Condensing H10: steam latent heat of vaporization
Materials:
hot plate with rod and flask clamp, Erlenmeyer flask with attached rubber hose,
large Styrofoam cup, can to secure cup, scales, temperature probes
Initial definitions and givens:
latent heat of vaporization (Lv) - the number of calories required to change one
gram of a substance from a liquid to a gas at its boiling point. Its also equal to
the number of calories released when one gram of a substance condenses from a
gas to a liquid.
Lv = 540 cal/g for water
Initial Instructions and questions:
Warning: Steam and hot water can cause serious burns! The hot plate is
slow to warm up, but it and the flask get very hot and stay hot for a long time
after the hot plate is turned off. Only heat the water when the flask is held
stable by the clamp. Be careful that the cup is stable inside the heavy can,
and the hose reaches to the bottom of the cup. When the experiment is over,
dont try to remove the flask from its clamp. The hose will also be hot.
You can boil water in the Erlenmeyer flask and create a stream of steam coming
out the rubber hose. This steam is to be bubbled through a Styrofoam cup of
room temperature water.
Design an experiment to determine Lv for water. You will use conservation of
energy, as we did in H9 to determine Lf. Here we produce steam at 100 C, it
condenses to liquid water in the cup and then the newly condensed water drops in
temperature until equilibrium reached is with the water in the cup. Discuss each
of the terms in the H1 = - H2. What is gaining heat and what is losing heat?
When you have it figured out, call over a teacher and explain it to them.
Hints: Use no more than 50 mL of hot water in the Erlenmeyer flask or it will take
too long to boil. Starting with hot water speeds up the process. You will not boil
it all away. Use 200 mL of room temperature water in the large Styrofoam cup.
Follow the temperature of the water in the cup from before the boiling starts until
the water reaches 60 C or more. The exact temperature is not important but
determine what it is. Stir occasionally, but keep the cup covered with another cup
most of the time to avoid loss of heat to the surroundings. Do you need to know
how much water is left in flask? Why not? Do you need to determine a final
weight of the water in the cup? Why?
H10 (continued)
- page 2 -
Guide to notes in your lab notebook:
1. How did your value of Lv compare with the accepted value?
2. Discuss possible sources of error?
3. How would you change your experiment to obtain a more reliable result?
Practice problems:
1. How much heat does it take to vaporize 10 g of water at 100C?
2. How much heat is required to turn 10 g of water at 90 C into 10 g of steam at +110
C? (Note: the specific heat of steam is 0.48 cal/gC.)
3. If 10 g of 100 C steam is mixed with 200 g of 20 water, what is the final temperature
of the 210 g mixture?