ChE102_Notes_-_Chap_4

ChE102_Notes_-_Chap_4 - ChE102 Div 9 10 Fall 2009 CHAPTER 4...

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Fall 2009 CHAPTER 4 – Liquids, Solids, and Solutions Petrucci: 12.1-12.4, 13.1-13.9 Enthalpy of Vapourization Vapourization (evaporation) – the passage of molecules from the surface of a liquid into the vapour state. Increases with: Increase in temperature Increase in liquid surface area Decrease in intermolecular forces Condensation – conversion of a vapour to a liquid Enthalpy of Vapourization (ΔHvapourization) – the quantity of heat that must be absorbed to vapourize a certain quantity of liquid at a constant temperature. ΔHcondensation = -ΔHvapourization Phase Equilibrium Water in an open vessel would evaporate completely At phase equilibrium (water in a closed vessel - (c) ): Equilibrium is a dynamic process No net conversion from one phase to another Properties are uniform throughout Course Notes 4 1
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Fall 2009 Vapour Pressure Vapour pressure ( P vap ) is the pressure measured when the system reaches phase equilibrium Dependent on temperature and gas type An intensive property (independent of system size) Example: H 2 O at 25°C has P vap = 23.8 mmHg For mixtures at equilibrium, the partial pressure of each component is given by P vap Vapour Pressure of Selected Liquids Boiling Point – where the pressure of the escaping molecules equals that of the molecules in the vapour phase above Normal Boiling Point (at P = 760 mmHg): a) Diethyl ether (C 4 H 10 O) b) Benzene (C 6 H 6 ) c) Water (H 2 O) d) Toluene (C 7 H 8 ) e) Aniline (C 6 H 7 N) And cuves become linear if expressed in terms of ln(P) α 1/T Antoine’s Equation for P vap Vapour pressure of a compound is a strong function of temperature The most important correlation for vapour pressure is Antoine’s equation: A , B and C are constants that depend on the type of substance in a given temperature range Antoine’s equation is an empirical equation – it is based on the observation that plots of log 10 P vap versus 1 / T are approximately linear Course Notes 4 2 T C B A P vap + - = 10 log
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Fall 2009 Energy Changes During Phase Changes Energy (heat) is necessary to accomplish a phase transition (vapourization, liquefaction or sublimation) H 2 O ( l ) + 44.01 kJ/mol H 2 O ( g ) @ 25°C and 1 atm: Δ H vap = 44.01 kJ/mol H 2 O ( g ) – 44.01 kJ/mol H 2 O ( l ) @ 25°C and 1 atm: Δ H liq = –44.01 kJ/mol Clausius-Clapeyron Equation for P vap Another useful equation correlating P vap to temperature is the Clausius-Clapeyron equation: T is in K Δ H vap is assumed not to vary with changes in temperature Clausius-Clapeyron equation is a theoretical equation that can also be applied to other phase transitions if the proper heat of phase transition (Δ H vap , Δ H sub , Δ H melt ) is used.
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ChE102_Notes_-_Chap_4 - ChE102 Div 9 10 Fall 2009 CHAPTER 4...

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