Enthalpy of vaporization of liquid

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

1. Enthalpy of Vaporization A detailed approach

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. 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. First Law of Thermodynamics Energy conservation law Describes change 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. 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. 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. 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. 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 Δ H = Δ U + Δ (PV) SI Unit – Joule Other conventional units – Btu and Calories

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. 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. 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. 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. 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. Characteristics of Enthalpy of Vaporization 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. Characteristics of Enthalpy of Vaporization (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. Temperature dependence of EOV

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. Physical model for vaporization (Diagrammatic representation)

19. Experimental determination

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. 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. 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. Clausius-Clapeyron relation (contd.) General form dP/dT = L/T Δ V Where dP/dT is slope of tangent to coexistence curve at any point L is latent specific heat T is temperature Δ V 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. 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. 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. 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. Sample Readings Temp (°C) h (mm Hg) Temp (K) (1/T) x 10 3 (1/K) P (mm Hg) (P = 768 –h) Log P 41.5 710 314.5 3.18 58.0 1.77 62.5 610 335.2 2.98 158 2.20 75.0 500 348.0 2.87 268 2.43 86.5 300 359.5 2.78 468 2.67 92.2 220 365.2 2.74 548 2.74 101.0 374.0 2.67 768 2.89

28. Graph between temperature and pressure

29. Calculations Molar latent heat / enthalpy of vaporization can be calculated from Clausius-Clapeyron relation as follows, ΔH v = -Rx[d(ln(P))/d(1/T )]] ln(P) = 2.303 log(P) Slope = m = d(ln(P))/d(1/T) ΔH v = -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. 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. Calculations (contd.) Applying values in equation 1 from previous slide ΔH v = 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. 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. Applications (contd.) Distillation Vapor Pressure Calculations

34. References http://en.wikipedia.org/wiki/First_law_of_thermodynamics http://en.wikipedia.org/wiki/Quasistatic_process http://en.wikipedia.org/wiki/Enthalpy http://en.wikipedia.org/wiki/Clausius-Clapeyron_relation

35. References http://en.wikipedia.org/wiki/Enthalpy_of_vaporization http://www.sciencedirect.com/science/article/pii/S0378381209002180 http://www2.selu.edu/Academics/Faculty/delbers/Heat%20of%20vaporization.htm

36. QUESTIONS ARE WELCOMED !!