PARADISE VALLEY COMMUNITY COLLEGE PHYSICS INTRODUCTION TO PHYSICS LABORATORY. Calorimetry

Transcription

1 PARADISE VALLEY COMMUNITY COLLEGE PHYSICS INTRODUCTION TO PHYSICS LABORATORY Calorimetry Equipment Needed: Large styrofoam cup, thermometer, hot water, cold water, ice, beaker, graduated cylinder, test tube, balance scale. Objectives: (a) To study heat flow. (b) To calculate the heat of melting of ice. Introduction: This experiment will introduce you to the idea that heat flow can be measured by observing the change in temperature of some standard substance, for example, liquid water. Using this method, you will learn that measured amounts of heat produce different temperature changes in different substances. The styrofoam cups used in this experiment are such good insulators that experiments can generally be performed with the cup uncovered. For example, when water originally 15 C below room temperature is placed in an uncovered cup, it increases in temperature by less than 0.2 C per minute. Since the marks on the thermometer scales are one degree apart, the error introduced by heat leakage in the brief experiments suggested here is not much more than the uncertainty in the temperature measurement. The cups are also very light (between 2 and 3 grams) and therefore absorb very little heat. Theory: When we touch a hot stove, thermal energy enters the hand because the stove is warmer than the hand. When we touch a piece of ice, however, thermal energy passes out of the hand and into the colder ice. In such instances, the direction of energy transfer is always from a warmer body to a neighboring cold body. The quantity that tells how warm or cold something is with respect to a standard body is called temperature. We say temperature is a measure of the random translational motion of atoms and molecules in a body; more specifically, it is a measure of the average kinetic energy of atoms and molecules in a body. If we continually add heat to a solid or liquid, the solid or liquid will eventually change state. A solid will liquefy, and a liquid will vaporize. Energy is required for liquefaction of a solid and vaporization of a liquid. The general behavior of 1

2 many substances can be illustrated with a description of the changes that occur with water. Suppose we have a 1 gram cube of ice at a temperature of 50 C in a closed container, and we put it on a stove to heat. A thermometer in the container reveals a slow increase in temperature up to 0 C. At 0 C, the temperature stops rising, yet heat is continually being supplied. This heat melts the ice. To melt the whole gram, 80 calories of heat are absorbed by the ice. Not until all the ice melts does the temperature begin to rise. Procedures: Part A: Heat Transfer You know from experience that when two quantities of water are put together, the temperature of the resulting mixture will be between the two starting temperatures. 1) Predict the final temperature of the mixture when the hot and cold water have identical masses. Accurately weigh 75 grams of hot water in a styrofoam cup. Using another styrofoam cup, accurately weigh another 75 grams of cold water. Measure the temperature of both samples, and note them below. 75 grams of cold water at C. 75 grams of hot water at C. Write down what you predict the resulting mixture temperature will be: C. 2) Explain below how you arrived at your prediction. 3) Mix the hot and cold water, and measure its temperature: C. Is there a difference between predicted and measured temperatures? Describe some possible sources of error in the measured resultant temperature. 4) Predict the final temperature of a mixture when the hot and cold water have different masses. Here s a hint about how to make such a prediction: (a) Is there more hot water than cold? Then the temperature of the mixture will be closer to the original temperature of the (cold/hot - circle one) water. (b) What is the ratio of hot to cold water? For example, let s say it s 4:1 (that is, 4 times as much hot water (at 60 C) as cold water (at 20 C)). In this case, the temperature of the mixture will be 52 C. Think about it... 2

3 5) Measure two samples of water (one from the cold tap, and the other from the hot tap): one 75 grams, and the other 25 grams. Measure the temperature of both samples. 75 grams of cold water at C. 25 grams of hot water at C. Write down what you predict the resulting mixture temperature will be: C. Explain below how you arrived at your prediction. 6) Mix the hot and cold water, and measure its temperature: C. 7) Is there a difference between predicted and measured temperatures? Explain possible sources of error. 8) State the Law of Conservation of Energy: Now that you ve performed these two experiments, answer the following question: 9) How do these experiments illustrate the Law of Conservation of Energy? Part B: Energy of Melting In Part A, you have shown that a loss of heat by one material produces a corresponding gain in another (the cold water was warmed). In this part, the loss of heat from hot water will be used to cause a solid to melt. By keeping track of some temperatures and amounts of materials used, you will be able to tell just how much heat it took to melt the solid. Again, as in Part A, we rely on the truth of the Law of Conservation of Energy, for you will show that the heat lost by the warm water was the amount of heat used to melt the solid. 1) Put exactly 200 cm 3 of warm water in a styrofoam cup, and hold a thermometer in it so that you can measure its temperature. 2) Have your lab partner fill a test tube half full of ice, and quickly put the 3

4 tube in an ice water bath to help cool down the test tube. 3) Leave tube and ice in the ice water bath for 1 minute. Remove the tube from the bath, and put it in the warm water, but just before doing so, measure the temperature of the warm water and record below. Temperature of water just before inserting test tube: C. 4) The solid ice will start turning into liquid as it absorbs heat from the warm water. Twirl the test tube to quicken melting. 5) At precisely the moment the last bit of ice melts, record the temperature of the warm water here. Temperature of warm water just as last bit of ice melts: C. 6) Pour the melted ice water from the test tube into an empty graduated cylinder, and record its volume here. Volume of ice water: cm 3. Now that you ve completed the experiment, you can tell exactly how much heat was given up by the warm water to melt the ice. But to do so, you need one definition (which you should memorize): The calorie is the amount of heat required to change the temperature of 1 gram of water by 1 degree Celsius. Fill in the following (show your work below for all calculations): Initial water temperature (in C):. Final water temperature (in C):. Temperature change as water cooled (in C):. Mass of warm water cooled (in grams):. Heat lost by the water (in heat calories):. Heat gained by melting ice (in heat calories):. Mass of melted ice (in grams):. calories/grams of ice melted:. 4

5 Discussion and Questions: 1) List some possible reasons for the differences between calorie your value and the accepted value. Be as specific as you can. 2) How can the transfer of energy from a hot material to a cold one be explained? 3) How can the process of melting absorb energy? 5