1. 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
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2. 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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3. 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.
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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.
Most chemical reactions and physical changes carried out in the laboratory are open to the atmosphere and thus occur at constant pressure.
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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.
In this textbook, the terms heat and enthalpy change are used interchangeably.
In other words, q = ΔH.
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8. 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).
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9. 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.
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10. 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
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11. 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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12. 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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13. 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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14. 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.
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15. 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.
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16. 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
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17. 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.
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18. 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
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19. 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.
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20. 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.
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21. 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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22. = –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
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23. 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
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24. 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
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25. BIG IDEA
Matter and Energy
The heat of reaction or process can be determined experimentally through calorimetry.
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26. END OF 17.2
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