Heats of Reactions
Laboratory Goals.
1.
2.
3.
4.
Determine the Heat Capacity of a Double Cup Styrofoam Calorimeter.
Determine the Heat of Neutralization for an Acid-Base reaction.
Determine the Heat of Solution for a Salt Dissolving in water.
Determine the Heat of Reaction for an Oxidation_Reduction reaction.
Abstract
The study of energy is a vital function of chemistry. Over 90 percent of the energy
we produce comes from chemical reactions such as the combustion of fossil fuels.
This study of energy and its transformations is known as thermochemistry. As
chemical bonds break and form in a chemical reaction, energy in the form of heat
is either released or absorbed. Under conditions of constant pressure, the energy
change of a reaction is called the heat of reaction or enthalpy change, ΔH . The
amount of heat either absorbed or released during a reaction can be measured if
the reaction is carried out in a container that insulates the reaction from its
surroundings. By convention if heat is released, the reaction is termed exothermic
and ΔH is negative. If heat is absorbed, the reaction is termed endothermic and
ΔH is positive.
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Exothermic: ΔH < 0
Endothermic: ΔH > 0
Heats of Reaction Lab
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In this lab, you will determine experimentally the value for ΔH for three reactions:
1.Acid-base neutralization reaction involving hydrochloric acid and sodium hydroxide
2. Dissociation of a salt in water
3. Oxidation/Reduction or "Redox" reaction of calcium and hydrochloric acid
Using an apparatus called a calorimeter, the heat lost or absorbed by a reaction in the
calorimeter can be calculated using the following equation:
ΔHreaction = - (ΔHcalorimeter + ΔHsolution)
The enthalpy change, ΔHsolution , is determined by measuring the temperature change, ΔT,
and the mass of the system. The product of the mass, the specific heat and temperature
change of the system will give the enthalpy change for the system.
ΔHsolution = (specific heat of water) x (grams of solution) x (ΔT)
The heat capacity of 1 g of a substance is called its specific heat capacity. The specific
heat of water is 4.18 J / g oC.
Prelab Assignment:
After reading the experimental lab procedure, prepare the following information in your
lab notebook:
1. A brief (2-3 sentence) introduction to the lab stating the goals of the lab.
2. A table of safety information including the chemicals used in the lab and their safety
handling precautions. This information can be obtained from the MSDS safety sheets.
3. In a calorimeter, 50.0mL of hot water at 60.0°C is added to 50.0mL of water at 20.0°C. the
calorimeter is covered and stirred. The following temperature and time data is recorded:
time, sec
10
20
30
40
50
60
70
80
Temp, oC
22.0
30.0
34.0
36.0
35.8
35.6
35.6
35.4
Plot the data and determine the Heat Capacity (Calorimeter Constant) of this
Calorimeter.
Give the information to your TA at the beginning of the lab. You will not be allowed
to work in the lab without this information.
Procedure
Chemicals
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Equipment and Supplies
2.0 M Hydrochloric acid (HCl)
2.0 M Sodium hydroxide (NaOH)
Ammonium nitrate (NH4NO3)
Calcium metal Ca
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Calorimeter
150 ml beaker
Thermometer
Graduated cylinder
Heats of Reaction Lab
Instruments
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Balance
2
I. Finding the Heat Capacity (Calorimeter Constant) of a Double
Cup Styrofoam Calorimeter.
A. Obtain two calorimeters (each consisting of a double Styrofoam cup and a cup lid with
one hole), a magnetic stirrer, a stir bar and a digital thermometer. Assemble as shown at
the top of page one of this Experiment.
Heat Capacity of First Calorimeter
1. Use a graduated cylinder to obtain exactly 50.0 mL of room temperature water and add it
to your calorimeter. Add a stir bar, allow a few minutes for the apparatus to come to
thermal equilibrium and measure and record the temperature of the water in the
calorimeter to the nearest 0.1 _C.
Record the exact temperature of this room temperature water.
2. Using the graduated cylinder measure 50.0 mL of water into a small beaker and heat until
it is about fifty (50o) degrees above room temperature. Mix the warm water thoroughly
and measure the exact temperature of the warm water.
Record the exact temperatureof this warmed water.
3. At time zero carefully pour the warm water into the calorimeter with the room
temperature water, replace the lid, insert the thermometer, and begin gently stirring the
mixture. Important: Temperature and Time measurements start upon mixing - this is zero
time!
4. After recording the initial temperature at mixing, record the temperature every five
seconds until it reaches a maximun and then every 15 seconds for two minutes more.
5. Empty your calorimeter, dry all parts completely, and allow it to return to room
temperature.
6. While the first calorimeter is cooling repeat the above procedure with the second
calorimeter.
Heats of Reaction Lab
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B. These two sets of data will be used to determine a value for the Heat Capacity of the Two
Cup Calorimeter, but first the temperature of the mixed water at time zero, Tmix@0, must be
determined. The temperatures at time zero of both the room temperature water, Troomtemp@0,
and the warmed water, Twarm@0, can be determined. However the temperature of the mixed
water at time zero, Tmix@0, cannot be measured with the therometer. In order to do this, we
can construct a plot as illustrated in the Figure below.
The plot graphs the temperature of the solution as a function of time. We can
estimate the temperature at the point of mixing, Tmix@0 , by constructing a curve as
shown in the plot, so that after mixing, we can extrapolated the temperature back to
time zero. Use the instruction below to construct your own plot..
1. Accurately plot Temp vs. Time for each of these two sets of data.
2. Extrapolate the cooling curve back to time zero to determine the temperature of the
mixture at time zero, Tmix@0. (This is as if the pouring, mixing, temperature
equilibrating and thermometer reading happened instanteously.)
3. Determine the change in the temperature, ΔT, from the initial room temperature to
the temperature of the mixture at time zero.
4. Determine the change in the temperature, ΔT, from temperature of
the warm water to the temperature of the
mixture at time zero.
C. The Heat Capacity of the calorimeter is the
amount of heat, in Joules (J), required to raise
the temperature of the calorimeter one degree
Celsius.
Calculate the Heat Capacity, Ccalorimeter, of
each of the Two Cup Styrofoam Calorimeters.
The heat lost by the warm water is equal to the
heat gained by both the room temperature water
and the room temperature calorimeter apparatus.
The heat lost by the warmer water would be:
ΔHlost = (specific heat of water) x (grams of
solution) x (ΔTlost)
where, ΔTlost = Tmix@0- Twarm@0
And the heat gained by the room temperature
water is:
ΔHgain = (specific heat of water) x (grams of
solution) x (ΔTgain)
where, ΔTgain = Tmix@0- Troomtemp@0
The heat gained by the calorimeter, ΔHcalorimeter, is
the difference between the heat lost by the warmer
water and the heat gained by the cooler water.
Because ΔHlost is a negative quantity, ΔHcalorimeter can be expressed as follows:
ΔHcalorimeter = - ( ΔHlost + ΔHgain)
Heats of Reaction Lab
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Now to calculate the Heat Capacity, Ccalorimeter, of a Two Cup Styrofoam Calorimeter, the
heat gained by the calorimeter, ΔHcalorimeter, is divided by the change in the temperature of
the calorimeter. Since the temperature of the calorimeter increased to Tmix@0, the change
in temperature will be ΔTgain.
Ccalorimeter = ΔHcalorimeter / ΔTgain
Calculate the Heat Capacity, Ccalorimeter, of the other calorimeter and find the
averae Heat Capacity, Ccalorimeter, for this type of calorimeter. (This value will be
needed to calculate the heat of reaction for each of the following reactions.)
An Example: To determine the heat capacity of the calorimeter, a student added 50 mL
warmed water to a calorimeter containing 50 mL of room temperature water and the temperature
vs. time data was collected and plotted.
The following temperatures at time zero were recorded or extrapolated from the cooling curve:
- Temperature of 50 mL warm water: 38.2 °C
- Temperature of 50 mL room temperature water: 21.2 °C
- Temperature after mixing: 29.2 °C with
using the following values are given: Density of water: 1 g/mL, Heat Capacity of water: 4.184
J/g-oC
The heat lost by the warm water:
ΔHlost = (50 g) x (4.184 J/g-oC) x (29.2- 38.2 _C) = - 1883 joules
The heat gained by the cool water:
ΔHgain = (50 g) (4.184 J/g- oC) (29.2 - 21.2_C) = + 1674 joules
The heat lost to the calorimeter is the difference between the heat
lost by the warm water and the heat gained by the room
temperature water:
ΔHcalorimeter = - (- 1883 + 1674 ) joules = + 209 joules
Therefore, the heat capacity of this calorimeter is:
Ccalorimeter = + 209 joules / (29.2 - 21.2_C) = + 26.1 joules / °C
II. Finding the Heat of Neutralization for an Acid-Base Reaction
Pour 50.0 mL of 2.0 M sodium hydroxide (NaOH), measured with your graduated
cylinder, into your calorimeter and determine the temperature. It should be close to Room
Temperature. Record the temperature.
2. Measure 50.0 mL of 2.0 M hydrochloric acid (HCl) in clean, graduated cylinder. Measure
the temperature of the HCl solution. It also should be Room Temperature. Record the
temperature. If the temperatures of the solutions are different allow the solutions to come
to Room Temperature.
3. Carefully, but quickly do the following:
1.
Heats of Reaction Lab
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1.
2.
3.
4.
5.
6.
7.
3.
Add the acid (HCl) to the base (NaOH) in the calorimeter,
Record the time this as time zero,
Replace the calorimeter lid,
Insert the therometer probe,
Turn the magnetic stirrer on low
Record the solution_s temperature (± 0.1 °C), as well as the time,
After recording the initial temperature at mixing,record the temperature every five
seconds until it reaches a maximun and then every 15 seconds for two minutes more.
Empty your calorimeter in the appropriate waste container, rinse and dry all parts
completely, and allow it to return to room temperature. Repeat the experiment once more
with the other calorimeter for a total of two trials.
The temperature changes that occur do not occur instantly. Before performing
the heat of neutralization experiment, the acid and base solutions are at room
temperature (20.5 _C in the example shown in Figure 1). After mixing, the
temperature rises sharply and then begins to fall. The descending portion of the plot is
called the cooling part of the curve. Extrapolation of the cooling curve back to time zero
(the point at which the solutions were mixed) yields the maximum change in
temperature that occurred (ΔT).
Figure 2. Extrapolation of temperature vs. time for an exothermic reaction.
Since you will be determining ΔT from your graph, it must be accurately plotted.
Therefore, your graph should take up a whole page of graph paper. The maximum
temperature is the intersection of two lines: the best line drawn through the data
points on the cooling portion of the curve, and a line drawn perpendicular to the
time-axis at time zero when the solutions are mixed. The maximum temperature,
Tmax, will be an estimate of the temperature at the instant of mixing. This Tmax is
calculated as shown in the graph.
Heats of Reaction Lab
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III. The Heat of Solution for Dissolving Ammonium Nitrate in
Water
1.
2.
3.
4.
5.
Set up and record the temperature changes for this reaction the same way as the AcidBase Reaction.
Pour 100.0 mL of distilled water (room temperature) into the calorimeter. Allow the
solution to reach thermal equilibrium and record the temperature.
Weigh out about 5 g (+ 0.1 g) of Ammonium nitrate, NH4NO3, in a plastic weigh boat.
Add the Ammonium nitrate to the calorimeter, stir, record the temperature immediately,
and every 5 seconds for one minute and then every 15 seconds for two minutes more.
Repeat again with the second calorimeter.
Graph the temperature versus time and extrapolate the Tmax from the graph for each trial.
IV. The Heat of Reaction for a Redox Reaction
1.
2.
3.
4.
5.
Set up and record the temperature changes for this reaction the same way as the AcidBase Reaction.
Pour 100.0 mL of 2.0 M HCl into the calorimeter. Allow the solution to reach thermal
equilibrium and record the temperature.
Weigh out about 1 g (+ 0.1 g) of the assigned metal.
Add it to the solution, stir and record the temperature immediately, and every 5 seconds
for one minute and then every 15 seconds for two minutes more.
Repeat again with the second calirometer.
Graph the temperature versus time and extrapolate the Tmax from the graph for each trial.
Post Lab Assignment
Include the following information in a lab report to give to your Instructor.
1. Determine the heat capacity of the calorimeter (Ccalorimeter ). Calculate the average heat
capacity of your calorimeters.
2. Construct graphs of temperature versus time for each of the Acid-Base Neutralization
Reactions to determine an average ΔT for this reaction. Calculate the heat of neutralization.
3. Write an equation that describes the acid-base neutralization reaction. Is this an exothermic
or endothermic reaction? Explain your answer.
Heats of Reaction Lab
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Example calculation:
The following data was obtained for reaction of an acid and a base:
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Temperature of 50 mL of acid before mixing: 23.0°C
Temperature of 50 mL of base before mixing: 23.0°C
Extrapolated temperature at time zero of 100 mL of solution after mixing: 28.0°C
Assume that the density (1 g/ml) and specific heat of the solution (4.184 J/g-oC) are the
same as pure water.
To determine the heat gained by the solution:
ΔHsolution = (100 g) (4.184 J/g-oC) (28.0 - 23.0_C) = + 2.09 x 103 joules
If the heat capacity of the calorimeter is 26.1 J/_C ( see example in procedure),
The heat gained by the calorimeter is:
ΔHcalorimeter= (26.1 J/_C) (28.0 - 23.0_C) = + 131 joules
Use, ΔH reaction = - (ΔH calorimeter + ΔH solution ), to calculate the heat of
neutralization:
ΔH = - (131 + 2092) joules = - 2.22 x 103 joules
4. Construct a graph of temperature versus time for the addition of ammonium nitrate to
water. Calculate the heat of solution. Write an equation that describes this reaction. Is this
an exothermic or endothermic reaction? Explain your answer.
5. From your data on the addition of a metal to HCl, calculate this heat of reaction. Write an
equation that describes this reaction. Is this an exothermic or endothermic reaction? Explain
your answer.
Laboratory Report
Instructions for completing your lab report are posted at:
http://umaine.edu/general-chemistry/files/2011/08/Lab_Grading_Guide_Spring_2012_V1.0.pdf
Here are some points to include in your lab report:
a) An introduction.
b) Your data including plots.
b) Your claim, underlined, following the introduction.
c) The reasoning to make your claim.
Heats of Reaction Lab
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