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Experiment 8 โ Calorimetry
INTRODUCTION
Heat flow between a system and its surroundings is something everyone experiences every day
without realizing it. Do you ever feel too hot or too cold? The level of thermal comfort is
dependent on the temperature of the air surrounding us, as well as the body temperature
regulated by metabolic processes. These reactions, which process food for energy, in turn
provide our bodies with heat as a byproduct.
Exchange of this heat with our surroundings is what determines our thermal comfort level. If the
exchange of heat is equal to the surroundings, we are comfortable. If the exchange of heat is
unequal, we can become either too hot (the surrounding temperature is higher than our body
temperature and as a result, sweating occurs) or too cold (the surrounding temperature is colder
than our body temperature and as a result, shivering occurs to build up heat energy).
Scientists and engineers often study the calorimetric interaction between human beings and their
surroundings in an attempt to maximize the thermal comfort level we experience on a day-to-day
basis. This is demonstrated through building design and standard air-conditioning systems.
Although the calorimeters applied in these endeavors are more sophisticated than the basic
coffee cups you will be using, the principles of the heat measurement experiments remain the
same.
COFFEE CUP CALORIMETER
A simple, but highly efficient set-up for a coffee cup calorimeter consists of two nested
Styrofoam coffee cups, a thermometer, and a cover. It is important to note which cup is used as
the inner cup and maintain this arrangement throughout the experiment. The same thermometer
should also be used to reduce variability in measurement.
Energy is defined as the capacity to do work and is involved in all physical and chemical
processes. Changes that absorb energy are referred to as endothermic, and changes that release
energy are exothermic. As a result, a system may gain or release energy, as indicated by a change
in temperature.
The Law of Conservation of Energy states that energy cannot be created nor destroyed, but may
be transferred from one object to another. With this principle in mind, it is important to look at
the concept of specific heat.
Specific heat (C) is the amount of heat that raises the temperature of 1 gram of substance by 1ยฐC.
This physical property varies depending on the substance and is represented by the equation:
In many textbooks, the amount of heat is referred to as โQโ or โqโ. We will use the latter, in our
case. Therefore, the specific heat equation is simplified as:
For water, the specific heat capacity is used to define the value of a calorie since it takes 1.00 cal
to raise the temperature of 1 gram of water by 1ยฐC. The units of measurement for the amount of
heat may be converted to the SI unit, joules, by the following conversion factor, 1 cal = 4.184 J.
CALORIMETRY
In theory, to measure the specific heat of a solid such as a metal, the temperature of the metal can
be measured before and after heating. With the initial mass of the metal, the specific heat can be
determined. However, this method requires that the amount of heat (q) transferred to the metal is
known. It is difficult to determine how many calories the metal absorbed.
Instead, if a solid piece of metal is heated and placed into a container of cool water, the metal
cools down while the water heats up. Eventually, both will reach the same temperature. The
metal transferred its energy to the water and the amount of energy lost by the metal is equal to
the amount of energy gained by the water and the container (calorimeter).
From the Law of Conservation of Energy, the following statement is made:
Thus, the energy transfer from the metal is represented in the equation below:
An important factor to pay attention to are the sign notations (+/-). These allows us to tell which
direction the heat is moving, whether into the object (+) or out of an object (-).
When we make a statement such as the one above, it must be properly translated into the
provided equation to accurately represent the transfer of energy. These values are equal, but
opposite in sign.
Thus, the equation is expressed with a negative sign in front of the q (metal).
The negative fixes this problem, allowing us to perform calculations where a positive value is set
equal to another positive value.
CALCULATION [Q (WATER)]
Example:
A metal sample at 100.0ยฐC is placed into water at 24.2ยฐC. The final temperature of the water
reached was 36.3ยฐC. How much heat was absorbed by the calorimeter if the heat capacity is 152
J/ยฐC?
q (cups) = Heat Capacity*โT = (152 J/ยฐC)(36.2ยฐC-24.2ยฐC) = 1840 J
PROCEDURE
Part I - Determining the Heat Capacity of a Calorimeter
- Obtain two Styrofoam coffee cups, a plastic lid, and a 100 mL graduated cylinder.
- Assemble the calorimeter by placing one cup inside of the other.
- Measure 50.0 mL of water in the graduated cylinder and pour this into the dry
calorimeter.
- Using a thermometer, measure the temperature of the water in the calorimeter and record
the value in your notebook.
- Place ~100 mL of water into a 250 mL beaker.
- Set the beaker on a hotplate with the temperature control turned to high and heat the
water until ~40ยฐC above the temperature of the water in the calorimeter.
- Using a paper towel, remove the beaker and pour 50.0 mL of the warmed water into the
100 mL graduated cylinder.
- Quickly record the volume and temperature of the water in the cylinder.
- Place the thermometer back into the calorimeter and allow it to cool down (~15- 20
seconds).
- Pour the warm water into the calorimeter and cover with your lid.
- Observe the thermometer reading in the calorimeter and note the highest temperature it
reaches. Record the value.
- Using the different in temperatures (โT) determine the heat capacity of the calorimeter.
- Repeat a steps 3-12 a second time your values, should reasonable agree with each other,
if not you may need to repeat a third time.
Part II โ Specific Heat Capacity of Nails/Metal Sample
- Prepare a boiling water bath by filling a 400 mL beaker with ~250 mL of hot water. Set
the beaker on a hotplate with the temperature control turned to high and allow heating till
the water starts boiling.
- Obtain four nails (or a Metal Sample) and place them on the balance to determine their
total mass. Record the value in your notebook.
- Place the four nails (metal Sample) into a tight bundle (two facing upwards and two
downwards) and place them into a test tube.
- Place the test tube into the boiling water and heat for 10 min.
- This allows the metal to heat without getting wet.
- While the sample is heating, dry your calorimeter.
- Add 3 0.0 mL of cool, room temperature water to the calorimeter.
- After the nails (metal Sample) have heated for 10 mins, add the nails into the water in the
calorimeter and immediately cover.
- Observe the thermometer in the calorimeter and note the highest temperature it reaches.
Record that value.
- Using the differences in temperatures (โT), along with the (heat capacity) of the
calorimeter, determine the specific heat capacity (C) of the metal nails.
- Repeat this Process with more nails or a different Metal Sample. Again, your specific
heat capacities should agree with each other. If not, you may want to do a third trial.
