Heat: To Release or to Absorb?

Background Information: The quantity of heat that is absorbed or released by water (q) is equivalent to the mass of the original water sample (in grams) multiplied by the specific heat capacity constant [4.184(J/g°C)]and the change in temperature (°C).

Eq #1: q = m × 4.184 × ΔT

Objective: Using Eq #1, we will calculate the quantity of heat absorbed or released by room temperature water when different substances are added.

Group Size: 2 to 3 students.

Materials needed:

Water (at least 400 mL)
1 Styrofoam cup of ice
Graduated cylinder to measure water in increments of 50 mL
Paper towels
At least 5 Styrofoam cups
Candle and matches, or other heat-producing source
Square of aluminum foil
At least 2 pennies
Tongs
2 Alka Seltzer tablets
Plastic knife, or other cutting utensil
Lab Pro and Graphing Calculator
Temperature Probe

Setting up the Lab Pro and calculator:

Insert the Lab Pro and TI calculator in plastic holder. Connect with the short link cord into the bottom of the calculator and Lab Pro. Lab Pro should be connected to an electrical outlet.
Insert one temperature probe into Channel 1 on the side of the Lab Pro.
In the TI calculator, go to “APPS” and find “DATAMATE”. The calculator should immediately recognize the probe.

Procedure: PART I

Put one of the pennies into the cup of ice to cool.
Fill two Styrofoam cups with 50 mL of water each. Fill these quantities into the data table for mass of H²O. (Hint: 50 mL of water = 50 g, since the density of water is 1.)
Place the candle on a piece of aluminum foil. Light the candle. With the tongs, one group member should heat a penny in the candle’s flame.
Another group member should use the temperature probe to measure the temperature of one of the Styrofoam cups of room temperature water. Have the recorder fill in the information in the data table as “Initial Temp”. Leave the probe in the water.
After the penny has been in the candle’s flame for about 1 to 1½ minutes, carefully drop the penny into the same cup with the probe. Stir the water with the probe until the temperature on the Lab Pro stabilizes. Have the recorder fill in the information in the data table as “Final Temp”.
Remove the probe and wipe it clean with a paper towel. Place it in the second cup of water and measure the temperature. Record it in the table. Leave the probe in the water.
Clean the ends of the tongs with a paper towel. Use the tongs to retrieve the penny from the cup of ice and place it in the cup with the temperature probe. Stir the water with the probe until the temperature on the Lab Pro stabilizes. Have the recorder fill in the information in the data table as “Final Temp”.
Using Eq #1 from above, calculate the quantity of heat absorbed or released, q, for both samples.

Data Table #1:

Substance	Mass H₂O (gm)	Initial Temp T_i(°C)	Final Temp T_f (°C)	Quantity of Heat q (joules)
Heated penny
Cooled penny

Additional Information:

Note that the SIGN of q is positive in one instance and will be negative is the other. A positive q value indicates that the water has absorbed heat. The potential energy of the water has increased. When a hot object is put into water, the water will absorb heat. The potential energy of the water will increase, and the water temperature will rise.

A negative value of q indicates that the water has released, or given up some of its potential energy. The water will thus end up with less potential energy that it originally possessed. When a cold object, such as ice or a cold penny, is added to water, the water will give up energy. The temperature of the water will then drop.
In which case does the potential energy of the water increase?
In which does the water’s potential energy decrease?
When the temperature of water rises, is q positive or is it negative?

Procedure: PART II

For the second part of the activity, we will measure the quantity of heat, q, transferred to water during a chemical reaction. We will be using the same substance each time. Although the amounts of water and Alka seltzer are changing in each trial, the ratio of Alka seltzer to water will remain the same.
Fill 3 Styrofoam cups with the following amounts of water: 50 g, 100 g, 150 g.
Cut one Alka Seltzer in half with the plastic knife. Fill in the “Mass” column of the data table, taking into account both the mass of the water and the mass of the Alka Seltzer. (One Alka Seltzer tablet ≈ 3.43 g
Measure the temperature of the 50 mL cup of water and record it in the table. Leave the probe in the cup and drop in ½ tablet of Alka Seltzer. Stir the probe in the cup until the temperature stabilizes. Record it in the table. Clean the end of the temperature probe with a paper towel.
Measure the temperature of the 100 mL cup of water and record it in the table. Leave the probe in the cup and drop in 1 tablet of Alka Seltzer. Stir the probe in the cup until the temperature stabilizes. Record it in the table. Clean the end of the temperature probe with a paper towel.
Measure the temperature of the 150 mL cup of water and record it in the table. Leave the probe in the cup and drop in 1½ tablets of Alka Seltzer. Stir the probe in the cup until the temperature stabilizes. Record it in the table. Clean the end of the temperature probe with a paper towel.
Using Eq #1 from above, calculate the quantity of heat absorbed or released, q, for both samples.

Data Table #2:

Substance	Mass H₂O (gm)	Initial Temp T_i(°C)	Final Temp T_f (°C)	Quantity of Heat q (joules)
50 g H2O with ½ tablet
100 g H2O with 1 tablet
150 g H2O with 1½ tablets

Exercises:

Plot the graph of mass (x) vs. quantity of heat (y). Label your axes in an appropriate manner.
What type of relationship does this graph model: linear, quadratic, exponential, etc. ?
Find the equation of the line or curve of best fit.

Extension: Chemistry

The slope of the linear model is a measure of enthalpy (ΔH). What are the units of the slope of our model?
What does the +/- sign on the slope mean? How does this relate to potential energy?