Lec-25-Chap-11-2P

Lec-25-Chap-11-2P - Heating Curve Convert 100 g of ice...

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1 Heating Curve Convert 100 g of ice at -20°C into vapor at 120°C. 2 TOTAL heat required = 308.9 kJ Heat the ice to 0°C. Δ H = mc Δ T = (100g)(2.06 Jg -1 °C -1 )(0-[-20]°C) = 4.1 kJ Convert the ice to water Δ H = n Δ H fus = (100g/18.02g mol -1 )(6.020 kJ/mol) = 33.4 kJ Heat the water from 0°C to 100°C. Δ H = mc Δ T = (100g)(4.184 Jg -1 °C -1 )(100 - 0°C) = 41.8 kJ Convert water to steam (at its normal bp). Δ H = n Δ H vap = (100g/18.02 g mol -1 )(40.7 kJ/mol) = 225.9 kJ Heat steam from 100 to 120°C. Δ H = mc Δ T = (100g)(1.84 Jg -1 °C -1 )(120 - 100°C) = 3.7 kJ Heating Curve

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3 Which burn would be worse, 1g of liquid H 2 O at 100°C or 0.1g water vapor at 100°C (skin = 37°C)? Cool the liquid to 37 °C. Δ H = mc Δ T = (1g)(4.184 Jg -1 °C -1 )(37-[100]°C) = -263J Convert the vapor to liquid Δ H = n Δ H fus = (0.1g/18.02g mol -1 )(-40.7 kJ/mol) = -226 J Cool the water from 100°C to 37°C. Δ H = mc Δ T = (0.1g)(4.184 Jg -1 °C -1 )(37-[100]°C) = -26.4J -252J A: 1g liquid B: 0.1g vapor C: no difference 4 Pressure (atm) Temperature (°C) Phase Diagrams Solid Liquid Gas critical point triple point sublimation deposition freezing reezing melting condensation condensation vaporization vaporization Melting point curve Vapor-pressure curve Supercritical fluid
5 Phase Diagrams Triple point Triple point Three phases in equilibrium. T and P are fixed. Water T = 0.01°C, P = 4.58 mmHg CO 2 T = -57°C, P = 5.2 atm Critical point The end of the liquid/gas equilibrium line. It occurs at the critical temperature, critical temperature, T c and critical pressure, critical pressure, P c Water T c = 374°C, P c = 218 atm CO 2 T c = 31°C, P c = 73 atm Above T c a gas cannot be liquefied (at any P ) 6 Supercritical CO 2 is an important solvent. It is used: Above T c and P c the substance is neither a liquid nor a gas. It is a supercritical fluid supercritical fluid . It has: • to extract caffeine from coffee beans • as a dry-cleaning fluid • a density characteristic of a liquid. • flow properties of a gas. Critical Temperature and Pressure

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7 Physical Comparison with Importance in Property Other Substances and Biological Environment Specific heat capacity Highest of all liquids and Moderates T in the environment and in (4.18 J g -1 °C -1 ) solids except NH 3 organisms; climate affected by movement of water (e.g., Gulf Stream) Heat of fusion Highest of all molecular Freezing water releases large quantity (333 J/g) solids except NH 3 of thermal E ; used to save crops from freezing by spraying them with liq. water Heat of vaporization Highest of all molecular Condensation of water vapor in clouds (2250 J/g) substances
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Lec-25-Chap-11-2P - Heating Curve Convert 100 g of ice...

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