1. Chapter 17 Thermochemistry 17.2 Measuring and Expressing
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
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2. How can you measure the amount of heat released when a match burns?
CHEMISTRY & YOU
How can you measure the amount of heat released when a match burns?
Remember: The concept of specific heat allows you to measure heat flow in chemical and physical processes.
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3. How can you measure the change in enthalpy of a reaction?
Calorimetry
Calorimetry
How can you measure the change in enthalpy of a reaction?
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4. 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.
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5. 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.
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6. 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.
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7. 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.
The terms heat and enthalpy change are used interchangeably in your textbook.
In other words, q = ΔH.
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8. Constant-Pressure Calorimeters
Calorimetry
Constant-Pressure Calorimeters
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
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9. 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
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10. 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
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11. 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.
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12. Constant-Volume Calorimeters
Calorimetry
Constant-Volume Calorimeters
Calorimetry experiments can also be performed at a constant volume using a device called a bomb calorimeter.
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13. Constant-Volume Calorimeters
Calorimetry
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.
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14. Constant-Volume Calorimeters
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.
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15. 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.
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