Honors Chemistry Energy and Lab Name Date Objectives: Calculate the amount of heat absorbed or released by a substance as its temperature changes. Describe how a calorimeter is used to measure energy that is absorbed or released. Make a hypothesis: What would you expect the final temperature to be if 30 grams of silver at 100 o C was mixed with 30 grams of water at 20 o C? Why? Background: To raise the temperature of an object energy must be added. This energy increases the average kinetic energy of the particles and increases the temperature of the object. Conversely, to decrease the temperature of an object, energy must be released. The particles average kinetic energy is decreased and the temperature of the object decreases. The amount of energy that is gained or released can be mathematically determined using the specific heat of the material. The specific heat is the amount of energy needed to raise the temperature of one gram of a substance one degree Celsius (or one degree Kelvin). Energy is measured in Joules (J) or calories (c). One calorie is the energy needed to raise the temperature of one gram of water one degree Celsius. 1 calorie = 4.184 J. Note: This calorie is different than the Calories for food. One Calorie for food is one kilocalorie in chemistry. 1000 calories (chemistry) = 1 Calorie (food). To determine the energy necessary to change the temperature of a material the following formula is used: Q = amount of energy that is gained or lost m mass of the object in grams cp specific heat of the material (J/g-K) (J/g- o C) ỎT = Final Temperature ( o C) Q = m c! T Q will be a positive or negative number. This will determine if energy has been gained or lost by the object. If energy has been gained then the final temperature will be (less than / greater than) the initial temperature. Then, the final temperature minus the initial temperature will be a (negative / positive) number. Q will then be a (negative / positive) number. Thus, when energy is gained, Q is (negative / positive). If energy has been lost then the final temperature will be (less than / greater than) the initial temperature. Then, the final temperature minus the initial temperature will be a (negative / positive) number. Q will then be a (negative / positive) number. Thus, when energy is lost, Q is (negative / positive). 1
When two objects that are at different temperatures are placed together, the object with a higher temperature will (gain / lose) energy to the object with a lower temperature. The object with a lower temperature will (gain / lose) energy to the object with a higher temperature. Energy will be gained by one object and lost by the other until the two objects are at the same temperature. The heat gained will be equal to the heat lost. The higher temperature will have a (negative / positive) Q and the lower temperature object with have a (negative / positive) Q. Q!"#$ = Q!"#$%& This equation can be used to determine the specific heat of an unknown object or the final temperature of two objects placed together assuming that there is no loss of energy to the surroundings. This means that the second object gains all of the energy that is lost by the first object. Mathematically, the equation would be: Q! = Q! Lab Activity: Go to the following website: m! c!! T! = m! c!! T! http://group.chem.iastate.edu/greenbowe/sections/projectfolder/flashfiles/thermochem/heat_metal.h tml Set the metal to silver, 30 grams, and a temperature of 100oC. Set the water temperature to 0oC. Note that the specific heat of silver is 0.234 J/g-K. Your screen should look like the picture to the right. Now hit start. Observe. When the temperature on the right is constant, record the temperature here:. Then, hit reset. Fill in the chart below: Final Temperature Silver 2
1. How much energy did the silver lose? Show ALL work. 2. How much energy did the water gain? Show ALL work. 3. Are the answers to questions one and two the same or different? Why? 4. Was your hypothesis correct? I am guessing that you chose a final temperature around 60 o C (halfway between 20 and 100). The final temperature was closer to 24 o C. This means that despite having the same mass and either losing or gaining the same amount of energy, the change in temperature was different. For the change in temperature to be different, something else must be different. What is that? 3
Gold and : Make a hypothesis. You have thirty grams of gold and thirty grams of water. The water starts at 20 o C and the gold starts at 170 o C. What will be the final temperature when the two are mixed? Set the metal to gold, 30 grams, and a temperature of 170 o C. Set the water temperature to 20 o C. Note that the specific heat of gold is 0.128 J/g-K. Your screen should look like the picture to the right. Now hit start. Observe. When the temperature on the right is constant, record the temperature here:. Then, hit reset. Fill in the chart below: Final Temperature Gold 5. How much energy did the gold lose? Show ALL work. 6. How much energy did the water gain? Show ALL work. 4
7. The final temperature of the gold and water is almost the same as the silver and water. How is that possible when the gold started at a much higher temperature than the silver? What else was different that may explain this? Identifying an unknown: Specific heat is different for each substance. If you have an unknown substance you can use calorimetry to determine the identity of the substance. Metal X Set up the experiment and record your starting points in the table below: Metal X Hit start and then record the final temperature here:. 8. How much energy did the water gain? Show ALL work. 9. How much energy did the unknown metal lose? Explain. 10. What is the specific heat of metal X? What could be the identity? Show ALL work. 5
Metal Y Set up the experiment and record your starting points in the table below: Metal Y Hit start and then record the final temperature here:. 11. How much energy did the water gain? Show ALL work. 12. How much energy did the unknown metal lose? Explain. 13. What is the specific heat of metal Y? What could be the identity? Show ALL work. Conclusion: Why does it take so much energy to raise the temperature of water? Use this to explain why it takes a long time for the ocean to heat up in the summer and then why it stays warm for the first two weeks in September. 6