Calorimetry POGIL.notebook December 06, 2012

Calorimetry POGIL.notebook

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December 06, 2012

POGIL ACTIVITY

Calorimetry ‐ Measurement of Heat Energy

Why?

The amount of heat energy released or absorbed by a chemical or physical change can be measured using an instrument called a calorimeter. This measurement is based on the law of energy conservaon. One important way of thinking about the law of energy conservaon is to recognize that if two bodies (body A and Body B) are in contact with each other, any heat lost by body A must equal heat gained by body B. Calculang the heat energy released or absorbed by a reacon is important in understanding the stability of compounds, predicng equilibrium concentraons, and idenfying condions that allow us to predict whether a reacon will occur.

Prerequisites• Law of energy conservaon• Exothermic change• Endothermic change


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Informaon

• Heat can be thought of as the flow of energy between two bodies because of a difference in temperature• Heat always flows from a body at a higher temperature to a body at a lower temperature unl both bodies are at the same temperature• Kinec energy is the energy associated with the moons of atoms or molecules• Temperature is a measure of the kinec energy of the atoms or molecules in a body; it is not energy


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ModelThe instrument chemists use to measure the heat energy involved in reacons is called a calorimeter. The calorimeter is composed of an insulated container (like a Styrofoam coffee cup) containing a measured mass of water. A thermometer rests in the water. A reacon vessel is also placed inside the water ‐ this is where the reacon actually occurs. The reacon vessel is thus surrounded by the water in the calorimeter.

Thermometer

waterinsulated container

reacon vessel


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Key Quesons

Case 1

1. If heat energy is released by the reacon occurring in the reacon vessel, where will this heat be transferred?

2. What will happen to the temperature of the surrounding water in the calorimeter?

3. What is the term given to a reacon that releases heat?


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Case 2

4. If heat is absorbed by the reacon occurring in the reacon vessel, where will the heat be absorbed from?

5. What will happen to the temperature of the surrounding water in the calorimeter?

6. What is the term given to a reacon that absorbs heat?


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7. State a relaonship, in words, between the heat lost or gained by a reacon occurring in the reacon vessel and the heat lost or gained by the surrounding water in the calorimeter.

8. State a relaonship, in words, between the heat lost/gained by a reacon and the change in temperature of the water in the calorimeter.


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9. How do you think the mass of the water in a calorimeter will affect the change in temperature of the water? That is, if a reacon released X amount of heat energy, how would the temperature of the water differ in a colorimeter containing 50 g of water versus one containing 100 g of water?

10. When using a calorimeter to measure the heat energy absorbed or released by a reacon, chemists use the symbol Q to represent the heat. What is the SI unit of heat energy?


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11. The heat released/absorbed by a reacon in a calorimeter (Q) is directly proporonal to the mass of the water in the calorimeter (given the symbol m) and directly proporonal to the change in the temperature of the water. The symbol used for the change in

temperature is T (where , to

represent a proportionality sign, can you write a mathemacal relaonship among Q, m and T?

12. What are the units of m (mass water) and T in this relaon?


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Key Equaon (try to answer number 13 and if you are not sure, ask!)

13. To change the proporonality sign, α, to an equality (= sign), we need to introduce a proporonality constant. This

proporonality constant is given the symbol c. Can you rewrite the relaon among Q, m and T into a mathemacal equaon by including c?

(Hint: put the Q term on the le of the equal sign and all others on the right).

14. Manipulate the above equaon to isolate c on the le. What are the units of c?

15. The proporonality constant c is known as the specific heat of a substance (in this case water). It is a physical property of a substance. What informaon does the specific heat of a substance provide?

us how much energy (T)to 1g of a substance by

1°C

e l l

raise


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Informaon• The specific heat of water has a value of 4.18


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Exercises

A calorimeter was used to measure the heat involved in a chemical reacon. The calorimeter contained 200. g of water and had an inial temperature of 25.0oC. When the reacon was finished, the temperature of the water increased to 60.0oC. Answer quesons 16‐19:

16. Was the chemical reacon endothermic or exothermic? How do you know?

17. What is the value for m, the mass of the water?

18. What is the change in temperature, T?

19. Calculate the amount of heat energy (Q) involved in the reacon and include the unit.


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20. Calculate the amount of heat energy required to raise the temperature of 150. g sample of water from 15.0oC to 75.0oC.


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The relaonship between Q, m, c and T can be applied to any pure substance.

21. A 100. g sample of pure lead is heated from 10.0oC to 197.5oC by the addion of 3,000J of heat energy.

a) Calculate the specific heat of lead. Be sure to include units.

b) Compare the specific heat of water with that of lead. If the same amount of energy is supplied to equal masses of each substance, would you expect both substances to have the same temperature change? Explain.

c) Which one will have a greater temperature change?


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Addional Calorimetry Problems

1. A reacon occurs in a calorimeter containing 500.0 g of water at a temperature of 22.0 oC. The reacon causes the temperature of the water to increase to 37.4 oC.

a) Is the reacon endothermic or exothermic? Explain.

b) Calculate the heat energy involved in the reacon.


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2. A reacon occurring in a calorimeter containing 200.0 g of water absorbs 850 J of heat energy. The inial temperature of the water is 24.5 oC.

a) Will the temperature of the water increase or decrease? Explain.

b) Calculate the final temperature of the water.


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3. Calculate the amount of heat energy needed to raise the temperature of a 50.0 g sample of aluminum from 21.0oC to 88.0oC. The specific heat capacity of aluminum is 0.89 J/g oC.


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4. Exactly 1.0 kJ of heat energy is added to individual 30.0 g samples of silver (specific heat capacity = 0.24J/g oC) and gold (specific heat capacity = 0.13 J/g oC). Both samples are at a temperature of 25.0 oC.

a) Predict which sample will undergo a greater change in temperature.

b) Calculate the change in temperature for each sample.


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Challenge Problem!

5. A 100.0 g sample of water at a temperature of 80.0 oC is mixed with a 60.0 g sample of water at a temperature of 25.0oC. Calculate the final temperature of the mixture.

a) Let’s first look at this problem conceptually. You have a sample of hot water (80.0 oC) and a sample of room temperature water (25 oC). When the two samples come in contact, in what direcon will heat flow?

b) How will the direcon of heat flow affect the temperature of the hot water? How will it affect the temperature of the cold water?

c) Which sample of water will lose heat and which sample will gain heat?

d) We know the heat will flow from one sample to the other unl they are both at the same temperature. How does the heat lost by one sample compare to the heat gained by the other sample?


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5e) Can you set up a quantave relaonship that will allow you to calculate the final temperature of both samples?


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Calorimetry POGIL.notebook December 06, 2012

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