- What energy changes occur when a liquid solidifies? When you studied phases of matter, you discovered two characteristic properties of substances: the temperature of melting is different for different pure substances, and the temperature of a melting solid or freezing liquid remains constant as long as both liquid and solid phases are present. The energy that was required to change a solid to a liquid is released when the substance changes from a liquid to a solid. Remember that the change that occurs when a solid melts or a liquid freezes is called a phase change.
- In this experiment, you will use para-dichlorobenzene C6H4Cl2. The heat given off as the substance turns from a liquid to a solid will be used to change the temperature of a measured amount of water.
MATERIALS NEEDED: Safety goggles Ring stand 250-ml graduated cylinder Balance (to 0.01g) Ring clamp Thermometer Test tube Ring support Test tube holder 12 to 13g of solid Wire gauze 250-ml beaker 7-oz polystyrene cup Ice Burner
PROCEDURE:1. Make a bath of boiling water by putting about 150 ml of water into a 250-ml beaker and heating the water to boiling. Use the standard ring stand and wire gauze set-up, with the gas burner below.
2. Find the mass of the empty test tube to the nearest 0.01g and record the mass of the empty tube on the DATA TABLE.
3. Put between 12 and 13g of the para-dichlorobenzene in the test tube and find the mass of the test tube containing the solid to the nearest 0.01 g. Record the mass on the DATA TABLE.
4. Using a 250-ml graduated cylinder, obtain approximately 125 ml of cold water and put it into the polystyrene cup that is to be used as the calorimeter. The level of the water in the cup should be above the level of the solid in the tube when the latter is in the cup. Place the thermometer in the cup. Read the temperature of the water, and if it is not 1 deg.C to 2 deg. C below room temperature, add an ice cube to the water and stir until the temperature has been sufficiently lowered. Remove any remaining ice (very important). The water will be measured accurately later, as melting ice will have increased it slightly.
5. Place the test tube containing the solid material into the boiling water until the material is just melted. AVOID OVERHEATING.
READ STEPS 6 AND 7 AS THEY GO TOGETHER QUICKLY!
6. Using the test tube holder, remove the test tube containing the melted material from the hot water. Allow the liquid material to cool until the first trace of solid is seen on the bottom of the test tube. If you miss this observation and more than a trace of solid is present, place the test tube back in the boiling water for just a moment until the solid melts. Then let it cool again until the first trace of solid appears.
7. While the liquid is cooling, measure and record on the DATA TABLE the temperature of the cold-water bath to the nearest 0.5 deg. C. Place the test tube containing the liquid and trace of solid into the plastic cup and stir gently with the test tube. Note the temperature of the water as you stir and continue to observe the temperature until no further increase is noted. (When reading the thermometer, immerse it in the cup midway between the tube and the side of the plastic cup.) Record the maximum temperature to the nearest 0.5 deg. C.
8. Carefully pour the water from the polystyrene cup into the graduated cylinder. Measure the volume of water to the nearest milliliter.
DATA TABLE:
Mass of empty test tube. = _______ g
Mass of test tube and solid. = _______ g
Initial temperature of the cold-water bath. = _______ deg. C
Final maximum temperature. = _______ deg. C
Volume of water. = _______ ml
CALCULATIONS AND RESULTS:
9. Calculate the mass of the para-dichlorobenzene. This is simply subtracting the mass of the empty test tube, from the mass of the test tube and solid.
Mass of para-dichlorobenzene. = _______ g
10. Calculate the mass of the water in the plastic cup (calorimeter). Remember, one milliliter of water has a mass of one gram, so the mass of the water is the same as the volume.
Mass of water in calorimeter. = _______ g
11. Calculate the temperature change of the water (
Twater). This is simply the final maximum temperature minus the initial temperature of the water in the calorimeter.
Change in temperature of water. = _______ g
12. Calculate the quantity of heat (number of calories) used in effecting the temperature change (neglect the heat required to change the temperature of the plastic cup itself). Remember the specific heat of water is 1 cal / g deg. C. You are calculating this for the water in the calorimeter, but it is also the quantity of heat given up from the para-dichlorobenzene as it changed states. Use the following formula:
heat associated with
temperature change
of the water= mass
of
waterx change
in
temperaturex specific
heat of
water
Quantity of heat. = _______ cal
13. Calculate the heat of solidification per gram of solid. The formula is as follows:
heat
of
solidification= quantity of heat (cal)
mass of para-dichlorobenzene (g)
Heat of solidification per gram. = _______ cal / g
14. Calculate the number of moles of the para-dichlorobenzene you used. Remember, this is just the mass in grams, divided by the formula weight. The formula weight for para-dichlorobenzene is 147.00.
Moles of the para-dichlorobenzene. = _______ mole
15. Calculate the heat of solidification in calories per mole of para-dichlorobenzene. The formula is the same as before, with different units.
heat
of
solidification= quantity of heat (cal)
moles para-dichlorobenzene
Heat of solidification per mole. = _______ cal / mole
- Look up the accepted value in a reference book and compare it to your experimental results. It will probably be called heat of fusion in the reference book.
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Back to Lab Dad's Laboratory
- In this experiment, you will use para-dichlorobenzene C6H4Cl2. The heat given off as the substance turns from a liquid to a solid will be used to change the temperature of a measured amount of water.
HEAT OF SOLIDIFICATION
INTRODUCTION: