Heat of fusion and evaporation
This page and the top half of the second page are to be completed before you report for lab. There are also things to be filled out in the body of your report. To answer the following questions read all of this lab, pages 113-119 in your text and use your lecture notes.
1) heat of fusion -
2) heat of evaporation -
3) specific heat -
4) calorimeter -
5) Explain, using complete sentences, what the equation below says. The symbols used are those from the lab sheet (so you must read the entire lab before answering).
The heat necessary to change the temperature of a certain quantity of water from Ti to Tf is given by the expression:
Mw (Tf - Ti) Sw
Read all of this experiment, then tell what each of the following symbols represents.
6) Mc =
7) Ti =
8) Tf =
9) Sc =
10) he =
11) hf =
12) Mw =
13) Sw = Tell what this is and give the value in cal/gr Sw =
14) When the lab asks for the value of the mass of the can, what other item is always to be included when the can is weighed? __________________________
(Be sure the thermometer is never in the can when it is being weighed!)
16) Before coming to the lab, do the questions on the last page of your lab that are numbered 14, 15, 16, 17, 18, and 19.
TO BE DONE IN LAB (Read before coming to class)
Object: To study how energy causes temperature changes and changes of phase (solid to liquid and liquid to gas).
Specific Heat: To do this experiment, you will need to understand the concept of specific heat. (The amount of energy necessary to change the temperature of 1 gram of substance, one degree Celsius).
General Procedure: The energy from melting ice and condensing steam will be used to change the temperature of a known amount of water, and the energies of fusion and evaporation of water will be calculated from this data.
A calorimeter is container that is designed to lose as small an amount of heat to its surroundings as possible. The calorimeter you will use uses an air layer as an insulator (it is essentially a can within a can). You will need to include the inside can in your calculations as it will absorb some heat energy as its temperature changes. The metal stirrer will always be included as part of the can when weighing the can, both when empty and when it contains water. The specific heat of this can is usually stamped in it. You will need to weigh it to determine its mass.
Caution: Do not put the ice in your mouth as it is not clean!
Procedure: Each person will work by themselves on this experiment.
1) Weigh the inner calorimeter cup with the stirrer.
(Be sure it is dry before weighing) Mass = mc = ____________ grams
2) Record its specific heat Sc = _____________ cal/g Co.
(Number stamped on can or ask instructor)
3) Fill the inner cup about half full of warm tap water (about 45 degrees Celsius), and weigh on the balance to determine the exact amount of water added.
a) Can + Water = _____________________ g
b) Empty can (see 1) = ________________ g
3) (a - b) mass of water = ____________________ g (Subtract)
4) Put the inter cup back into the ring in the calorimeter. Put the lid on the calorimeter and place a thermometer in the large hole, and the stirring rod in the small hole. Stir it well.
5) Record the temperature of water just before adding ice
6) Take your calorimeter to the front of the room. Your instructor will add some ice when you request him to do so. Do not add the ice yourself.
7) Stir, constantly observing the temperature, record the lowest temperature reached after all the ice has been melted Tf =______
8) Weigh the can with the water and melted ice, subtract its previous weight to determine the mass of the ice.
M (can+water+ice) = _____________
Write this same number in the blanks in question 10a and in 12b, place a check in the following blanks when you have finished. 10a __ 12b__
8b) M(can+water+ice)_________ - M (can+water)__________ = MI (ice) _____________.
When you are in the lab, you may skip down to (Eq. 1) on the next page. Write your values on top of the symbols in that equation. The purpose of the explanation below is to show you how the equations for determining the heats of fusion and vaporization are obtained.
9) The heat absorbed by the melting ice cools the water and the can. The equation is :
heat gained by ice = heat lost by water + heat lost by can
a) heat lost by water = (temperature change of water) x (mass of the water) x (specific heat of water)
= (Tf - Ti) x Mw x Sw
b) heat lost by can = (temperature change of can) x (mass of the can) x (specific heat of can)
= (Tf - Ti) x Mc x Sc
c) heat gained by ice water = (heat of fusion) x (Mass of ice) + (temperature change of the water after melting) x (mass of water)
= (hf) x MI + MI x (Tf - O oC) x Sw
The full equation then becomes
hf x MI + MI (Tf - O oC) x Sw = (Tf - Ti) x Mc x Sc + (Tf - Ti) x Mw x Sw
And the heat of Fusion is given by: (Write your numbers on top of the symbols in the equation below.)
(Ti - Tf) (Mc x Sc + Mw x Sw) - MI Tf
(Eq. 1) hf =__________________________________________
use this equation to calculate hf
(show your work) Write your answer below
hf = heat of fusion of water = _________________________
10) If you have not changed the contents of you cup since the last weighing, record the values below. If something has changed, re-weigh and record the values.
a) weight of can + water = ____________ (from item 8, the melted ice is now water also!)
b) Mc = weight of empty can = ___________ (from item 3)
(a - b) Mw = weight of water = _____________
(Subtract these two to obtain the weight of the water.)
11) Record the temperature of the water, just before adding the steam
Ti = ____________C.
When using the steam generator the utmost caution must be used to ensure no one is burned!!! Do not attempt to use this unless instructed to do so be the instructor!
Run steam from the generator through your solution until the temperature reaches about 40 oC. Stir well for a few seconds and read the final temperature Tf = __________.
12) Again weigh the inner can, stirrer and water. ______________
M (can+water+steam) - M (can+water) = Ms (steam)
Ms (steam) = ____________________ -_____________ = ________________
(12 a) (12 b)
When in lab, you may now skip to (Eq. 2) on the next page.
13) The heat lost by the steam is gained by the water and the can. The equation for this is (heat lost by steam) + heat lost by water (formed when the stem condenses) and as it changes from 100 oC to Tf = heat gained by water.
heat lost by steam = (mass of steam) x (heat of evaporation of water) = Ms x he
heat gained by water = mass of water x temperature change x specific heat
= Mw (Tf - Ti) x Sw
heat gained by can = mass of can x temperature change x specific heat
= Mc x (Tf - Ti) x Sc
heat lost by the water after condensing from steam
= Ms x (100 - Tf) x Sw
The full equation is then:
Ms x he + Ms (100 - Tf) x Sw = Mc (Tf - Ti) Sc + Mw x (Tf - Ti) x Sw
And solving for he, we have: [Write your numbers on top of the symbols below.
Note that the values of Tf, Ti and Mw are all different than in the previous problem. Mc and Sc are unchanged. (Write your numbers on top of the symbols in the equation below.
(Tf - Ti) (Mc Sc + Mw Sw) - Ms (100 - Tf)
(Eq. 2) he = ________________________________________________
(show your work) use this equation to calculate he = ____________________
14) Look up the following information in your text or use your lecture notes. Do the calculations indicated. (include units in all your answers)
15) Heat of fusion of water (in Joules/gram) __________________
16) Number of Joules/calorie ___________________
17) Use the number in 16) to convert the heat of fusion (item 15) to calories/gram._____________________________
18) The heat of evaporation of water in Joules/gr. from your text (including units) is ___________________________________________
19) Change this to calories/gram. ________________________
20) Calculate the % error of hf (clearly show your work)
% error = Correct answer(from question 17) - Answer from Eq. 1
Correct answer (from question 17)
In the first blank substitute the numbers from your experiment.
% error in hf = _____________________________________ = ____________
Substitute in your values then show the final result.
19) Repeat the calculation to find the % error of he.
(Compare your answer from Eq. 2 to that from question 19)
Show your work as in 20.
% error in he = _____________________________________ = _________
Tell some things that can cause errors in this experiment and what can be done to improve them.