Lab 8: Chemical Processes

 

Lab 8: Chemical Processes
Introduction
Have you ever needed to place a cold pack on
a sprained muscle?
It’s the final seconds of the community league champion‐
ship basketball game, and your team is behind by one
point. One of your team’s players takes a shot and scores.
The game is over, and your team won! But something is
wrong: the player is sitting on the floor, and appears to be
in a lot of pain. The coach quickly brings a cold pack to the
player, squeezes it, and places it on the swelling ankle. The
bag immediately becomes cold—but how?
Though we often use them interchangeably, heat and tempera‐
ture have different definitions—though they are closely related
in the study of thermodynamics. Heat is the transfer of energy
from one object to another due to a difference in tempera‐
ture. Temperature, on the other hand, describes how much energy the atoms and molecules in a substance have. This en‐
ergy, often called internal energy, describes how quickly the atoms or molecules in a substance move or vibrate around.
When an object gains heat its molecules vibrate with more energy, which we can sense or measure as an increase in tem‐
perature. When you touch a hot object, it feels hot because a heat moves from the hot object (higher energy) to your skin
(lower energy). Similarly, an object feels cold when heat is lost by your hand and gained by the cold object. Heat always
transfers in the direction of high temperature to low temperature—high energy to low energy.
Both physical processes and chemical reactions can release or absorb energy in the form of heat. When a reaction or physical
change gives off energy it is called an exothermic process. To remember exothermic, think of ‘exiting’ as in leaving or going out. An
endothermic process does just the opposite—it takes in energy from its surroundings. The generalized chemical equations for exo‐
thermic and endothermic reactions are:
The direction energy moves determines whether the process is considered endothermic or exothermic, and tells you how the tem‐
perature of a system changes. In an endothermic reaction or physical change, energy is absorbed and the overall temperature of
the system decreases. Some examples of endothermic processes include the melting of water in a soft drink or the evaporation of a
liquid. Similarly, an endothermic reaction takes in energy for chemical changes to occur. One example is what occurs in an instant
cold pack like the ones used to decrease the swelling caused from a sports injury. These types of cold packs utilize the chemical
exothermic:
endothermic:
reactants → products + energy
reactants + energy → products
Figure 1: The combustion of fuel, such as wood or coal, is a com‐
mon example of an exothermic reaction. Under the right condi‐
tions (usually the application of enough heat), a chemical reac‐
tion occurs between wood and the oxygen in air. Fire is the re‐
Concepts to explore:
 Understand the difference between endothermic and exothermic
processes
 Understand the concept of enthalpy90
Pre‐lab Questions
1. Define enthalpy:
2. What is the relationship between the enthalpy of a reaction and its classification as endothermic or exother‐
mic?
3. With instant hot compresses, calcium chloride dissolves in water and the temperature of the mixture in‐
creases. Is this an endothermic or exothermic process?
Lab 8: Chemical Processes
process of ammonium nitrate (NH4NO3 ) dissolving in water. The ammonium nitrate needs to absorb heat from the surrounding
water to dissolve, so the overall temperature of the mixture decreases as the reaction occurs.
In contrast, energy is released in an exothermic process. An example of an exothermic reaction is what occurs in common hand
warmers. The increase in temperature is the result of the chemical reaction of rusting iron:
4 Fe(s) + 3 O2(g)  2 Fe2O3(s) + energy
Iron usually rusts fairly slowly so that any heat transfer is not easily noticed. In the case of hand warmers, common table
salt is added to iron filings as a catalyst to speed up the rate of the reaction. Hand warmers also have a permeable plastic
bag that regulates the flow of air into the bag, which allows just the right amount of oxygen in so that the desired tempera‐
ture is maintained for a long period of time. Other ingredients that are found in hand warmers include a cellulose filler,
carbon to disperse the heat, and vermiculite to insulate and retain the heat.
Enthalpy is a quantity of energy contained in a chemical process. In the cases we will be dealing with, the energy released or ab‐
sorbed in a reaction is in the form of heat. Enthalpy by itself does not have an absolute quantity, but changes in enthalpy can be
observed and recorded. For example, if you stick your finger into a glass of cold tap water, it probably feels pretty cold. However,
after being outside on a freezing winter day for a long period of time, the same glass of water might actually feel warm to touch. It
would be difficult to measure the absolute quantity of energy in the water in either case, but it is relatively easy to notice the move‐
ment of energy from one object to another. In exothermic reactions, heat energy is released and the change in enthalpy is negative,
while in endothermic reactions, energy is absorbed and the change in enthalpy is positive.
Note: the energy term on the right side shows that the reaction is exothermic, but is not required.91
Lab 8: Chemical Processes
Experiment: Cold Packs vs. Hand Warmers
In this lab you will observe the temperature changes for cold packs and hand warmers. Since temperature is defined as the
average kinetic energy of the molecules, changes in temperature indicate changes in energy. You will use simply a Styro‐
foam cup as a calorimeter to capture the energy. The customary lid will not be placed on the cup since ample oxygen from
the air is needed for the hand warmer ingredients to react within a reasonable amount of time.
Procedure
Part 1: Cold Pack
1. Measure 10 mL of distilled water into a 10 mL graduated cylinder.
2. 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.
3. Place a thermometer and a stirring rod into the calorimeter (Styrofoam cup). CAUTION: Hold or secure the
calorimeter AND the thermometer to prevent breakage.
4. Pour the 10 mL of water into the calorimeter containing the ammonium nitrate, (NH4NO3) taken from the cold
pack.
5. Immediately record the temperature and the time.
6. Quickly begin stirring the contents in the calorimeter.
7. 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.
8. 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.
9. 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.
Materials
Safety Equipment: Safety goggles, gloves Scale
Entire contents of a hand warmer Stir rod
1/4 contents of a cold pack Spatula
Calorimeters (2 Styrofoam cups) Stopwatch
Thermometer (digital) Distilled water*
10mL Graduated cylinder *You must provide92
Lab 8: Chemical Processes
2. Carefully place and hold the thermometer in another Styrofoam cup.
3. Cut open the inner package of a hand warmer and quickly transfer all of its contents into the calorimeter. Immedi‐
ately 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 reac‐
tion 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.
Data
Time (sec) Temp. (0C) Time (sec) Temp. in (0C)
Initial 240
30 270
60 * 300
90 330
120 360
150 390
180 420
210 450
Minimum Temperature (0C) : __________
Table 1: Cold pack data93
Lab 8: Chemical Processes
Time (sec) Temp. (0C) Time (sec) Temp. in (0C)
Initial 240
30 270
60 * 300
90 330
120 360
150 390
180 420
210 450
Maximum Temperature (0C) : __________
Table 2: Hand warmer data94
Graph
Graph the data from Tables 1 and 2 as two sepa‐
rate 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.
Cold Packs vs. Hand Warmers
-10
0
10
20
30
40
50
60
70
0 30 60 90 120 150 180 210 240 270 300 330
Time in seconds
Temperature inoC
Cold Packs Hand Warmers
Use the following space to graph your data:
Lab 8: Chemical Processes95
Calculations
Calculate the overall temperature change for the cold and hot pack substance. HINT: This is the difference in the maximum
temperature and minimum temperature of each.
a. Cold pack ΔT:
b. Hand warmer ΔT:
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.
Lab 8: Chemical Processes96
3. Which pack had the greatest change in enthalpy? How do you know?
Lab 8: Chemical Processes

 

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