Slide 1 of 35 Chemistry 17.3

© Copyright Pearson Prentice Hall Slide 2 of 35 Heat in Changes of State During a race, an athlete can burn a lot of calories that either do work or are released as heat. This section will help you to understand how the evaporation of sweat from your skin helps to rid your body of excess heat. 17.3

© Copyright Pearson Prentice Hall Heat in Changes of State > Slide 3 of 35 Heats of Fusion and Solidification How does the quantity of heat absorbed by a melting solid compare to the quantity of heat released when the liquid solidifies? 17.3

© Copyright Pearson Prentice Hall Heat in Changes of State > Slide 4 of 35 Heats of Fusion and Solidification The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies; that is, H fus = –H solid. 17.3

Slide 5 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Fusion and Solidification The molar heat of fusion (H fus ) is the heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature (in KJ/mol). The molar heat of solidification (H solid ) is the heat lost when one mole of a liquid solidifies at a constant temperature (in KJ/mol). Note: H fus values (in KJ/mol) are listed in Table 17.3 on p.522 of your textbook. 17.3

Slide 6 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Vaporization and Condensation 17.3

Slide 7 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Fusion and Solidification Key Equation:H phase change =  H n* * n = m FOR PURE SUBSTANCES M W here: H phase change = molar enthalpy of the phase change (in KJ/mol)  H = enthalpy change (heat) in kJ n = amount of substance (in mol) m = mass of the substance (in g) M = molar mass of the substance (in g/mol) 17.3

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© Copyright Pearson Prentice Hall Slide 12 of 35 Practice Problems for Sample Problem 17.4 Problem Solving Solve Problem 21 with the help of an interactive guide tutorial.

© Copyright Pearson Prentice Hall Heat in Changes of State > Slide 13 of 35 Heats of Vaporization and Condensation How does the quantity of heat absorbed by a vaporizing liquid compare to the quantity of heat released when the vapor condenses? 17.3

© Copyright Pearson Prentice Hall Heat in Changes of State > Slide 14 of 35 Heats of Vaporization and Condensation The quantity of heat absorbed by a vaporizing liquid is exactly the same as the quantity of heat released when the vapor condenses; that is, H vap = –H cond. 17.3

Slide 15 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Vaporization and Condensation The amount of heat necessary to vaporize one mole of a given liquid is called its molar heat of vaporization (H vap ). It is measured in KJ/mol. The amount of heat released when 1 mol of vapor condenses at the normal boiling point is called its molar heat of condensation (H cond ). It is measured in KJ/mol. Note: ∆H vap values (in KJ/mol) are listed in Table 17.3 on p.522 of your textbook. 17.3

Slide 16 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Vaporization and Condensation 17.3

© Copyright Pearson Prentice Hall Heat in Changes of State > Slide 17 of 35 Heats of Vaporization and Condensation Animation 21 Observe the phase changes as ice is converted to steam when heat is added.

Slide 18 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Vaporization and Condensation Enthalpy changes accompany changes in state. 17.3

Slide 19 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heats of Vaporization and Condensation 17.3

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© Copyright Pearson Prentice Hall Slide 24 of 35 Practice Problems for Sample Problem 17.5 Problem-Solving Solve Problem 24 with the help of an interactive guided tutorial.

© Copyright Pearson Prentice Hall Heat in Changes of State > Slide 25 of 35 Heat of Solution What thermochemical changes can occur when a solution forms? 17.3

Slide 26 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heat of Solution During the formation of a solution, heat is either released or absorbed. The enthalpy change caused by dissolution of one mole of substance is the molar heat of solution (H soln ). 17.3

Slide 27 of 35 © Copyright Pearson Prentice Hall > Heat in Changes of State Heat of Solution When ammonium nitrate crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve. 17.3

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© Copyright Pearson Prentice Hall Slide 32 of 35 Practice Problems for Sample Problem 17.6 Problem-Solving Solve Problem 26 with the help of an interactive guided tutorial.

© Copyright Pearson Prentice Hall Slide 33 of 35 Section Quiz -or- Continue to: Launch: Assess students’ understanding of the concepts in Section 17.3 Section Quiz

© Copyright Pearson Prentice Hall Slide 34 of Section Quiz. 1. The molar heat of condensation of a substance is the same, in magnitude, as its molar heat of a.formation. b.fusion. c.solidification. d.vaporization.

© Copyright Pearson Prentice Hall Slide 35 of The heat of condensation of ethanol (C 2 H 5 OH) is  43.5 kJ/mol. As C 2 H 5 OH condenses, the temperature of the surroundings a.stays the same. b.may increase or decrease. c.increases. d.decreases Section Quiz

© Copyright Pearson Prentice Hall Slide 36 of Section Quiz 3. Calculate the amount of heat absorbed to liquefy 15.0 g of methanol (CH 3 OH) at its melting point. The molar heat of fusion for methanol is 3.16 kJ/mol. a.1.48 kJ b.47.4 kJ c.1.52  10 3 kJ d.4.75 kJ

© Copyright Pearson Prentice Hall Slide 37 of How much heat (in kJ) is released when 50 g of NH 4 NO 3 (s), moles, are dissolved in water?  s soln =  25.7 kJ/mol a kJ b.13.1 kJ c.25.7 kJ d.1285 kJ 17.3 Section Quiz

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