Lec-26-Chap-11-2

Lec-26-Chap-11-2 - Phase Changes: Solids, Liquids & Gases...

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1 21 Vaporization low T velocity or energy number of molecules Threshold for escape high T • High E k molecules leave. • The average E k of the liquid drops. • The liquid cools. Vaporization is endothermic - Requires energy to break interactions Condensation The reverse process is exothermic- gives off energy 22 Phase Changes Δ vap = – Δ cond Molar heat of vaporization, Δ vap Heat required to vaporize 1 mol ( P = 1 bar). Molar heat of condensation, Δ cond Heat released when 1 mol condenses ( P = 1 bar). For H 2 O( l ) H 2 O(g) Δ = Δ vap = +40.7 kJ/mol For H 2 O(g) H 2 O( l ) Δ = Δ cond = -40.7 kJ/mol 23 Melting and Freezing The molecules in a solid are closely packed and in constant motion. They ± vibrate about fixed positions ± do not have enough E to slide past each other. A solid melts when E k > forces holding the particles in their fixed positions. Melting is endothermic. Freezing (solidification, crystallization) is exothermic. 24 Phase Changes Δ fus = – Δ cryst Molar heat of crystallization, Δ cryst Heat released when 1 mol crystallizes ( P = 1 bar). For H 2 O(s) H 2 O( l ) Δ = Δ fus = +6.02 kJ/mol For H 2 O( l ) H 2 O(s) Δ = Δ cryst = -6.02 kJ/mol Normal melting or freezing occurs at P = 1 atm Molar heat of fusion, Δ fus Heat required to melt 1 mol ( P = 1 bar). 25 Phase Changes Sublimation – the direct conversion of a solid to a gas Examples Solid CO 2 (dry ice) sublimes. Ice (v.p. = 4.6 mmHg @ 0°C) in a frost-free ‘fridge is removed by blowing dry air over it… Deposition is the reverse process: (g) (s) Example : plating A l onto the surface of a CD Δ sub = – Δ dep 27 Phase Changes endothermic process (heat added) exothermic process (heat released)
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2 28 Phase Changes 29 Phase Changes Substance mp(°C) Δ H fus (kJ/mol) bp(°C) Δ H vap (kJ/mol) O 2 (16 e - ) –248 0.445 –183.0 6.8 F 2 (18 e - ) –220 1.020 –188.1 6.54 C l 2 (34 e - ) –103 6.406 –34.6 20.39 Br 2 (70 e - ) –7.2 10.794 59.6 29.54 Nonpolar molecules London forces increase as the number of e - increase. The data support this trend… 30 Phase Changes Substance mp(°C) Δ H fus (kJ/mol)* bp(°C) Δ H vap (kJ/mol)* SO 2 (32 e - ) –76 7.4 –10.0 24.9 HC l (18 e - ) –115 2.0 –85.1 16.2 HBr (36 e-) –87 2.4 –66.8 17.6 H 2 O (10 e - ) 0 6.0 +100.0 40.7 HF (10 e - ) –83 4.6 +19.5 7.5 NH 3 (10 e - ) –78 5.6 –33.5 23.4 Polar molecules More difficult to explore trends. Values depend on
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This note was uploaded on 04/20/2008 for the course CHE 131 taught by Professor Kerber during the Spring '08 term at SUNY Stony Brook.

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Lec-26-Chap-11-2 - Phase Changes: Solids, Liquids & Gases...

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