1.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 Describeschange in internal energyof 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 gothrough a sequence of states that are infinitesimally closeto equilibrium (so the system remains in quasistaticequilibrium), in which case the process is typicallyreversible 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 systems pressure and volume H = U + (PV) SI Unit Joule Other conventional units Btu and Calories 9. Why Enthalpy is measured? Total enthalpy of a system cant be measured directly Enthalpy change of a system is measured instead It is measured to, Calculate useful work obtainable from a closedthermodynamic system under constant pressure Determine nature of reaction e.g., exothermic orendothermic 10. Enthalpy is not necessarily heat !! Enthalpy is sometimes described as heat content of asystem Heat is defined as thermal energy in transit For the description that enthalpy is in-fact heat to bevalid, no energy exchange must occur withenvironment 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 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. 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. Temperature dependence of EOV 17. Physical model for vaporization Proposed by professor Jozsef Garai, Florida InternationalUniversity, USA Energy required to free an atom from liquid is equivalent toenergy 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 Bunsenburner 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 phasetransition between two phases of a single constituent On a P-T diagram, line separating two phases is knownas coexistence curve This relation gives the slope of the tangents to thiscurve 23. Clausius-Clapeyron relation (contd.) General formdP/dT = L/TV Where dP/dT is slope of tangent to coexistence curve at any point L is latent specific heat T is temperature 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 ReadingsTemp (C) h (mm Hg)Temp (K)(1/T) x 103 P (mm Hg) Log P(1/K)(P = 768 h)41.5710 314.5 3.18 58.0 1.7762.5610 335.2 2.98 1582.2075.0500 348.0 2.87 2682.4386.5300 359.5 2.78 4682.6792.2220 365.2 2.74 5482.74101.0 0 374.0 2.67 7682.89 28. Graph between temperature andpressureTemperature and Log P3.5 32.5 2Log P1.510.5 02.6 2.7 2.82.9 3 3.1 3.2 3.3 1/T x 103 29. Calculations Molar latent heat / enthalpy of vaporization can be calculatedfrom Clausius-Clapeyron relation as follows, Hv = -Rx[d(ln(P))/d(1/T)]]ln(P) = 2.303 log(P)Slope = m = d(ln(P))/d(1/T)Hv = -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 graphand 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 Hv = 10.21 cal/mol Accepted value for water is 9.72 cal/mol Deviation is 4.79 % Resultsobtained 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_thermodyn amics 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_vaporizatio n http://www.sciencedirect.com/science/article/pii/S03 78381209002180 http://www2.selu.edu/Academics/Faculty/delbers/He at%20of%20vaporization.htm 36. QUESTIONSAREWELCOMED !!