Thermodynamics WS 2
Using a Calorimeter
Look at the diagram of a calorimeter on p. 3 of your Thermodynamics notes.
1. The calorimeter shown on the transparency is used to measure the caloric content of foods. To do this, a sample of food is burned inside the reaction chamber of the calorimeter. What is the system? What are the surroundings?
System = reaction inside the chamber
Surroundings = everything else
2. What besides food must be added to the chamber? Explain why.
O2 – combustion requires O2
3. What are the products of the reaction that takes place in the reaction chamber?
CO2, H2O, heat
4. Why is the calorimeter isolated?
So heat doesn’t escape
5. What does the thermometer measure?
Temperature of water surrounding the reaction chamber
6. Describe the movement of heat as the reaction takes place inside the chamber.
Heat moves from reaction chamber to water surrounding chamber.
7. Assuming that no heat escapes from the calorimeter, what equation would you use to determine the amount of heat released by the burning food in the reaction chamber? Define all variables in the equation.
q = cp x m x DT, all of these variables for H2O
8. Does the answer obtained from the equation in question 7 have a positive or negative value? Explain why. What is the sign of DH for the reaction?
Positive, because the water is taking in energy. This means the reaction itself is exothermic.
9. What is the energy content of a 1.25 g sample of oatmeal that raises the temperature of 2.50 kg of water in a calorimeter from 25.0oC to 27.2oC? Assume that the specific heat of water is 4.18 J/g oC.
(A: 18 kJ/g oatmeal)
10. A calorimeter that contains 2.5 kg of surrounding water is used to measure the heat associated with the reaction of 520 g of a reactant. Calculate the energy, in joules per gram of reactant, associated with the reaction if the temperature of the water changes from 27.40oC to 20.28oC. Assume that the specific heat of water is 4.18 J/g oC. Is the reaction exothermic or endothermic? (A: 140 J/g; endothermic)
11. A calorimeter that contains 6.050 kg of surrounding water is used to measure the heat associated with the reaction of 0.500 mol of a substance. If the initial temperature of the water is 15.0oC and the final temperature is 19.5oC, how much heat, in joules per mole of reactant, is associated with the reaction? Also express this heat in calories. Assume that the specific heat of water is 4.18 J/g oC, and that
4.184 J = 1.000 cal. Is the reaction exothermic or endothermic?
(A: 2.2 x 105 J/mol; 5.3 x 104 cal/mol; exothermic)
12. Predict the final temperature of 2.50 kg of water within a calorimeter if the water is at 25.0oC before a 1.8 g piece of dried peach with an energy content of 18.5 kJ is burned. Assume that the specific heat of water is 4.18 J/g oC. (A: 26.8oC)
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