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**Question of the Day: Day 4 2-8**

1. What type of ΔH favors a spontaneous reaction? 2. What type of ΔS is favors a spontaneous reaction? Day 4 2-8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**How can you measure the change in enthalpy of a reaction?**

Calorimetry Calorimetry How can you measure the change in enthalpy of a reaction? Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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Calorimetry Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. In a calorimetry experiment involving an endothermic process, the heat absorbed by the system is equal to the heat released by its surroundings. In an exothermic process, the heat released by the system is equal to the heat absorbed by its surroundings. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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Calorimetry Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. The insulated device used to measure the absorption or release of heat in chemical or physical processes is called a calorimeter. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters Foam cups can be used as simple calorimeters because they do not let much heat in or out. Most chemical reactions and physical changes carried out in the laboratory are open to the atmosphere and thus occur at constant pressure. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters The enthalpy (H) of a system accounts for the heat flow of the system at constant pressure. The heat absorbed or released by a reaction at constant pressure is the same as the change in enthalpy, symbolized as ΔH. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at constant pressure. In this textbook, the terms heat and enthalpy change are used interchangeably. In other words, q = ΔH. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters To measure the enthalpy change for a reaction in aqueous solution in a foam cup calorimeter, dissolve the reacting chemicals (the system) in known volumes of water (the surroundings). Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters Measure the initial temperature of each solution, and mix the solutions in the foam cup. After the reaction is complete, measure the final temperature of the mixed solutions. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**qsurr = m C ΔT Constant-Pressure Calorimeters Calorimetry**

You can calculate the heat absorbed or released by the surroundings (qsurr) using the formula for the specific heat, the initial and final temperatures, and the heat capacity of water. qsurr = m C ΔT Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters qsurr = m C ΔT m is the mass of the water. C is the specific heat of water. ΔT = Tf – Ti Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Constant-Pressure Calorimeters**

Calorimetry Constant-Pressure Calorimeters The heat absorbed by the surroundings is equal to, but has the opposite sign of, the heat released by the system. qsurr = –qsys Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**qsys = ΔH = –qsurr = –m C ΔT**

Calorimetry Constant-Pressure Calorimeters The enthalpy change for the reaction (ΔH) can be written as follows: qsys = ΔH = –qsurr = –m C ΔT The sign of ΔH is positive for an endothermic reaction and negative for an exothermic reaction. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Enthalpy Change in a Calorimetry Experiment**

Sample Problem 17.3 Enthalpy Change in a Calorimetry Experiment When 25.0 mL of water containing mol HCl at 25.0°C is added to 25.0 mL of water containing mol NaOH at 25.0°C in a foam-cup calorimeter, a reaction occurs. Calculate the enthalpy change (in kJ) during this reaction if the highest temperature observed is 32.0°C. Assume that the densities of the solutions are 1.00 g/mL and the volume of the final solution is equal to the sum of the volumes of the reacting solutions. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Analyze List the knowns and the unknown.**

Sample Problem 17.3 Analyze List the knowns and the unknown. 1 Use ΔH = –qsurr = –m C ΔT to solve for ΔH. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Analyze List the knowns and the unknown.**

Sample Problem 17.3 Analyze List the knowns and the unknown. 1 KNOWNS UNKNOWN Cwater = 4.18 J/(g·°C) Vfinal = VHCl + VNaOH = 25.0 mL mL = 50.0 mL Ti = 25.0°C Tf = 32.0°C densitysolution = 1.00 g/mL ΔH = ? kJ Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Calculate Solve for the unknown.**

Sample Problem 17.3 Calculate Solve for the unknown. 2 First calculate the total mass of the water. mwater = 50.0 mL = 50.0 g 1 mL 1.00 g Assume that the densities of the solutions are 1.00 g/mL to find the total mass of the water. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Calculate Solve for the unknown.**

Sample Problem 17.3 Calculate Solve for the unknown. 2 Now calculate ΔT. ΔT = Tf – Ti = 32.0°C – 25.0°C = 7.0°C Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Calculate Solve for the unknown.**

Sample Problem 17.3 Calculate Solve for the unknown. 2 Use the values for mwater, Cwater, and ΔT to calculate ΔH. ΔH = –qsurr = –mwater Cwater ΔT = –(50.0 g)(4.18 J/(g·oC))(7.0°C) = –1500 J = –1.5 kJ Use the relationship 1 kJ = 1000 J to convert your answer from J to kJ. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Evaluate Does the result make sense?**

Sample Problem 17.3 Evaluate Does the result make sense? 3 The temperature of the solution increases, which means that the reaction is exothermic, and thus the sign of ΔH should be negative. About 4 J of heat raises the temperature of 1 g of water 1°C, so 200 J of heat is required to raise 50 g of water 1°C. Raising the temperature of 50 g of water 7°C requires about 1400 J, or 1.4 kJ. This estimated answer is very close to the calculated value of ΔH. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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The initial temperature of the water in a constant-pressure calorimeter is 24°C. A reaction takes place in the calorimeter, and the temperature rises to 87°C. The calorimeter contains 367 g of water, which has a specific heat of 4.18 J/(g·°C). Calculate the enthalpy change during this reaction. Move solution up so it doesn’t run off the slide. Also, the text wraps differently in this slide versus the previous one. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**= –367 g 4.18 J/(g·°C) (87°C – 24°C)**

The initial temperature of the water in a constant-pressure calorimeter is 24°C. A reaction takes place in the calorimeter, and the temperature rises to 87°C. The calorimeter contains 367 g of water, which has a specific heat of 4.18 J/(g·°C). Calculate the enthalpy change during this reaction. ΔH = –m C ΔT = –367 g 4.18 J/(g·°C) (87°C – 24°C) = –97000 J = –97 kJ Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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**Key Concepts & Key Equation**

The value of ΔH of a reaction can be determined by measuring the heat flow of the reaction at a constant pressure. qsys = ΔH = –qsurr = –m C ΔT Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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Glossary Terms calorimetry: the precise measurement of heat flow out of a system for chemical and physical processes calorimeter: an insulated device used to measure the absorption or release of heat in chemical or physical processes Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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BIG IDEA Matter and Energy The heat of reaction or process can be determined experimentally through calorimetry. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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END OF 17.2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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