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CHEMISTRY & YOU Chapter 17 Thermochemistry 17.1 The Flow of Energy 17. Measuring and Expressing Enthalpy Changes 17.3 Heat in Changes of State 17.

The evaporation of sweat is your body s way of cooling itself to a normal temperature. 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

All solids absorb heat as they melt to become liquids. The gain of heat causes a change of state instead of a change in temperature.

, or its affiliates. All Rights Reserved. The heat absorbed by one mole of a solid substance as it melts to a liquid at constant temperature is the molar heat of fusion (ΔH fus ).

1 CHEMISTRY & YOU Chapter 17 Thermochemistry 17.1 The Flow of Energy 17. Measuring and Expressing Enthalpy Changes 17.3 Heat in Changes of State 17.4 Calculating Heats of Reaction Why does sweating help cool you off? When your body heats up, you start to sweat. The evaporation of sweat is your body s way of cooling itself to a normal temperature. 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. What is the relationship between molar heat of fusion and molar heat of solidification? All solids absorb heat as they melt to become liquids. The gain of heat causes a change of state instead of a change in temperature. The temperature of the substance undergoing the change remains constant. 3 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 4 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. The heat absorbed by one mole of a solid substance as it melts to a liquid at constant temperature is the molar heat of fusion (ΔH fus ). The molar heat of solidification (ΔH solid ) is the heat lost when one mole of a liquid substance solidifies at a constant temperature. The quantity of heat absorbed by a melting solid is exactly the same as the quantity of heat released when the liquid solidifies. ΔH fus = ΔH solid 5 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 6 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 1

The melting of 1 mol of ice at 0 C to 1 mol of liquid water at 0 C requires the absorption of 6.01 kj of heat. The conversion of 1 mol of liquid water at 0 C to 1 mol of ice at 0 C releases 6.

2 The melting of 1 mol of ice at 0 C to 1 mol of liquid water at 0 C requires the absorption of 6.01 kj of heat. The conversion of 1 mol of liquid water at 0 C to 1 mol of ice at 0 C releases 6.01 kj of heat. H O(s) H O(l) H O(l) H O(s) ΔH fus = 6.01 kj/mol ΔH solid = 6.01 kj/mol Using the Heat of Fusion in Phase- Change Calculations How many grams of ice at 0 C will melt if.5 kj of heat are added? 7 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 1 Analyze List the knowns and the unknown. Find the number of moles of ice that can be melted by the addition of.5 kj of heat. Convert moles of ice to grams of ice. KNOWNS Initial and final temperature are 0 C ΔH fus = 6.01 kj/mol ΔH =.5 kj UNKNOWN m ice =? g Start by expressing ΔH fus as a conversion factor. 1 mol H O(s) 6.01 kj Use the thermochemical equation H O(s) kj H O(l). 9 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 10 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Express the molar mass of ice as a conversion factor. Multiply the known enthalpy change by the conversion factors g H O(s) 1 mol H O(s) 1 mol H O(s) m ice =.5 kj 6.01 kj = 6.74 g H O(s) 18.0 g H O(s) 1 mol H O(s) 11 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved.

3 3 Evaluate Does the result make sense? To melt 1 mol of ice, 6.01 kj of energy is required. Only about one-third of this amount of heat (roughly kj) is available. So, only about one-third mol of ice, or 18.0 g/3 = 6 g, should melt. This estimate is close to the calculated answer. Calculate the amount of heat absorbed to liquefy 15.0 g of methanol (CH 4 O) at its melting point. The molar heat of fusion for methanol is 3.16 kj/mol. 13 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 14 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Heats of Vaporization and Condensation Calculate the amount of heat absorbed to liquefy 15.6 g of methanol (CH 4 O) at its melting point. The molar heat of fusion for methanol is 3.16 kj/mol. 1 mol 3.16 kj ΔH = 15.6 g CH 4 O 3.05 g CH 4 O 1 mol = 1.54 kj Heats of Vaporization and Condensation What is the relationship between molar heat of vaporization and molar heat of condensation? 15 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 16 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Heats of Vaporization and Condensation Interpret Data A liquid that absorbs heat at its boiling point becomes a vapor. The amount of heat required to vaporize one mole of a given liquid at a constant temperature is called its molar heat of vaporization (ΔH vap ). This table lists the molar heats of vaporization for several substances at their normal boiling point. Heats of Physical Change Substance ΔH fus (kj/mol) ΔH vap (kj/mol) Ammonia (NH 3 ) Ethanol (C H 6 O) Hydrogen (H ) Methanol (CH 4 O) Oxygen (O ) Water (H O) Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 18 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 3

Heats of Vaporization and Condensation Condensation is the exact opposite of vaporization. When a vapor condenses, heat is released.

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.

Explain why the evaporation of sweat off your body helps cool you off. Energy is required to vaporize (or evaporate) a liquid into a gas.

4 Heats of Vaporization and Condensation Condensation is the exact opposite of vaporization. When a vapor condenses, heat is released. The molar heat of condensation (ΔH cond ) is the amount of heat released when one mole of vapor condenses at its normal boiling point. 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. ΔH vap = ΔH cond 19 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 0 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. CHEMISTRY & YOU CHEMISTRY & YOU Explain why the evaporation of sweat off your body helps cool you off. Explain why the evaporation of sweat off your body helps cool you off. Energy is required to vaporize (or evaporate) a liquid into a gas. When liquid sweat absorbs energy from your skin, the temperature of your skin decreases. 1 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Interpret Graphs A heating curve graphically describes the enthalpy changes that take place during phase changes. Remember: The temperature of a substance remains constant during a change of state. Using the Heat of Vaporization in Phase- Change Calculations How much heat (in kj) is absorbed when 4.8 g H O(l) at 100 C and kpa is converted to H O(g) at 100 C? 3 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 4 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 4

5 1 Analyze List the knowns and the unknown. First convert grams of water to moles of water. Then find the amount of heat that is absorbed when the liquid is converted to steam. KNOWNS Initial and final conditions are 100 C and kpa UNKNOWN ΔH =? kj Mass of liquid water converted to steam = 4.8 g ΔH vap = 40.7 kj/mol Start by expressing the molar mass of water as a conversion factor. 1 mol H O(l) 18.0 g H O(l) 5 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 6 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Express ΔH vap as a conversion factor kj 1 mol H O(l) Use the thermochemical equation H O(l) kj H O(g). Multiply the mass of water in grams by the conversion factors. 1 mol H O(l) ΔH = 4.8 g H O(l) 18.0 g H O(l) = 56.1 kj 40.7 kj 1 mol H O(l) 7 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 8 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 3 Evaluate Does the result make sense? Knowing that the molar mass of water is 18.0 g/mol, 4.8 g H O(l) can be estimated to be somewhat less than 1.5 mol H O. The calculated enthalpy change should be a little less than 1.5 mol 40 kj/mol = 60 kj, and it is. The molar heat of condensation of a substance is the same, in magnitude, as which of the following? A. molar heat of fusion B. molar heat of vaporization C. molar heat of solidification D. molar heat of formation 9 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 30 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 5

The molar heat of condensation of a substance is the same, in magnitude, as which of the following? A. molar heat of fusion B. molar heat of vaporization C. molar heat of solidification D.

, or its affiliates. All Rights Reserved. During the formation of a solution, heat is either released or absorbed.

6 The molar heat of condensation of a substance is the same, in magnitude, as which of the following? A. molar heat of fusion B. molar heat of vaporization C. molar heat of solidification D. molar heat of formation What thermochemical changes can occur when a solution forms? 31 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 3 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. During the formation of a solution, heat is either released or absorbed. During the formation of a solution, heat is either released or absorbed. The enthalpy change caused by the dissolution of one mole of substance is the molar heat of solution (ΔH soln ). 33 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 34 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. A practical application of an exothermic dissolution process is a hot pack. In a hot pack, calcium chloride, CaCl (s), mixes with water, producing heat. CaCl (s) Ca + (aq) + Cl (aq) ΔH soln = 8.8 kj/mol The dissolution of ammonium nitrate, NH 4 NO 3 (s), is an example of an endothermic process. The cold pack shown here contains solid ammonium nitrate crystals and water. Once the solute dissolves, the pack becomes cold. The solution process absorbs energy from the surroundings. NH 4 NO 3 (s) NH 4+ (aq) + NO 3 (aq) ΔH soln = 5.7 kj/mol 35 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 36 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 6

Calculating the Enthalpy Change in Solution Formation How much heat (in kj) is released when.50 mol NaOH(s) is dissolved in water? 1 Analyze List the knowns and the unknown.

7 Calculating the Enthalpy Change in Solution Formation How much heat (in kj) is released when.50 mol NaOH(s) is dissolved in water? 1 Analyze List the knowns and the unknown. Use the heat of solution for the dissolution of NaOH(s) in water to solve for the amount of heat released (ΔH). KNOWNS ΔH soln = 44.5 kj/mol amount of NaOH(s) dissolved =.50 mol UNKNOWN ΔH =? kj 37 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 38 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Start by expressing ΔH soln as a conversion factor. Multiply the number of moles by the conversion factor kj 1 mol NaOH(s) Use the thermochemical equation NaOH(s) Na + (aq) + OH (aq) kj/mol. ΔH =.50 mol NaOH(s) = 111 kj 44.5 kj 1 mol NaOH(s) 39 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 40 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 3 Evaluate Does the result make sense? ΔH is.5 times greater than ΔH soln, as it should be. Also, ΔH should be negative, as the dissolution of NaOH(s) in water is exothermic. How much heat (in kj) is absorbed when 50.0 g of NH 4 NO 3 (s) are dissolved in water if ΔH soln = 5.7 kj/mol? 41 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 4 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 7

8 Key Concepts How much heat (in kj) is absorbed when 50.0 g of NH 4 NO 3 (s) are dissolved in water if ΔH soln = 5.7 kj/mol? 1 mol 5.7 kj ΔH = 50.0 g NH 4 NO g NH 4 NO 3 1 mol = 16.1 kj 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, ΔH fus = ΔH solid. 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, ΔH vap = ΔH cond. During the formation of a solution, heat is either released or absorbed. 43 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 44 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. Glossary Terms Glossary Terms molar heat of fusion (ΔH fus ): the amount of heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature molar heat of solidification (ΔH solid ): the amount of heat lost by one mole of a liquid as it solidifies at a constant temperature molar heat of vaporization (ΔH vap ): the amount of heat absorbed by one mole of a liquid as it vaporizes at a constant temperature molar heat of condensation (ΔH cond ): the amount of heat released by one mole of a vapor as it condenses to a liquid at a constant temperature molar heat of solution (ΔH soln ): the enthalpy change caused by the dissolution of one mole of a substance 45 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 46 Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. END OF Copyright Pearson Education, Inc., or its affiliates. All Rights Reserved. 8

Name Date Class THE FLOW OF ENERGY HEAT AND WORK

Name Date Class THE FLOW OF ENERGY HEAT AND WORK 17.1 THE FLOW OF ENERGY HEAT AND WORK Section Review Objectives Explain the relationship between energy, heat, and work Distinguish between exothermic and endothermic processes Distinguish between heat