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Thermodynamic property relations IP University Thermal science

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Thermodynamic Property Relations
Properties of pure substances
Maxwell Relations
P-v diagram of a pure substance
pvt surface diagram
How to read steam table using an example

Thermodynamic Property Relations
Properties of pure substances
Maxwell Relations
P-v diagram of a pure substance
pvt surface diagram
How to read steam table using an example

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Thermodynamic property relations IP University Thermal science

  1. 1. Unit 2 1) Thermodynamic Property Relations & 2) Properties of pure substances By Ankit Saxena
  2. 2. Some Mathematical theorem
  3. 3. Maxwell Relations
  4. 4. Gibbs function The gibbs function is useful when evaluating the availability of system in which Chemical reaction occurs. The change in availability of a system is equal to the change in the gibbs function of the system at constant temperature and Pressure.
  5. 5. Helmholtz function
  6. 6. Maxwell relation Cont..
  7. 7. Pure Substances? • A substance that has a fixed chemical composition throughout is called a pure substance. • A pure substance does not have to be of a single chemical element or compound, however. A mixture of various chemical elements or compounds also qualifies as a pure substance as long as the mixture is homogeneous. • A mixture of liquid and water vapor is a pure substance, but a mixture of liquid and gaseous air is not.
  8. 8. PHASE CHANGE PROCESSES OF PURE SUBSTANCES • Compressed liquid (sub-cooled liquid) A substance that it is not about to vaporise. • Saturated liquid A liquid that is about to vaporise. • Saturated vapour A vapour that is about to condense. • Saturated liquid–vapour mixture The state at which the liquid and vapour phases coexist in equilibrium. • Superheated vapour A vapour that is not about to condense (i.e., not a saturated vapour)
  9. 9. Let's consider the results of heating liquid water from 20 degree C, 1 atm. while keeping the pressure constant  We will follow the constant pressure process  First place liquid water in a piston-cylinder device where a fixed weight is placed on the piston to keep the pressure of the water constant at all times  As liquid water is heated while the pressure is held constant, the following events occur
  10. 10. If the entire process between state 1 and 5 described in the figure is reversed by cooling the water while maintaining the pressure at the same value, the water will go back to state 1, retracing the same path, and in so doing, the amount of heat released will exactly match the amount of heat added during the heating process. T-v diagram for the heating process of water at constant pressure SATURATED TEMPERATURE AND SATURATED PRESSURE? The temperature at which water starts boiling depends on the pressure; therefore, if the pressure is fixed, so is the boiling temperature. Water boils at 100°C at 1 atm pressure. Saturation temperature Tsat The temperature at which a pure substance changes phase at a given pressure. Saturation pressure Psat The pressure at which a pure substance changes phase at a given temperature.
  11. 11. • Latent heat  The amount of energy absorbed or released during a phase- change process. • Latent heat of fusion  The amount of energy absorbed during melting. It is equivalent to the amount of energy released during freezing. • Latent heat of vaporisation  The amount of energy absorbed during vaporisation and it is equivalent to the energy released during condensation.  The magnitudes of the latent heats depend on the temperature or pressure at which the phase change occurs.  At 1 atm pressure, the latent heat of fusion of water is 333.7 kJ/kg and the latent heat of vaporisation is 2256.5 kJ/kg.  The atmospheric pressure, and thus the boiling temperature of water, decreases with elevation.
  12. 12. Property diagrams for phase-change processes The variations of properties during phase-change processes are best studied and understood with the help of property diagrams such as the T- v, P-v, and P-T diagrams for pure substances.
  13. 13. T-v diagram of constant pressure phase-change processes of a pure substance at various pressures (numerical values are for water). Critical Point: The point at which the saturated liquid and saturated vapour states are identical. At supercritical pressures (P > Pcr), there is no distinct phase-change (boiling) process.
  14. 14. P-v diagram of a pure substance Fig: P-v diagram of a pure substance
  15. 15. Extending the diagrams to include the Solid Phase P-v diagram of a substance that contracts on freezing P-v diagram of a substance that expands on freezing (Such as water)
  16. 16. P-T diagram P-T diagram is often called the phase diagram since all three phases are separated from each other by three lines. Solid and vapor region is separated by Sublimation line. Liquid and vapor region is separated by vaporization line. And the melting or fusion line separates the solid and liquid regions. These three lines meet at the triple point, where all three phases coexist in equilibrium. Substances that expand or contract on freezing differ only in the melting line on P-T diagram.

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