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Enthalpy of vaporization of liquid

A presentation on how to determine the Enthalpy of Vaporization using specific laboratory apparatus.

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Enthalpy of vaporization of liquid

  1. 1. A detailed approach
  2. 2. Presented by Ahmed Asad – CIIT/SP10-BEC-003/LHR Muhammad Usama – CIIT/SP10-BEC-017/LHR Mohammad Abubakar – CIIT/SP10-BEC-022/LHR Noaman Ahmed – CIIT/SP10-BEC-037/LHR Saad Wazir – CIIT/SP10-BEC-043/LHR Saim Khan – CIIT/SP10-BEC-044/LHR Waqar Farooq – CIIT/SP10-BEC-050/LHR
  3. 3. Presentation Outline Restatement of first law of thermodynamics Definition of enthalpy Some common enthalpy changes Enthalpy of vaporization Characteristics of enthalpy of vaporization Physical model for vaporization Experimental determination Sample readings and calculations Applications
  4. 4. First Law of Thermodynamics Energy conservation law Describeschange in internal energy of a thermodynamic system Clausius’ statement: In a thermodynamic process, the increment in the internal energy of a system is equal to the difference between the increment of heat accumulated by the system and the increment of work done by it.
  5. 5. First Law of Thermodynamics (contd.) In any incremental process, the change in the internal energy is considered due to,  Heat added to the system  Work done by the system dU = dQ - dW
  6. 6. First Law of Thermodynamics (contd.) For a quasistatic process (infinitely slow process), dU = dQ – PdV NO real process is quasistatic A quasistatic process ensures that the system will go through a sequence of states that are infinitesimally close to equilibrium (so the system remains in quasistatic equilibrium), in which case the process is typically reversible
  7. 7. Quasistatic and Reversibility Any reversible process is a quasistatic process Any quasistatic process may not be reversible  Due to heat flow  Due to entropy generation Example of an irreversible quasistatic process  Compression against a system with a piston subject to friction
  8. 8. Enthalpy Measure of total energy of a thermodynamic system A state function Includes  Internal energy (energy required to create a system)  Amount of energy required to establish system’s pressure and volume ΔH = ΔU + Δ(PV) SI Unit – Joule Other conventional units – Btu and Calories
  9. 9. Why Enthalpy is measured? Total enthalpy of a system can’t be measured directly Enthalpy change of a system is measured instead It is measured to,  Calculate “useful work” obtainable from a closed thermodynamic system under constant pressure  Determine nature of reaction e.g., exothermic or endothermic
  10. 10. Enthalpy is not necessarily heat !! Enthalpy is sometimes described as heat content of a system Heat is defined as thermal energy in transit For the description that enthalpy is in-fact heat to be valid, no energy exchange must occur with environment other than heat or expansion work
  11. 11. Common Enthalpy Changes Enthalpy of reaction Enthalpy of formation Enthalpy of combustion Enthalpy of neutralization Enthalpy of solution Enthalpy of vaporization Enthalpy of sublimation
  12. 12. Vaporization Phase transition from liquid phase to gas phase Two types  Evaporation  Occurs at temperatures below boiling temperature  Usually occurs on surface  Boiling  Occurs at or above boiling temperature  Occurs below the surface
  13. 13. Enthalpy of Vaporization (EOV) Enthalpy change required to completely change the state of one mole of substance between liquid and gaseous states Energy required to transform a given quantity of a substance from a liquid into a gas at a given pressure Usually measured at boiling point of a substance
  14. 14. Characteristics of Enthalpy ofVaporization It is temperature dependent EOV decreases with increase in temperature EOV diminishes completely at critical temperature beyond which liquid and vapor phase no longer co- exist Units – J/mol or kJ/mol, kJ/kg, Btu/lb, kcal/mol
  15. 15. Characteristics of Enthalpy ofVaporization (contd.) Enthalpy of condensation is same as enthalpy of vaporization but with opposite sign Enthalpy change of vaporization is always positive Enthalpy change of condensation is always negative
  16. 16. Temperature dependence of EOV
  17. 17. Physical model for vaporization Proposed by professor Jozsef Garai, Florida International University, USA Energy required to free an atom from liquid is equivalent to energy required to overcome surface resistance of liquid This model states,Latent heat = (Max. surface area) x (Surface tension) x (No. of atoms in liquid)
  18. 18. Physical model for vaporization(Diagrammatic representation)
  19. 19. Experimental determination
  20. 20. Apparatus Round bottom boiling flask Distillation condenser or multiple condensers Heat source (a burner or a heating mantle) A vacuum gauge (Bourdon type gauge) Aspirator or trapped vacuum pump Pressure-regulating device (a needle valve that is part of a Bunsen burner base) Thermometer
  21. 21. Basic Goal To determine boiling point of the liquid (water) under study at different pressure values To determine enthalpy of vaporization using the Clausius-Clapeyron relation
  22. 22. Clausius-Clapeyron relation A relation used to characterize a discontinuous phase transition between two phases of a single constituent On a P-T diagram, line separating two phases is known as coexistence curve This relation gives the slope of the tangents to this curve
  23. 23. Clausius-Clapeyron relation (contd.) General form dP/dT = L/TΔV  Where  dP/dT is slope of tangent to coexistence curve at any point  L is latent specific heat  T is temperature  ΔV is specific volume change of phase transition For transitions between a gas and condensed phase, the expression may be rewritten as, ln(P) = (-L/R) x (1/T) + C
  24. 24. Procedure Maintain lowest possible pressure by closing bleed valve Set water flow to the aspirator at maximum level to provide highest vacuum Place few boiling stones in round bottom flask to minimize bumping
  25. 25. Procedure (contd.) Temperature increases until boiling starts When boiling occurs, allow the thermometer reading to stabilize for 1 to 2 minutes and note the temperature Note the pressure reading on the manometer at this temperature
  26. 26. Procedure (contd.) Increase the pressure of the vessel by slightly opening the bleed valve Repeat the same procedure as described previously and then increase pressure again Take at least five readings and plot a graph between reciprocal of temperature and log of pressure difference
  27. 27. Sample ReadingsTemp (°C) h (mm Hg) Temp (K) (1/T) x 103 P (mm Hg) Log P (1/K) (P = 768 –h)41.5 710 314.5 3.18 58.0 1.7762.5 610 335.2 2.98 158 2.2075.0 500 348.0 2.87 268 2.4386.5 300 359.5 2.78 468 2.6792.2 220 365.2 2.74 548 2.74101.0 0 374.0 2.67 768 2.89
  28. 28. Graph between temperature andpressure Temperature and Log P 3.5 3 2.5 2Log P 1.5 1 0.5 0 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 1/T x 103
  29. 29. Calculations Molar latent heat / enthalpy of vaporization can be calculated from Clausius-Clapeyron relation as follows, ΔHv = -Rx[d(ln(P))/d(1/T)]] ln(P) = 2.303 log(P) Slope = m = d(ln(P))/d(1/T) ΔHv = -2.303(R)(m) - - - - Eq. (1)  Where R is ideal gas constant = 1.987 cal/K  m is slope of line obtained from graph
  30. 30. Calculations (contd.) Slope (m) can be obtained from linear regression A convenient method is to draw a trend-line on the graph and select the option to display an equation of line The equation of line of the sample experiment graph is, y = -2.233x+8.859 From equation, value of slope (m) = -2.233
  31. 31. Calculations (contd.) Applying values in equation 1 from previous slide ΔHv = 10.21 cal/mol Accepted value for water is 9.72 cal/mol Deviation is 4.79 % Results obtained from this experiment seldom increase 5% deviation from expected value
  32. 32. Applications Major application in conversion of water into steam Steam is used in  Power generation (steam turbines)  Agriculture  Energy storage  Wood treatment  Cleaning purposes  Sterilization
  33. 33. Applications (contd.) Distillation Vapor Pressure Calculations
  34. 34. References http://en.wikipedia.org/wiki/First_law_of_thermodyn amics http://en.wikipedia.org/wiki/Quasistatic_process http://en.wikipedia.org/wiki/Enthalpy http://en.wikipedia.org/wiki/Clausius- Clapeyron_relation
  35. 35. References http://en.wikipedia.org/wiki/Enthalpy_of_vaporizatio n http://www.sciencedirect.com/science/article/pii/S03 78381209002180 http://www2.selu.edu/Academics/Faculty/delbers/He at%20of%20vaporization.htm
  36. 36. QUESTIONSAREWELCOMED !!

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