CALORIMETRY

There is a pan of water that been equilibrating with the room for a sufficient time that it has reached room temperature at the back of the lab.  Use this for your 'room temperature' water.

In this lab you will study heat energy as it is transferred between objects. The historical measure of heat energy is the calorie, which is the amount of heat energy that must be added to 1 gram of water at 14.5 oC to raise its temperature by 1oC.

For the first part of the experiment, you will be investigating the transfer of heat energy from warm water to cool water. Heat lost by the warm water will be gained by the cool water until thermal equilibrium is attained. The amount of heat exchanged depends on the kind of material (water in this case); the amount of material; and the temperatures involved. Heat energy is given the symbol Q. The heat gained of lost by an object is defined by:

where M is the mass of the object, c is its specific heat capacity (1 cal/g oC for water),  Delta-T is the change in temperature of the object. You will use Logger Pro to acquire a graph of temperature versus time.  The graph should show three distinct regions: a horizontal line starting at t = 0 that gives you room temperature; an increasing region with a fairly steep (and possibly irregular) slope when you are adding the hot water; and a gradual rise to a maximum followed by a slight fall in temperature due to losing heat from the warmer water into the cooler room. You have the information to calculate the mass of the room temperature water and the mass of the hot water that was added.  By applying conservation of energy - i.e. Qrt + Qh = 0, you can find the temperature of the hot water before it was mixed with the room temperature water.

In the second part of the experiment, you will measure the specific heat of a metallic object by investigating how the object effects the temperature of a known amount of water.   In preparation, start a beaker of water heating on the electric hot plate. We will use boiling water to bring our metallic sample for the second part to a known initial temperature. In this experiment you will measure all of the temperatures and masses.  Using the accepted value for the specific heat of water you will be able to find the specific heat capacity of the metal mass.  You will compare your results to the accepted values for various metals and allows and choose the most likely candidate for the metal that makes up your mass.

Before you begin either experiment, check the room temperature reading of your temperature probe against the liquid-in-glass thermometer.  The two need not exactly agree, but they should be close.  Once this has been done you are ready to start the first part of the experiment.

#### Part 1 - Mixtures of water.

• Measure the mass of a stacked pair of dry Styrofoam cups. Add about 100 mL of ROOM TEMPERATURE water to the cups, and again measure the mass. Calculate the mass of water.
• Place your temperature probe into the cup. Open Logger Pro and set the experiment time to be 300 seconds. Allow the probe to equilibrate for about a minute before proceeding.
• Using the other pair of cups, get a similar amount of HOT WATER. The water may have to run for a while to get hot.  It is not necessary to make a measurement with the graduated cylinder. Sometimes there is a problem with the lab's hot water.  If so, your instructor will give you directions.
• Start the computer taking temperature data.
•  Quickly, return to your experimental area and carefully pour the hot water into the cup with the temperature probe.
• Use the temperature sensor as a stirring rod and continue to stir the water until the temperature has reached its maximum value and started to drop, or until the 300 seconds is up. Do not allow the sensor to rest on the sides or bottom of the cup.
• Find the change in temperature from the initial room temperature water to the maximum temperature of the mixture. (You may find rescaling your graph and using the 'Analyze', 'Examine' option to be useful.) You should be cautious if you use the statistics option to find the maximum that there are no spikes or other anomalies in your data - otherwise the reported maximum might not be the physical maximum temperature.
• How much heat was gained by the room temperature water? How much heat was lost by the hot tap water?
• Again measure the mass of the cups that contain the water and determine how much hot water was added.
• Finally, calculate the initial temperature of the hot tap water.

#### Part 2 - Finding the specific heat of a metal

• Once the water that you are heating starts to boil, suspend the 200 gram mass from a piece of string so that it is fully immersed. Allow the water to boil for AT LEAST 5 Minutes. Do not allow the mass to rest on the bottom of the beaker.
• In the meantime, empty your cups and put about 100 grams of room temperature water in a stacked pair. There are two approaches: The first is to add some water and then measure the mass.  Subtract off the mass of the dry cups and you know how much water is present.  The 100 grams is approximate, 90 to 110 will do.  You just need to know the mass.   The second method is to realize that the density of water is 1 g/ml, so if you add 100ml of water to your cups, the mass of water should be (approximately) 100g.
• Use the liquid-in-glass thermometer to measure the temperature of the boiling water - do not allow the thermometer to touch the sides or bottom of the beaker.  We have all been taught that water boils at 100 oC. Did you MEASURE the temperature of the boiling water to be EXACTLY 100 oC? If not, why do you think that your temperature might have been different?
• Place your temperature probe in the cup of room temperature water and allow it to come to equilibrium. Wait at least a minute before having the computer start taking data. Stir gently and do not allow the probe to rest against the wall or bottom. You should observe a horizontal region on your plot.
• Quickly and CAREFULLY bring the mass over to your cup by means of the string. Move your temperature probe to the side of the cup (but not touching the wall) and immerse the mass in your water. Gently lift and lower your mass to stir the water while taking care that the hot mass does not touch the temperature probe.
• Continue until the temperature starts to decline or until the 300 seconds is up.
• Scale your graph so that the portion of interest fills the graph, then print your graph.
• Again analyze your data and determine the heat capacity of your mass. What material do you think the mass is made of?   (hint: search the web for the specific heat of metals/alloys). What do you think could be done to make the experiment more accurate?