author: User
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    4.
    Volume work

    During chemical reactions, volume changes often occur
    .
    Caused by the volume change of the work environment system called volume work done, generally indicated by W is


    .


    As shown in Figure 2-2, the piston is used to seal the gas in the cylindrical cylinder, and the gas expansion opposes the external force F to push the piston from position I to position II, and the displacement is △ l
    .
    If the mass of the piston itself and the friction between the piston and the cylinder wall are ignored, the work done by the piston against the external force F is

    W=F·△l

    Suppose the cross-sectional area of ​​the piston is S, then there is

    Figure 2-2 Schematic diagram of work done by volume change

    In the formula, it is the external pressure p resisted when the volume expands; (S△l) is the volume change △V
    .
    The system does work to the environment, and the work is negative, so

    W=-p·△V

    From the above definition of volume work, it can be seen that if the external pressure p=0 or the volume change △V=0, the volume work W=0
    .
    The systems and processes studied in this chapter do not do non-volume work, that is, the work done by the system change process is all volume work


    .


    From the unit of p, Pa, and the unit of △V, m 3 , the unit of volume work can be derived

    Pa·m 3 =(N·m -2 )·m 3 =N·m=J

    Therefore, the unit of volume work is J or kJ
    .

    [Example 2-1] Ideal gas expands from p 1 =16×10 5 Pa, V 1 =1dm 3 to p 2 =1×10 5 Pa, V 2 =16dm 3 under constant temperature conditions , the process is as shown in Figure 2-1 The three ways to complete, find the volume work of each way
    .

    Solve the ideal gas expansion, the system does work to the environment, and the volume work is a negative value

    W=-p·△V

    (1) Route F first resists external pressure 8×10 5 Pa expansion, and then resists external pressure 1×10 5 Pa expansion

    W F =[-8×10 5 Pa×(2-1)×10 -3 m 2 ]+[-1×10 5 Pa×(16 -2 )×10 -3 m 3 ]

    =-2200J

    (2) Path G resists external pressure 1×10 5 Pa one time expansion

    W G =-1×10 5 Pa×(16-1)×10 -3 mm=-1500J

    (3) Route H first resists external pressure 4×10 5 Pa expansion, and then resists external pressure 1×10 5 Pa expansion

    W H =[-4×10 5 Pa×(4-1)×10 -3 m 3 ]+[-1×10 5 Pa×(16 -4 )×10 -3 m 3 ]

    =-2400J

    It can be seen that volume work is a physical quantity related to a pathway, not a state function, and the value of work performed by different pathways may be different
    .

    Volume work can also be calculated using the pV diagram method
    .
    The external pressure p external is plotted against the volume V of the system, and the resulting curve is called the pV line


    .


    Figure 2-3 Use pV diagram to show volumetric work

    Figure 2-3 shows the path F in Figure 2-1 as a pV line
    .
    Point A represents the initial state, V 1 =1×10 -3 m 3 , p 1 =16×10 5 Pa; Point E represents the final state, V 2 =16×10 -3 m 3 , p 2 =1×10 5 Pa


    .


    S=8×(2-1)+1×(16-2)=22 (unit area)

    5.
    Thermodynamic energy

    Thermodynamic energy is also called internal energy, which is the sum of all energy in the system


    .


    Although the thermodynamic energy of the system cannot be obtained yet, the thermodynamic energy of the system is a fixed value when the state of the system is certain
    .
    Therefore, the thermodynamic energy U is the state function of the system


    .


    △U=U end-U beginning

    Thermodynamic energy is a measuring property of the system, and it is additive
    .

    Ideal gas is the simplest system, and its thermodynamic energy is only a function of temperature
    .
    If the temperature does not change, the thermodynamic energy of the system does not change, that is, the amount of change in the thermodynamic energy of the system is zero (△T=0, then △U=0)


    .


    Related Links: Basic Concepts of Thermodynamics (1)

     

     

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