S water = J/gC

Transcription

1 EXPERIMENT 7 Specific Heat Capacity of A Metal INTRODUCTION One of the properties of matter is that it requires a certain amount of heat energy to raise the temperature of a unit mass of a substance. This property is termed the heat capacity of a substance. If one measures the amount of heat needed to raise 1 gram of a substance 1 Celsius degree, this property is termed the specific heat capacity. For water, the specific heat capacity, S, is Joules per gram per Celsius degree, or S water = J/gC One of the properties of metals is that they are good conductors of heat. This means that metals have low specific heat capacities, and that it takes small amounts of heat energy to raise their temperatures. A heat transfer experiment is carried out within a well-insulated vessel known as a calorimeter. Because it is insulated, the calorimeter can effectively confine heat transferred during an experimental procedure. In the calorimeter, one substance will lose heat energy and another substance will gain heat energy. To differentiate between heat energy gained and heat energy lost, the following algebraic sign convention is used: heat energy gained by a substance is reported as positive, and heat energy lost by a substance is reported as negative. In this experiment we will find the specific heat capacity of an unknown metal. If a heated sample of hot metal is dropped into a calorimeter containing cool water, the metal will lose heat energy, causing the temperature of the metal to decrease, and the cool water will gain the heat energy from the metal, causing the temperature of the water increase. This process occurs unitl both the metal and the water reach the same intermediate temperature. The heat energy gained by the water in the calorimeter (Q water ) can be calculated from the specific heat capacity of the water solution (S water ), the mass of the water solution (m water ), and the change in temperature of the water solution ( T water ): Q water = S water m water Twater Because the water is gaining heat energy, Q water should be a positive value. For this to happen the change in temperature must be calculated as T = T final - T initial. If we assume that no heat energy escapes from the calorimeter, then the heat energy gained by the water must have come from the heat energy lost by the metal. Because the metal is losing heat energy, Q metal should be a negative value. Therefore, the heat energy lost by the metal (Q metal ) is equal in magnitude but opposite in sign to the heat energy gained by the water (Q water ): Q metal = -Q water 51

2 From the relationship for the metal: Q metal = S metal m metal T metal by knowing the heat energy lost by the metal (Qmetal), the specific heat capacity of the metal (S metal ) can be calculated from the mass of the metal (m metal ) and the change in temperature of the metal ( T metal ), where T = T final - T initial : S metal = Q metal m metal T metal PROCEDURE 1. Students will work individually for this experiment. Except for the laboratory handout, remove all books, purses, and such items from the laboratory bench top, and placed them in the storage area by the front door. For laboratory experiments you should be wearing closed-toe shoes. Tie back long hair, and do not wear long, dangling jewelry or clothes with loose and baggy sleeves. Open you lab locker. Put on your safety goggles, your lab coat, and gloves. 2. Place a clean, dry 250-mL beaker on a milligram balance. Tare the beaker by pressing one of the keys with the symbol " T/0 ". This will zero out the mass of the beaker. 2. Remove the beaker from the balance chamber, add about 25 g of metal pellets to the beaker, place it back on the balance, read the mass, and record it in your Data Table. CAUTION: Never transfer chemicals to a container that is on the pan in the balance chamber remove the container from the balance chamber, add the chemical, and then return the container to the pan in the balance chamber. 3. Attach an iron ring to a ring stand, and place a ceramic-centered wire gauze on the iron ring. Adjust the height of the iron ring so that when a laboratory burner is placed on the ring stand there is 5 cm of space between the top of the burner and the ceramic-centered wire gauze. Add about 400 ml of water to a 600-mL beaker, and place the beaker on the ceramic-centered wire gauze. Heat the water to boiling. 52

3 4. Pour the metal pellets into the extra large test tube. Obtain a 2-pronged clamp from drawer 022 at the back of the lab room and attach it the ring stand. Insert the extra large test tube through the 2-pronged clamp and into the beaker of boiling water. Carefully set the bottom of the extra large test tube on the bottom of the beaker. Do not tighten the 2-pronged clamp to the test tube; you want the test tube to be able to slide easily through the clamp. Have the 2-pronged clamp low enough so that at least 5 cm of test tube is exposed above it, as shown below. Heat the metal in the boiling water for 5 minutes. 5. While the metal pellets are in the boiling water, place a clean, dry 150-mL beaker on a milligram balance and tare its mass. Place about 50 to 60 ml of deionized water into the beaker, place it on the balance, read its mass, and record it as mass of calorimeter water in your Data Table. 53

4 6. Obtain three styrofoam cups and a lid from the box on the back counter. Place the three styrofoam cups in a 250-mL beaker for support as shown below. This is your calorimeter. Add the 50 to 60 ml of water to the top styrofoam cup, and using a thermometer measure the temperature of the water in the styrofoam cup. Record it in your Data Table as the initial temperature of calorimeter water. 7. Measure the temperature of the boiling water in the 600-mL beaker. The metal should now be at the same temperature, so record it in your Data Table as initial temperature of hot metal. Obtain a 3- pronged clamp from drawer 023 in the back of the lab room, and attach the 3-pronged clamp to the test tube above the 2-pronged clamp. Using the 3-pronged clamp as a handle, lift the test tube out of the boiling water and quickly pour the metal from the test tube into the top styroform cup. 8. Stir the water in the styrofoam cup with a stirring rod, then place the lid on the calorimeter and insert the thermometer into the water through a hole in the lid. Observe the temperature rise for several minutes, occasionally swirling the water, and record the maximum temperature the water achieves as the final temperature of calorimeter water and metal. 9. Empty the water from the calorimeter into the sink, and place the wet metal in the Pyrex casserole dish. Dry the Styrofoam cups and lid, and return the dry Styrofoam cups and lid to the box on the back counter. Do not throw them away. 10. Clean and wipe dry your laboratory work area and all apparatus. When you have completed your lab report have the instructor inspect your working area. Once your working area has been checked your lab report can then be turned in to the instructor. 54

5 EXPERIMENT 7 LAB REPORT Name: Student Lab Score: Date/Lab Start Time: Lab Station Number: DATA TABLE Mass of Metal. g Mass of Calorimeter Water. g Initial Temperature of Calorimeter Water. C Initial Temperature of Hot Metal. C Final Temperature of Calorimeter Water and Metal. C 1 Temperature Change of Calorimeter Water. C 2 Heat Gained by Calorimeter Water. J 3 Heat Lost by Metal. J 4 Temperature Change of Metal. C 5 Specific Heat Capacity of Metal. J/g C CALCULATIONS 1. 55

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8 POSTLAB QUESTIONS 1. Calculate the number of joules of heat energy necessary to raise 250. g of water from 19.6 C to 65.4 C Box your answer. 2. Calculate the mass of a piece of copper that releases 545 joules of heat energy when cooled from C to 26.3 C. The specific heat capacity of copper is J/gC. Box your answer. 3. Find the specific heat capacity of an unknown metal if a 36.5 g sample at 99.6 C produces a resulting temperature of 25.0 C when placed in a calorimeter containing g of water at 20.2 C. Box your answer. 58