Heat can be thought of as the flow of energy between two bodies because of a difference in temperature

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1 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 conservation. One important way of thinking about the law of energy conservation 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. Calculating the heat energy released or absorbed by a reaction is important in understanding the stability of compounds, predicting equilibrium concentrations, and identifying conditions that allow us to predict whether a reaction will occur. Prerequisites Law of energy conservation Exothermic change Endothermic change Information 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 until both bodies are at the same temperature Kinetic energy is the energy associated with the motions of atoms or molecules Temperature is a measure of the kinetic energy of the atoms or molecules in a body; it is not energy 1

2 Model The instrument chemists use to measure the heat energy involved in reactions 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 reaction vessel is also placed inside the water this is where the reaction actually occurs. The reaction vessel is thus surrounded by the water in the calorimeter. 2

3 Key Questions Case 1 1. If heat energy is released by the reaction occurring in the reaction 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 reaction that releases heat? 3

4 Case 2 1. If heat is absorbed by the reaction occurring in the reaction vessel, where will the heat be absorbed from? 2. What will happen to the temperature of the surrounding water in the calorimeter? 3. What is the term given to a reaction that absorbs heat? 4

5 7. State a relationship, in words, between the heat lost by a reaction occurring in the reaction vessel and the heat gained by the surrounding water in the calorimeter. State a relationship, in words, between the heat lost by a reaction occurring in the reaction vessel and the heat gained by the surrounding water in the calorimeter. 8. State a relationship, in words, between the heat lost by a reaction and the change in temperature of the water in the calorimeter. State a relationship, in words, between the heat lost by a reaction and the change in temperature of the water in the calorimeter. 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 reaction 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 reaction, chemists use the symbol Q to represent the heat. What is the SI unit of heat energy? 5

6 11. The heat released/absorbed by a reaction in a calorimeter (Q) is directly proportional to the mass of the water in the calorimeter (given the symbol m) and directly prop 12. What are the units of m (mass water) and T in this relation? Key Equation (try to answer number 13 and if you are not sure, ask!) 13. To change the proportionality sign, α, to an equality (= sign), we need to introduce a proportionality constant. This proportionality constant is given the symbol c. 14. Manipulate the above equation to isolate c on the left. What are the units of c? 15. The proportionality constant c is known as the specific heat of a substance (in this case water). It is a physical property of a substance. What information does the spec 6

7 Information The specific heat of water has a value of 4.18 Exercises A calorimeter was used to measure the heat involved in a chemical reaction. The calorimeter contained 200. g of water and had an initial temperature of 25.0 o C. When the reaction was finished, the temperature of the water increased to 60.0 o C. Answer questions 16 19: 1. Was the chemical reaction endothermic or exothermic? How do you know? 2. What is the value for m, the mass of the water? 3. What is the change in temperature, T? 4. Calculate the amount of heat energy (Q) involved in the reaction and include the unit. W 7

8 1. Calculate the amount of heat energy required to raise the temperature of 150. g sample of water from 15.0 o C to 75.0 o C. The relationship between Q, m, c and T can be applied to any pure substance. 2. A 100. g sample of pure lead is heated from 10.0 o C to o C by the addition of 3.00 X 10 3 J 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 s c) Which one will have a greater temperature change? 8

9 Additional Calorimetry Problems 1. A reaction occurs in a calorimeter containing g of water at a temperature of 22.0 o C. The reaction causes the temperature of the water to i a) Is the reaction endothermic or exothermic? Explain. b) Calculate the heat energy involved in the reaction. 2. A reaction occurring in a calorimeter containing g of water absorbs 850. J of heat energy. The initial temperature of the water is o C. a) Will the temperature of the water increase or decrease? Explain. b) Calculate the final temperature of the water. 9

10 3. Calculate the amount of heat energy needed to raise the temperature of a 50.0 g sample of aluminum from 21.0 o C to 88.0 o C. The specific heat ca 4. Exactly 1.00 kj of heat energy is added to individual 30.0 g samples of silver (specific heat capacity = 0.24J/g o C) and gold (specific heat capacity = 0 a) Predict which sample will undergo a greater change in temperature. b) Calculate the change in temperature for each sample. 10

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