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LESSON 3 Heat in Changes of State. Heats of Fusion and Solidification. Molar Heat of Fusion ( H fus ) - the heat absorbed by one mole of a substance in melting from a solid to a liquid Molar Heat of Solidification ( H solid ) heat lost when one mole of liquid solidifies.
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Heats of Fusion and Solidification • Molar Heat of Fusion (Hfus) - the heat absorbed by one mole of a substance in melting from a solid to a liquid • Molar Heat of Solidification(Hsolid) heat lost when one mole of liquid solidifies
Heats of Fusion and Solidification • Heat absorbed by a melting solid is equal to heat lost when a liquid solidifies • Thus, Hfus = -Hsolid
Heats of Fusion and Solidification • H20(s) H20(l) Hfus = 6.01kJ/mol • H20(l) H20(s) Hsolid = -6.01kJ/mol
Using the Heat of Fusion in Phase-Change Calculations How many kilojoules of heat are required to melt a 10.0g popsicle at 0°C? Assume the popsicle has the same molar mass and heat of fusion of water. ∆Hfus= 6.01kJ/mol
Using the Heat of Fusion in Phase-Change Calculations How many grams of ice at 0°C could be melted by the addition of 0.400 kJ of heat?
Heats of Vaporization and Condensation • When liquids absorb heat at their boiling points, they become vapors. • Molar Heat of Vaporization(Hvap) the amount of heat necessary to vaporize one mole of a given liquid.
Heats of Vaporization and Condensation • Condensation is the opposite of vaporization. • Molar Heat of Condensation(Hcond) amount of heat released when one mole of vapor condenses • Hvap = - Hcond
Heats of Vaporization and Condensation • H20(g) H20(l) Hcond = - 40.7kJ/mol • H20(l) H20(g) Hvap = 40.7kJ/mol
Using the Heat of Vaporization in Phase-Change Calculations How much heat is absorbed when 63.7g of liquid water at 100°C and 101.3Kpa is converted to steam at 100°C? ∆Hvap = 40.7 kJ/mol
Using the Heat of Vaporization in Phase-Change Calculations How many kilojoules of heat are absorbed when 0.46 g of Chloroethane (C2H5Cl) vaporizes at its normal boiling point? The molar heat of vaporization of chloroethane is 26.4kJ/mol.
Heating Curve of Water vaporization/melting = endothermic H positive (energy absorbed to get hotter). Solidification/condensation = exothermic H negative (energy released as it gets cooler).
Heat of Solution • Heat changes can also occur when a solute dissolves in a solvent. • Molar Heat of Solution (Hsoln) heat change caused by dissolution of one mole of substance • Sodium hydroxide provides a good example of an exothermic molar heat of solution:
Heat of Solution NaOH(s) Na1+(aq) + OH1-(aq) Hsoln = - 445.1 kJ/mol • The heat is released as the ions separate and interact with water, releasing 445.1 kJ of heat as Hsoln thus becoming so hot it steams! H2O(l)
Heat of Solution NaOH(s) Na1+(aq) + OH1-(aq) Hsoln = - 445.1 kJ/mol • How much heat (in kJ) is released when 0.677 mol of NaOH(s) is dissolved in water? H2O(l)
Heat of Solution NaOH(s) Na1+(aq) + OH1-(aq) Hsoln = - 445.1 kJ/mol • How many kJ of heat are released when 25.0 g of NaOH are dissolved in water? H2O(l)