CHM 110 - CHEMISTRY AND ISSUES IN THE ENVIRONMENT


List of Chemicals for the Home Labs

Home Laboratory # 2
C. Ophardt, Elmhurst College, c. 2002

COMBUSTION, HEAT, AND CALORIES

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?

INTRODUCTION:


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.