Do you do chemistry lab works for the high school 10th graders? One of them is attached.Lab: Chemical Processes
Objectives
•
Understand the difference between endothermic and exothermic processes.
•
Understand the concept of enthalpy.
Introduction
If you’ve participated in sports, spent anytime
snowboarding, camping, or fishing; chances
are, you’ve seen either an instant hot pack or
cold pack. Cold packs are great ways to treat
minor sprains. You simply squeeze the cold
pack and in just a few minutes, the selfcontained device delivers ice cold relief to the
injured area. A similar product, available at
many large grocery stores, was developed by
a world famous chef – it’s the instant cup of
hot coffee. You simply press a button on the
bottom of the container, shake the contents,
and wait about five minutes before drinking a
steaming cup of hot, gourmet coffee.
Figure 1: NASA engineers test the Russian
RD-180 rocket engine in order to measure the
performance of the Atlas III rocket propulsion
system. The combustion of a mixture of liquid
oxygen and kerosene acts as a powerful
propellant producing an extreme exothermic
reaction.
These devices rely on chemical reactions and
consequently the transfer of heat during that
reaction. During a chemical reaction new
products are formed as the reactants are
brought together. This transformation of
matter requires a transfer of heat. Energy bonds which hold the original material
together must be broken, this can only happen with an application of energy. Creating a
new substance also requires energy to form new bonds. During any chemical reaction
energy must either be released or absorbed.
© KC Distance Learning
Both the cold pack and the instant cup of hot coffee combine chemicals to transfer
energy. Energy can be released in a variety of forms including heat, light, or sound;
however, almost every chemical reaction transfer energy as heat. During a reaction,
energy is transferred between the “system” (the area where the reaction takes place)
and the “surroundings” (everything outside the system). Energy can “exit” the system
and move to the surroundings as in an exothermic reaction; however, energy can also
move in the opposite direction. In an endothermic reaction energy “enters”, or is drawn
into the system from the surroundings. Since most energy transfer occurs as heat,
exothermic reactions may produce warmth while endothermic reactions may produce a
cooling affect in the surroundings.
Exothermic Reaction:
reactants → products + energy
Endothermic Reaction:
reactants + energy → products
The reaction of the chemicals in the
cold pack, the hand warmers, and
the instant hot coffee require a
transfer of energy. Most common
cold pack products use ammonium
nitrate (NH4NO3) to create a
reaction with water (H2O). While
most chemical hand warmers rely
on the addition of salt (NaCl) to
accelerate the oxidation reaction of
iron (Fe) and oxygen (O2), usually
a very slow process. One
interesting design component of
hand warmers is the permeable
plastic mesh bag which regulates the flow of oxygen to control the desired amount of
heat generated during the reaction. Visit this website to see the technology behind the
instant cup of hot coffee — the flash animation on the home page of the site is also very
informative.
Enthalpy describes the quantity of energy contained in a chemical process. Enthalpy
does not have an absolute quantity, but a change in enthalpy can be observed and
recorded. On most occasions, if you stick your finger into a glass of cold tap water, it
probably feels cold. Try the same simple experience after spending a time building a
© KC Distance Learning
snowman on a freezing winter day. The same glass of water may actually feel warm to
touch. Measuring the absolute quantity of energy in the water would be difficult;
however, it is relatively easy to notice the movement of energy between finger and
water. Heat energy released in exothermic reactions produces a negative enthalpy.
Conversely, heat energy absorbed in an endothermic reaction produces a produces a
positive enthalpy.
Pre-lab Questions
1. In your own words, what is the definition of enthalpy?
2. What is the relationship between the classification of a reaction as endothermic
or exothermic and the enthalpy of the reaction?
3. Chemicals combine in instant cold packs and the temperature of the mixture
decreases. Is this an exothermic or endothermic process?
© KC Distance Learning
Experiment: Hand Warmers vs. Cold Packs
During this lab activity you will observe temperature changes produced by the activation
of chemicals in instant cold and hot packs. Temperature is the measurement of the
average kinetic energy of the molecules within a substance and can therefore be used
to indicate a change in energy. You will create a simple calorimeter using a Styrofoam
cup and thermometer to capture and measure the energy of the reaction. Follow all
safety precautions and use the proper safety equipment during this experiment. Be sure
to make careful observations during each phase of the experiment.
Materials
•
Safety Equipment: Eye goggles, gloves
•
Plastic stir stick or chop stick
•
Contents of hand-warmer packet
•
Spatula made from plastic straw
•
¼ contents of cold packet
•
Measuring cup
•
Calorimeter (1 12oz. Styrofoam cup)
•
Measuring spoons
•
Stop watch (tracking time in seconds)
•
Distilled water
•
Thermometer
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Procedure
1. Prepare a clean area in your kitchen to safely conduct the experiment.
2. Collect all of the materials from the materials list – do not combine materials until
you have completely read through the lab instructions.
3. Be sure to wear both your safety goggles and gloves when handling any
chemicals.
Part 1: Cold Pack
4. Measure 2 teaspoons (10 mL) of distilled water into a measuring cup.
5. Place about 1/4 (or approximately 10.0 g) of the ammonium nitrate crystals found
in the solid inner contents of a cold pack into a Styrofoam cup. The Styrofoam
cup is used as a simple calorimeter.
6. Place a thermometer and a stirring rod into the calorimeter (Styrofoam cup).
CAUTION: Hold or secure the calorimeter AND the thermometer to prevent
breakage.
7. Pour the 10 mL of water into the calorimeter containing the ammonium nitrate,
(NH4NO3).
8. Immediately record the temperature and the time.
9. Quickly begin stirring the contents in the calorimeter.
10. Continue stirring and record the temperature at thirty second intervals in Table 1.
You will need to stir the reaction the entire time you are recording data.
11. Collect data for at least five minutes and until after the temperature reaches its
minimum and then begins to rise. This should take approximately 5 to 7 minutes.
12. Record the overall minimum temperature in the appropriate place on the data
table.
Part 2: Hand Warmer
1. Wash and dry the thermometer. HINT: Remember to rinse it with distilled water
before drying.
2. Carefully place and hold the thermometer in another Styrofoam cup.
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3. Cut open the inner package of a hand warmer and quickly transfer all of its
contents into the calorimeter. Immediately record the initial temperature of the
contents and being timing the reaction. HINT: Data collection should start quickly
after the package is opened because the reaction will be activated as soon as it
is exposed to air.
4. Quickly insert the stirring rod into the cup and begin stirring the contents in the
calorimeter.
5. Continue stirring and record the temperature at thirty second intervals in Table 2.
You will need to stir the reaction the entire time you are recording data.
6. Let the reaction continue for at least five minutes and until the temperature has
reached its maximum and then fallen a few degrees. This should take
approximately 5 to 7 minutes.
7. Record the overall maximum temperature in the appropriate place in the data
table.
8. Dispose of the components of both solutions through approved household
methods.
9. Dispose of the solid components through approved household methods.
10. Clean your equipment and work area using approved cleaning procedures.
© KC Distance Learning
Data and Observations
Table 1
Time (sec)
Initial
Cold Pack Data
Temp. (°C)
Time (sec)
240
30
270
60
* 300
90
330
120
360
150
390
180
420
210
450
Temp. (°C)
Minimum Temperature (°C): _______________
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Table 2
Time (sec)
Initial
Hand Warmer Data
Temp. (°C)
Time (sec)
240
30
270
60
* 300
90
330
120
360
150
390
180
420
210
450
Temp. (°C)
Maximum Temperature (°C): _______________
© KC Distance Learning
Graph
Graph the data from Tables 1 and 2 as two separate lines on the same chart. Be sure to title your graph
and label each axis. Include the units in the axis labels. An example is shown here.
Use the following space to graph your data:
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Post-lab Questions
1. Which pack works by an exothermic process? Use experimental data to support
your answer.
2. Which pack works by an endothermic process? Use experimental data to support
your answer.
3. Which pack had the greatest change in enthalpy? How do you know?
© KC Distance Learning
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