Constructing Heating/Cooling Curve

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Chemistry Lab Report|
Constructing Heating/Cooling Curve|
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Salman Ishaq 12-E|
1/27/2013|

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BACKGROUND

As energy flows from a liquid, its temperature drops. The entropy, or random ordering of its particles, also decreases until a specific ordering of the particles results in a phase change to a solid. If energy is being released or absorbed by a substance remaining at the same temperature, this is evidence that a dramatic change in entropy, such as a phase change, is occurring. Because all of the particles of a pure substance are identical, they all freeze at the same temperature, and the temperature will not change until the phase change is complete. If a substance is impure, the impurities will not lose energy in the same way that the rest of the particles do. Therefore, the freezing point will be somewhat lower, and there will be a range of temperatures instead of a single temperature.

PROBLEM

To evaluate the samples, you will need a heating/cooling curve for pure Na2S2O35H2O that you can use as a standard. To create and use this curve, you must do the following. • Obtain a measured amount of pure Na2S2O35H2O.

• Melt and freeze the sample, periodically recording the time and temperature. • Graph the data to determine the melting and freezing points of pure Na2S2O35H2O.
• Interpret the changes in energy and entropy involved in these phase changes. • Verify the observed melting point against the accepted melting point found in reference data from two different sources. • Use the graph to qualitatively determine whether there are impurities in a sample of Na2S2O35H2O.

OBJECTIVES

Observe the temperature and phase changes of a pure substance. Measure the time needed for the melting and freezing of a specified amount of substance. Graph experimental data and determine the melting and freezing points of a pure substance. Analyze the graph for the relationship between melting point and freezing point. Identify the relationship between temperature and phase change for a substance. Infer the relationship between energy and phase changes.

Recognize the effect of an impurity on the melting point of a substance. Analyze the relationship between energy, entropy, and temperature.

MATERIALS

• Balance, centigram
• Beaker tongs
• Beakers, 600 mL (3)
• Chemical reference books
• Forceps
• Gloves
• Graph paper
• Hot mitt
• Ice
• Lab apron
• Na2S2O35H2O
• Plastic washtub
• ring clamps (3)
• ring stands (2)
• Ruler
• Safety goggles
• Stopwatch or clock with a second hand
• Test-tube clamp
• Test tube, Pyrex, medium
• Thermometer clamp
• wire gauze with ceramic center (2)
• Wire stirrer
Bunsen burner option
• Bunsen burner
• Gas tubing
• Striker
Hot plate option
• Hot plate
Probe option
• Thermistor probes (2)
Thermometer option
• Thermometers, nonmercury (2)

Procedure

PART 1–PREPARATION
1. Put on safety goggles, gloves, and a lab apron.
2. Fill two 600 mL beakers three-fourths full of tap water.
3. Heat water for a hot-water bath. If you are using a Bunsen burner, attach to a ring stand a ring clamp large enough to hold a 600 mL beaker. Adjust the height of the ring until it is 10 cm above the burner. Cover the ring with wire gauze. Set one 600 mL beaker of water on the gauze. If you are using a hot plate, rest the beaker of water directly on the hot plate. 4. Monitor the temperature of the water with a thermometer or a thermistor probe. Complete steps 5–8 while the water is heating. 5. Cool the water for a cold-water bath. Fill a small plastic washtub with ice. Form a hole in the ice that is large enough for the second 600 mL beaker. Insert the beaker and pack the ice around it up to the level of the water in the beaker. 6. Bend the piece of wire into the shape of a stirrer, as shown in Figure 1. One loop should be narrow enough to fit into the test tube, yet wide enough to easily fit around the thermometer without touching it. 7. Prepare the sample....
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