Physical Sciences 11
Experiment 1
How bad is that snack anyway?
Monday, 2/10 – Wednesday, 2/12
Science Center Room 117
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Please read this entire document and complete the attached prelab before your lab.
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This page and all subsequent pages should be stapled together and submitted to
your Lab TF before you leave the laboratory.
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All students must submit their own lab reports.
TF Name:
Student Name:
Lab Partner(s):
Grading:
Prelab:
PS11 2014 Experiment 1 / 10
Lab Report:
/ 20
Safety:
/3
Cleanup:
/2
Total:
/ 35
TF check
1 Introduction
Most of the calories in a snack such as a cheese ball are concentrated in carbohydrates (80%) and
fats (15%). Your body metabolizes these classes of compounds to create energy that helps run
cellular processes. In the first half of this lab you will attempt to determine the caloric content of a
snack using simple yet effective laboratory procedures.
Food scientists use high quality, sensitive devices called calorimeters to measure the energy content
of foods in units of Calories (Cal; 1 Cal = 1,000 cal). In general, calorimetric measurements
involve a complete burning of food in a vessel immersed in water, and the rise in temperature of the
entire assembly is monitored. While we will use a much simpler setup to measure the caloric content
of a snack in this experiment, the basic principles are similar.
Before you measure the calories in a snack, you will first determine the efficiency of your
“calorimeter” by measuring the energy density (i.e. energy content per unit mass) of a candle and
comparing it to the known value. Your calorimeter will allow you to determine the temperature rise
of water that is in contact with the object giving off energy (the candle). Using the temperature
information and the relationship below, you will calculate the energy output of the candle. Also,
we can safely assume all of the energy is in the form of heat because <1% of a candle’s output
goes into the generated light.
where qrxn
qcalor
m
Cs
Tf
Ti
=
=
=
=
=
=
qrxn = − qcalor =− m Cs ΔT = −m Cs (T f − Ti )
the heat of reaction
the thermal energy received by the calorimeter
mass of water
specific heat capacity of water
the final temperature of water
the initial temperature of water
For more information on Heat and Heat Capacity, please refer to our textbook page 3.18 - 3.20.
You will then determine the calories in a snack by combusting it in a similar manner and taking
into account the efficiency of your calorimeter.
In the second half of this lab, you will use a coffee cup calorimeter to determine the enthalpy change of
the following two chemical reactions.
(1)The neutralization of citric acid with sodium hydroxide.
(2) Dissolving ammonium chloride in water.
Please read about Enthalpy in our textbook page 3.22-3.24, including the text in the blue box titled
“Reactions in the Liquid Phase: An Important Example of Reactions at Constant Pressure” and “Check
Yourself 5 – Measuring ∆Hrxn in a Coffee-Cup Container”.
PS11 2014 Experiment 1 2 Procedure
Safety Precautions
• Safety glasses and nitrile gloves and lab coats must be worn at all times.
• You will be working with fire. Long hair must be tied back. Keep all flammable
materials (notebooks, lab reports, etc.) off of your lab bench during the experiment.
•
Please see Laboratory Safety Manual for PS11 2014 for detailed safety precautions.
Part Ia. Determining the efficiency of the calorimeter
1) Measure the mass of a weighing dish and thumbtack using an analytical balance. Secure a
candle to the weighing dish with the thumbtack. Measure the mass of the candle using the same
balance.
2) Fill an empty soda can with 60 g of water (approximately 1/3 of a can). Determine the mass of
the water in the can. Clamp the filled can one inch above the candle.
3) Clamp a digital thermometer in the can, making sure the tip of the thermometer is submerged
in the water but not touching the bottom of the can. Record the initial temperature of the
water. To do this, record the water temperature at 10s intervals for 40s. Is the temperature
constant? If no, please wait for one more minute before re-measuring the temperature. Repeat this
step until the temperature stabilizes before proceeding to step 4.
4) Light the candle and gently slide it under the can. Take temperature readings every 30
seconds till the temperature reaches around 80 o C. Do NOT stir with the thermometer.
5) Measure the mass of the extinguished candle (subtract the mass of the weigh boat and
thumbtack).
Part Ib. Determining the calories in a snack
1) Repeat the procedure with new water using a cheese ball secured onto a cork and wire in a
weighing dish, but take temperature readings every 15 seconds until the cheese ball is
completely burned. In addition, take temperature readings for an additional 30 seconds after
the snack has extinguished.
2) Be careful when you carry the burned residue over to an analytical balance (you may want to
carry it in a weighing dish).
PS11 2014 Experiment 1 3 Part IIa. Determining the ΔHrxn Enthalpy of Reaction for Acid/Base Neutralization
1) In this part of the experiment, you will measure the enthalpy of reaction between citric acid (H3Cit)
and sodium hydroxide (NaOH) using a procedure of your own design.
2) You will be given 0.50 M solutions of each, a digital thermometer, and Styrofoam® coffee cups. Be
sure to check the bottom of your cups for any holes.
3) As part of your prelab report, you wrote a detailed procedure for this part of the experiment based on
a stoichiometric ideal; however, for the actual experiment you will collect a random volume of the
solutions. The total quantity of NaOH and citric acid must be 30 mL and you should you not use
less than 10 mL of each solution.
4) After you collect your data, you will share it with other groups in your lab section.
Part IIb. Determining the ΔHrxn Enthalpy of Reaction for the Dissolution of Ammonium Chloride
(NH4Cl)
1) In this part of the experiment, you will measure the enthalpy of reaction between ammonium
chloride and water using a procedure of your own design.
2) You will be given solid ammonium chloride, distilled water, a digital thermometer, and Styrofoam®
coffee cups. Be sure to check the bottom of your cups for any holes.
3) As part of your prelab report, you wrote a detailed procedure for this part of the experiment. The
quantity of ammonium chloride should be between 6 and 9 grams and the water should be roughly
30 grams.
4) After you collect your data, you will share it with other groups in your lab section.
Waste Disposal and Clean-up
• Pour water from the soda cans down the drain and leave the empty can at your station.
• Snacks, ashes, used matches, and burnt candles go into the trash. Before disposing of used
candles and used matches, run them under tap water in the sink to make sure they are
extinguished before you dispose of them in the trash.
•
Pour the liquid waste from Part II in the liquid waste container in the back.
•
•
Leave corks, wires, and empty weigh boats back at your station.
Wipe down your lab bench.
PS11 2014 Experiment 1 4 Lab Report Part Ia. Determining the efficiency of the calorimeter
Mass of water
Initial mass of candle
Final mass of candle
Initial Temperature recordings:
0s_________
10s_________ 20s__________ 30s________ 40s__________
Temperature recorded while burning
Time (s)
Temperature (°C)
Observations
0
30
60
90
120
150
180
210
240
270
300
330
360
390
420
450
PS11 2014 Experiment 1 5 1. Use Excel program on the lab computer, input the temperature v.s. time data, make a
plot and find the initial slope of the curve. Send the Excel file to your TF’s email box
before the end of the lab. Determine the ratio of the change in mass of the candle (∆m) to
the change in temperature of water (∆T). You may assume that the burn rate is constant.
2. Calculate the apparent energy density of the candle in J/kg.
3. Given that the actual energy density of the candle is approximately 4.4 × 107 J/kg, determine
the efficiency of the calorimeter, i.e. the fraction of thermal energy released by the candle that gets
converted into heat in the water.
Efficiency =
PS11 2014 Experiment 1 6 Part Ib: Determining the calories of a cheese ball
Mass of water
Initial mass of snack
Final mass of snack
Time (s)
Temperature (°C)
Observations
0
15
30
45
60
75
90
105
120
135
150
165
180
195
210
225
240
PS11 2014 Experiment 1 7 4. Calculate the energy density of the cheese ball (in J/kg and Cal/g), taking into
account the efficiency of the calorimeter that you calculated above.
Energy Density =
5. State all the assumptions that you made in determining the caloric content of your snack.
6. What are some sources of error in the procedure or assumptions made?
PS11 2014 Experiment 1 8 Part IIa. Determining the ΔHrxn for Acid/Base Neutralization
7. Please consider what data you would like to collect in order to find the ΔHrxn for the reaction as
written below, then create a data table. After you perform the experiment, please calculate ΔHrxn in
terms of kJ per “mole of reaction”. Is the reaction exothermic or endothermic? (Please use 4.184 J/ g·°C
for the specific heat of water)
H3Cit (aq) + 3 NaOH (aq)
Data Table
Na3Cit (aq) + 3 H2O (l)
Calculations
Part IIb. Determining the ΔHrxn for the Dissolution of Ammonium Chloride
8. Please consider what data you would like to collect in order to find the ΔHrxn for the reaction as
written below, then create a data table. After you perform the experiment, please calculate ΔHrxn in
terms of kJ per “mole of reaction”. Is the reaction exothermic or endothermic? (Please use 4.184 J/ g·°C
for the specific heat of water)
NH4Cl (s) + H2O (l)
Data Table
PS11 2014 Experiment 1 !"# NH4+ (aq) + Cl- (aq) + H2O (l)
Calculations
9 Prelab
To be completed prior to lab and checked off by your TF as you enter the lab.
1. Given the following data:
Initial mass of snack food: 0.9873 g
Final mass of residue: 0.4568 g
Mass of water in can: 249.7 g
Initial temperature: 24.8 °C
Final temperature: 33.2 °C
Calculate the heat released, in calories per gram of oil, for this snack food.
2. The addition of hydrochloric acid to a silver nitrate solution precipitates silver chloride according
to the following reaction:AgNO3 (aq) + HCl (aq) → AgCl (s) + HNO3 (aq). When 50.0 mL of 0.100
M AgNO3 is combined with 50.0 mL of 0.100 M HCl in a coffee-cup calorimeter, the temperature
changes from 23.40 °C to 24.21 °C. Calculate ΔHrxn (kJ per mole of reaction) for the reaction as
written. Use 1.00 g/mL as the density of the solution and C = 4.18 J/g · °C as the specific heat
capacity. (Hint: this is the practice question on page 3.24 of our textbook. Reading through pages
3.22-3.24 should be helpful.)
PS11 2014 Experiment 1 10 3. Write out a detailed procedure for determining the change in enthalpy for the reaction between
citric acid (H3Cit) and sodium hydroxide (NaOH). H3Cit + 3 NaOH
Na3Cit + 3 H2O.
Be sure to calculate what volume of each 0.50 M solution should be mixed to give a total volume of
30.0 mL and the reactants in the correct stoichiometry. Also, write a data table in the space given
on page 9.
4. Please write out a data table for the dissolving of ammonium chloride on page 9.
5. You and your lab partners will use different amounts of solution in Part IIa and different amounts
of ammonium chloride in Part IIb. Theoretically, should you find the same result for ΔHrxn (kJ per
mole of reaction) for both reactions? Please explain your answer.
PS11 2014 Experiment 1 11