Chemistry 17.3

Heat in Changes of State 17.3 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.

Heats of Fusion and Solidification 17.3 Heats of Fusion and Solidification 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?

Heats of Fusion and Solidification 17.3 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.

Heats of Fusion and Solidification 17.3 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, ∆Hfus = –∆Hsolid.

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for Sample Problem 17.4 Problem Solving 17.21 Solve Problem 21 with the help of an interactive guide tutorial.

Heats of Vaporization and Condensation 17.3 Heats of Vaporization and Condensation 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?

Heats of Vaporization and Condensation 17.3 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).

Heats of Vaporization and Condensation 17.3 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, ∆Hvap = –∆Hcond.

Heats of Vaporization and Condensation 17.3 Heats of Vaporization and Condensation

Heats of Vaporization and Condensation Animation 21 Observe the phase changes as ice is converted to steam when heat is added.

Heats of Vaporization and Condensation 17.3 Heats of Vaporization and Condensation Enthalpy changes accompany changes in state. Enthalpy changes accompany changes in state. Fusion and vaporization are endothermic processes. Solidification and condensation are exothermic processes. Interpreting Diagrams Which arrows represent processes that release heat to the surroundings?

Heats of Vaporization and Condensation 17.3 Heats of Vaporization and Condensation A heating curve graphically describes the enthalpy changes that take place during phase changes. INTERPRETING GRAPHS a. Identify In which region(s) of the graph is temperature constant? b. Describe How does the amount of energy required to melt a given mass of ice compare to the energy required to vaporize the same mass of water? Explain. c. Apply Concepts Which region of the graph represents the coexistence of solid and liquid? Liquid and vapor?

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for Sample Problem 17.5 Problem-Solving 17.24 Solve Problem 24 with the help of an interactive guided tutorial.

17.3 Heat of Solution Heat of Solution What thermochemical changes can occur when a solution forms?

17.3 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).

17.3 Heat of Solution When ammonium nitrate crystals and water mix inside the cold pack, heat is absorbed as the crystals dissolve. The cold pack shown has two sealed plastic bags, one inside the other. The outer bag contains ammonium nitrate crystals. The inner bag contains liquid water. When the pack is squeezed, the inner bag breaks, allowing the ammonium nitrate and water to mix. Inferring How would you define the system and the surroundings in this process?

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for Sample Problem 17.6 Problem-Solving 17.26 Solve Problem 26 with the help of an interactive guided tutorial.

17.3 Section Quiz. 17.3.

17.3 Section Quiz. 1. The molar heat of condensation of a substance is the same, in magnitude, as its molar heat of formation. fusion. solidification. vaporization.

may increase or decrease. increases. decreases. 17.3 Section Quiz 2. The heat of condensation of ethanol (C2H5OH) is 43.5 kJ/mol. As C2H5OH condenses, the temperature of the surroundings stays the same. may increase or decrease. increases. decreases.

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. 1.48 kJ 47.4 kJ 1.52  103 kJ 4.75 kJ

17.3 Section Quiz 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 12.85 kJ 13.1 kJ 25.7 kJ 1285 kJ

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