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PART 1: Pre-lab Introduction and Questions
THE STORY OF A BATH TUB OF HOT WATER
1. There is a simple pleasure in taking a hot bath or
a hot shower. As the faucet is turned on, first cold water pours
out, but shortly warm and then very hot water pours out. For this
example, we will think about a bath tub about one third full of
water (approximately that much water is also used in a shower).
2. For most of the Chicago area, the water source is Lake Michigan
or deep wells. Electrical energy is required to pump the water
from the lake or the well to the filtration treatment plant. The
general scheme of events that happens at the treatment plant before
the water pours out of the faucet is as follows. First, chlorine,
as the gaseous diatomic element, is added for disinfection to
kill disease causing bacteria. This is followed by the addition
of "alum", aluminum potassium sulfate, which makes flocs
of gelatinous aluminum hydroxide precipitate, which aids in the
coagulation and settling of suspended solids. Then the water very
slowly travels through large areas of settling ponds and sand
filters to remove various small solid particles. The clarified
and purified water is then treated with: lime, calcium hydroxide,
to neutralize acids; with sodium fluoride to prevent tooth decay;
and possibly some more chlorine to maintain a slight residual
level of disinfection ability. Electrical energy is again used
to pump the water into the tall water storage tanks.
3. In the house a hot water tank is used to heat the water. The
energy to heat the water most likely results from the chemical
energy released as heat in the combustion of natural gas, methane.
The combustion of natural gas, methane, is a very common chemical
reaction requiring oxygen. The end product is carbon dioxide and
water; and excess energy in the form of heat. In other cases,
the water may be heated by electricity, which is generated using
heat from the combustion of coal or nuclear energy (this is another
whole story).
ProfO Notes:
Methane Combustion Reaction Graphic
4. The excess heat resulting from the chemical reaction is a consequence
of the thermal energy of rapidly moving gaseous product molecules.
The hot gas molecules collide with the outside of the steel walls
of the hot water tank and transfer their thermal energy to the
iron atoms in the steel to make them move faster. At the same
time water molecules inside of the tank collide with the iron
atoms in the steel tank, and again energy is transferred from
the iron atoms to the water molecules by making them move faster.
Thus the water "absorbs" the excess thermal energy.
5. The hot water from the tank moves through pipes to the bath tub. Some of the hot water "cools" as it first heats the pipes, the bathtub itself, and finally as it stands for any length of time.
6. The story of the bathtub of hot water continues with the addition
of soap or bath detergent to the water. The purpose of soap or
detergents is to interact with "dirt". grease, and oils,
because water alone is unable to make them soluble. (The details
of detergent action will be discussed later.) The story concludes
as the bath is completed, the water becomes cool, and the dirty
water is released into the drain. The water travels through underground
pipes to the sewage treatment plant. There the water undergoes
primary and secondary treatment to remove "dirt" and
biologically change organic wastes into carbon dioxide and water.
This last reaction is the biological equivalent of combustion
carried out by bacteria. Finally, the water is released into a
nearby river or lake.
COMBUSTION REACTION IN MORE DETAIL:
Excess heat from the combustion reaction results from a complex
consideration of the potential energy or chemical energy
in the bonds of the molecules before and after the reaction. During
the course of any chemical reaction, bonds in the reactant molecules
are broken which requires energy, an endothermic process.
In the combustion of methane, the carbon-hydrogen bonds are broken,
as well as, the diatomic oxygen bonds. As bonds are reformed in
the product molecules, energy is given off, an exothermic
process. In this case carbon-oxygen bonds are formed
to make carbon dioxide; and hydrogen-oxygen bonds are formed to
make water molecules. In a combustion reaction, more energy is
given off in the bond forming products than is required by the
bond breaking reactants, thus the overall reaction is exothermic.
ProfO Notes:
Methane Combustion Reaction Graphic
The exothermic chemical energy of the combustion reaction is transferred
to gaseous products of carbon dioxide and water in the form of
heat. Thermal energy is the consequence of the
rapid motion of molecules. Temperature is the
a measure of the average speed of of the molecular motion. If
the average speed of the molecules increases, the temperature
increases. The measurement of temperature is independent of the
amount of substance present.
Heat is the transfer of thermal energy from a
hot object to a cooler object through the collisions of rapidly
moving molecules. The quantity of heat absorbed or released depends
upon the quantity of substance, as well as, the change in temperatures
of the hot and cold objects. Heat is measured in units of calories*
, which is defined as the amount of energy needed to raise the
temperature of one gram of water one degree Celsius. The amount
of heat transferred to the water is related to its specific heat.
For copper the specific heat is 0.093 cal required to raise the
temperature one degree Celsius for one gram of copper.
* There are 1000 of these small calories in one kilocalorie which
is actually the unit to measure the large Calories in foods.
Prelab Questions: (3 points)
Define of terms:
Ques. 1: The combustion of natural gas results in chemical energy being used to heat the bath tub of water - True or False.
Ques. 2: Measurement of heat does not depend on the quantity of water involved - True or False.
Ques. 3: Heat is transferred from a cold object to a warm object by thermal motions - True or False.
Ques. 4: Temperature is is related to the average speed of the molecules - True or False.
Ques. 5: Temperature of a solution is independent of the volume of the solution - True or False.
Ques. 6: In an exothermic reaction, heat is required for the overall reaction - True or False.
Ques. 7: The bond forming part of the combustion reaction of methane is exothermic - True or False.
Ques. 8: The bond breaking part of the combustion reaction of methane is endothermic because energy is required for this process - True or False.
Ques. 9: Which has more heat content, a hot cup of coffee or a hot bath tub of water? Explain your answer.
Ques. 10: Which has faster moving molecules, a hot cup of coffee or a hot bath tub of water, at the same temperature? Explain your answer.
PART 2. Online Computer Lab (6 points)
HOW MUCH ENERGY IS REQUIRED TO HEAT WATER?
In this experiment, you will determine the amount of energy (specific
heat) that is required to heat water. To determine the specific
heat of water, you will heat a zinc bar to a known temperature
(that of boiling water - 100 degrees C) and then place the zinc
bar into a known volume of water. The zinc bar loses heat to the
water. The water gains heat feom the zinc bar. From the temperature
rise of the water you can calculate the amount of heat energy
transferred from the zinc metal to the water.
EXPERIMENTAL PROCEDURE:
****All Data for Part 2 is given online****
****You do not actually have to complete these procedures.
Dr. Ophardt did them for you and took pictures of the results.
You should read the procedures to see what was done, record the
data, and answer the questions.*****
MEASURE ALL TEMPERATURES IN CELSIUS (Right side of the
thermometer)
Measure (estimate to the nearest tenth of a Celsius degree) and
record the temperature of this water using a thermometer; leave
the thermometer in the cup so that it and the water are both the
same temperature.
1. First obtain a zinc bar and record the weight in the data table.
2. Measure exactly one eighth cup (30 milliliters) of room
temperature tap water. Put this water into a Styrofoam cup.
Assume that 1 ml of water equals 1 gram of water since the density
of water is 1.0 g/ml. Record the weight of water in the data table.
Computer online data of initial weights.
3. Measure the temperature of the tap water in the cup after waiting for 2-3 minutes for the temperature to stabilize after putting it into the cup.
4. a. Place the zinc bar into a beaker containing 1-2 cups
of water. Leave the zinc bar in the boiling water beaker at least
2 minutes after the water boils.
b. Use your hand to hold the thermometer in the boiling water
to measure and record the temperature of the boiling water. Do
this relatively early in the 2 minute period. This boiling water
temperature will also be assumed to be the same for the zinc bar
after 2 minutes.
Computer online data of initial temperatures
4. Use tongs to remove only the zinc
bar from the boiling water. Quickly and without delay dump only
the hot zinc bar into the polystyrene cup containing the 30
ml of tap water from step # 2.
5. Immediately, use the thermometer to stir slightly, and read
the temperature every 30 seconds until the temperature no longer
rises in 60 seconds. Record the final temperature of the water
and the zinc bar.
Computer online
data of final temperatures
QUES. 11: Refer to the fourth
paragraph in Part 1, and use the concepts given in "Combustion
Reaction in More Detail" to describe at a molecular level
what is happening to the zinc bar after it is dumped and the tap
water in the cup. Think in terms of thermal energy, temperature,
and heat. Where is the heat moving? From ? To ?
Compare the movement the zinc atoms in the zinc bar before and
after the zinc bar is transferring energy to the water? Compare
the movement of the water molecules before and after contact with
the zinc bar.
Dr. O, Help!
QUES. 12: DATA: Copy all of the data in the message box form.
Weight of zinc bar ______________________
Temperature (Celsius) of boiling water and zinc bar ______________________
Volume and Weight of water in cup _______ ml __________ g
Original/starting temp. of water in cup ______________________
Final temperature of water in cup; ______________________
At the end of the temperature change for the water, what else
in the cup is also at this final temperature? __________
Dr. O, Help!
CALCULATIONS: Show the work for all calculations.
| General formula needed several times during
the lab: Heat in calories = (weight of object) x (temp. change) x (specific heat of object) |
Ques.13: Copy all of your data and show calculations for
13a-d
Ques.13a Change in water temperature ______________________
Ques.13b. Change in zinc bar temperature ______________________Dr. O, Help!
Ques.13c. Calories of heat transferred from zinc bar
equals
(weight zinc bar) X (zinc bar temp. change) X (0.092 ) = ______________
cal Dr. O, Help!
The specific heat of zinc is: 0.092 cal per gram per degree
Units of gram and degree cancel in calculation
(approximate answer = 50-100 cal)
Ques.13d. Calories of heat transferred from zinc bar
EQUALS (no calculation necessary)
Calories of heat absorbed by water ________________
cal
Ques. 14: Experimental Specific Heat of water
equals
Cal. heat absorbed by water divided by change
water temp. divided by weight of tap water ______________________
(This answer should be in the range of 0.3 to 1.5)
QUES. 15: The actual/true specific
heat of water is 1.0 cal per gram per degree. What is the percent
error in your experimental determination? Dr. O, Help
percent error = (Actual Specific Heat minus Exp. Spec. Heat)
multiply by 100 and divide by the Actual Specific Heat of water
which is 1.0. Again the units will cancel. This is actual crude
experimental data and does contain a rather large error.
QUES. 16: Application: Use the actual specific
heat of water (1.0 cal per g per degree).
16a. If you have 70 grams of water, how many calories of heat
are required to raise the temperature 1 degree?
16b. If you have 2 grams of water, how many calories of heat are
required to raise the temperature 40 degrees?
PART 3. At home collection of lab data (6 points)
HEAT OF COMBUSTION
INTRODUCTION:
What is the efficiency of using a candle to heat water?
In this experiment you will attempt to measure the efficiency
of using a candle to heat water. Again you will measure the temperature
change in a known amount of water. In addition we will measure
the loss in weight of a candle after burning.
EXPERIMENTAL PROCEDURE:
1. Light the white candle provided in the baggie to produce
a good clean burning flame and to burn the tapered end until it
is flat.
2. Measure exactly one cup of tap water (240 ml) into a thin walled
small porcelain coffee cup. Again assume that 1 ml of water equals
1 gram of water since the density of water is 1.0 g/ml. Record
the weight of water in the data table.
3. Measure the temperature of the tap water in degrees Celsius.
4. Note the time and light the candle. Unless you have some type
of metal stand to support the cup, you will have to hold the cup
about 1-2 inches above the height of the candle. Record the temperature
of the water at 1 minute intervals until exactly 10 minutes have
passed. (May increase 15-40 degrees)
5. Blow out the candle, let it cool. For our purposes, the very
large type of candle that you are using will lose 0.84 g of wax
per 10 minutes of burning. Record the weight of candle wax that
undergoes the combustion reaction and is used up.
QUES 17: a. Complete the combustion
reaction as follows (no need to balance):
Candle wax plus oxygen produces carbon dioxide plus water plus
heat
C22H46 + O2 ====> _______ + ______ + heat energy
ProfO Notes:
Chemical Reaction Graphic
Ques 17b. Use the information in the "Combustion
Reaction in More Detail", to list specifically which
molecules have bonds that are broken and which ones have bonds
that are reformed for the reaction in 17a. Then be specific and
list which bonds between which atoms are broken (there are 3 bonds
broken) and formed (there are 2 kinds of formed) for each molecule
from the above equation - be sure to look at the chemical
reaction graphic as there is one more type of bond broken
for the wax as compared to methane
and methane in more detail
Ques 17c. Is the overall reaction exothermic or endothermic?
QUES. 18: DATA:
Weight of candle "burned" ______________________
Weight of water ______________________
Temperature of water in degrees Celsius at beginning ______________________
Temperature of water in degrees Celsius at the end ______________________
Change in temperature of water ______________________
(Most people get 15 to 40 degrees change)
QUES. 19: CALCULATIONS - Show Work for calculations
for partial credit:
Ques 19a. Heat absorbed by the water equals
Wgt. water X Temp. Change X Actual Specific
Heat Water ________________ Dr.
O, Help
(Approximate answer = 3000 - 6000 cal)
Ques 19b. Experimental Heat of Combustion of candle equals
Heat absorbed by water divided by weight of candle "burned"
____________________
Ques 19c. Theoretical Heat of Combustion of the
candle equals
Wgt. candle "burned" X 9000 cal. per gram ____________________
QUES. 20a. Percent Heating Efficiency
equals
(Experimental Heat Combustion of the candle)
divided by (Theoretical Heat of Combustion of the candle) times
100 __________________
QUES. 20b. What does the percent heating efficiency say
about how effective the candle was in heating the water?
QUES. 20c. What happened to the rest of the heat from
the candle that was not used to heat the water?
QUES. 21a. Application Question: A bath tub has
dimensions of 25 cm deep, 60 cm wide, and 120 cm long. Do you
remember the volume formula? length x width x height; 1 cubic
cm = 1 ml; 1 ml water = 1 gram water. What is the weight of water
in the bathtub?
(Approximate answer =150,000 - 200,000 grams)
QUES. 21b. How many calories of heat are needed
to be absorbed to heat this bath tub of water from 20 C to 50
C?
(apply the general formula
to obtain heat absorbed by the water in the tub)
QUES. 21c. Then find the grams of candle required
to heat the bathtub of water:
(calories of heat absorbed by bath tub of water (ans. 21b.) )
divided by (Experimental Heat of combustion of the candle(ans.
19b).
As a point of information, this same bath tub of water requires
20.8 cubic feet of natural gas to be burned.
