chemistry 17.3

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Chemistry 17.3

Chapter 17

Section 3

© Copyright Pearson Prentice Hall

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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.

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Heat in Changes of State >

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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?

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>Heat in Changes of State Heats of Fusion and Solidification

The molar heat of fusion (∆Hfus) is the heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature.

The molar heat of solidification (∆Hsolid) is the heat lost when one mole of a liquid solidifies at a constant temperature.

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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, ∆Hfus = –∆Hsolid.

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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SAMPLE PROBLEM

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Practice Problems for Sample Problem 17.4

Problem Solving 17.21 Solve Problem 21 with the help of an interactive guide tutorial.



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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?

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>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 (∆Hvap).

The amount of heat released when 1 mol of vapor condenses at the normal boiling point is called its molar heat of condensation (∆Hcond).

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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, ∆Hvap = –∆Hcond.

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Animation 21

Observe the phase changes as ice is converted to steam when heat is added.

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Enthalpy changes accompany changes in state.

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SAMPLE PROBLEM

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Problem-Solving 17.24 Solve Problem 24 with the help of an interactive guided tutorial.



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Heat of Solution

Heat of Solution

What thermochemical changes can occur when a solution forms?

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>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 (∆Hsoln).

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>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.

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SAMPLE PROBLEM

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Problem-Solving 17.26 Solve Problem 26 with the help of an interactive guided tutorial.


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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

17.3 Section Quiz.

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17.3 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.


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2. The heat of condensation of ethanol (C2H5OH) is 43.5 kJ/mol. As C2H5OH condenses, the temperature of the surroundings

a. stays the same.

b. may increase or decrease.

c. increases.

d. decreases.

17.3 Section Quiz


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17.3 Section Quiz

3. Calculate the amount of heat absorbed to liquefy 15.0 g of methanol (CH3OH) 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. 1.52 103 kJ

d. 4.75 kJ


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4. How much heat (in kJ) is released when 50 g of NH4NO3(s), 0.510 moles, are dissolved in water? ssoln = 25.7 kJ/mol

a. 12.85 kJ

b. 13.1 kJ

c. 25.7 kJ

d. 1285 kJ

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END OF SHOW

chemistry 17.3

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