17.2 Measuring and Expressing Enthalpy Changes 1 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 17 Thermochemistry 17.1 The Flow of Energy 17.2 Measuring and Expressing Enthalpy Changes 17.3 Heat in Changes of State 17.4 Calculating Heats of Reaction

17.2 Measuring and Expressing Enthalpy Changes 2 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 3 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry is the measurement of the heat flow into or out of a system for chemical and physical processes. In 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. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 4 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 5 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 6 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 7 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 8 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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). Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 9 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 10 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters You can calculate the heat absorbed or released by the surroundings (q surr ) using the formula for the specific heat, the initial and final temperatures, and the heat capacity of water. q surr = m  C  ΔT Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 11 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters q surr = m  C  ΔT m is the mass of the water. C is the specific heat of water. ΔT = T f – T i Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 12 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters The heat absorbed by the surroundings is equal to, but has the opposite sign of, the heat released by the system. q surr = –q sys Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 13 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Pressure Calorimeters The enthalpy change for the reaction (ΔH) can be written as follows: q sys = ΔH = –q surr = – m  C  ΔT The sign of ΔH is positive for an endothermic reaction and negative for an exothermic reaction. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 14 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Volume Calorimeters Calorimetry experiments can also be performed at a constant volume using a device called a bomb calorimeter. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 15 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Constant-Volume Calorimeters In a bomb calorimeter, a sample of a compound is burned in a constant-volume chamber in the presence of oxygen at high pressure. Calorimetry

17.2 Measuring and Expressing Enthalpy Changes 16 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calorimetry Constant-Volume Calorimeters The heat that is released warms the water surrounding the chamber. By measuring the temperature increase of the water, it is possible to calculate the quantity of heat released during the combustion reaction.

17.2 Measuring and Expressing Enthalpy Changes 17 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What type of calorimeter would you use to measure the heat released when a match burns? Describe the experiment and how you would calculate the heat released. CHEMISTRY & YOU

17.2 Measuring and Expressing Enthalpy Changes 18 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. What type of calorimeter would you use to measure the heat released when a match burns? Describe the experiment and how you would calculate the heat released. CHEMISTRY & YOU A constant-volume, or bomb, calorimeter would be used to measure the heat released when a match burns. The match would be ignited in the chamber. By measuring the temperature increase in the water and using the equation q = –m  C  ΔT, the heat released, q, can be calculated.

17.2 Measuring and Expressing Enthalpy Changes 19 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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. Sample Problem 17.3 Enthalpy Change in a Calorimetry Experiment

17.2 Measuring and Expressing Enthalpy Changes 20 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Analyze List the knowns and the unknown. 1 Use dimensional analysis to determine the mass of the water. You must also calculate ΔT. Use ΔH = –q surr = –m  C  ΔT to solve for ΔH. Sample Problem 17.3

17.2 Measuring and Expressing Enthalpy Changes 21 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Analyze List the knowns and the unknown. 1 KNOWNSUNKNOWN C water = 4.18 J/(g·°C) V final = V HCl + V NaOH = 25.0 mL mL = 50.0 mL T i = 25.0°C T f = 32.0°C density solution = 1.00 g/mL ΔH = ? kJ Sample Problem 17.3

17.2 Measuring and Expressing Enthalpy Changes 22 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. First calculate the total mass of the water. Calculate Solve for the unknown. 2 m water = 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. Sample Problem 17.3

17.2 Measuring and Expressing Enthalpy Changes 23 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Now calculate ΔT. Calculate Solve for the unknown. 2 ΔT = T f – T i = 32.0°C – 25.0°C = 7.0°C Sample Problem 17.3

17.2 Measuring and Expressing Enthalpy Changes 24 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Use the values for m water, C water, and ΔT to calculate ΔH. Calculate Solve for the unknown. 2 Use the relationship 1 kJ = 1000 J to convert your answer from J to kJ. Sample Problem 17.3 ΔH = –q surr = –m water  C water  ΔT = –(50.0 g)(4.18 J/(g· o C))(7.0°C) = –1500 J = –1.5 kJ

17.2 Measuring and Expressing Enthalpy Changes 25 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. 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.

17.2 Measuring and Expressing Enthalpy Changes 26 > Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. ΔH = –m  C  ΔT = –367 g  4.18 J/(g·°C)  (87°C – 24°C) = –97000 J = –97 kJ 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.