Constant Pressure Calorimetry

Objective
To become acquainted with the use of a coffee cup calorimeter and determine the
heat of reaction of a neutralization reaction.

Introduction
Chemical reactions are accompanied by heat change. When heat is released, the reaction is
called exothermic. When heat is absorbed, the reaction is called endothermic. If
substances mixed in a flask undergo an exothermic reaction, the contents of the flask
become warmer. If the substances undergo an endothermic reaction, the flask contents
become colder. The heat change of a reaction is generally called the heat of reaction. For a
reaction performed at constant pressure, the heat of reaction is equal to the enthalpy change,
DH, for the reaction.

Every substance has an enthalpy, H. Generally, the sum of the enthalpies of the products
differs from the sum of the enthalpies of the reactants. The enthalpy change, DH, is equal
to the sum of the enthalpies of the products minus the sum of the enthalpies of the reactants.
When DH is negative, heat is released by the reaction and thus, the reaction is exothermic.

Heat is commonly measured in units of calories. One calorie (cal) is the amount of heat
needed to raise the temperature of one gram of water one degree Celsius. One kilocalorie
(kcal) equals 1000 calories. The SI unit of heat is the joule; one calorie is equal to 4.184
joules.

Calorimetry is the study of heat transferred in a chemical reaction, and a calorimeter is the
tool used to measure this heat. Calorimetry can be used to find heats of reaction. In a
calorimeter, a chemical reaction is generally performed in a water bath. The heat of reaction
will change the temperature of the calorimeter. For an exothermic reaction, the temperature
of the calorimeter will increase. For an endothermic reaction, the temperature of the
calorimeter will decrease. The heat change associated with the temperature increase of the
calorimeter is equal to the heat capacity of the calorimeter (Ccalorimeter) times the temperature
change (DT = Tfinal – Tinitial):

𝑞”#$%&'()*)& = 𝐶”#$%&'()*)& × ∆𝑇 (𝟏)

The heat of reaction is equal to the negative of the heat change of the calorimeter because
heat flows out of the reaction into the calorimeter (notice the change of direction):

𝑞&67 = −𝑞”#$%&'()*)& (𝟐)

In today’s experiment, you will determine a heat of reaction in a coffee cup calorimeter. A
coffee cup calorimeter consists of two Styrofoam cups, a lid, and a thermometer. Two
solutions are mixed in the calorimeter and the temperature change of the mixed solution is
measured. In a coffee cup calorimeter, the heat capacity of the calorimeter is essentially
equal to the heat capacity of the solution. We will assume that the heat capacity of the
solution is equal to the heat capacity of water. The heat capacity of water is equal to:

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𝐶”#$%&'()*)& = 𝑚;%$ × 𝐶;,=#*)& (𝟑)

Where m is mass in grams, g, and Cs,water is the specific heat of water 1.00
“#$
A ∙ ℃

. We will

also assume that the density of the solution is about 1.00 A
(D

. For example:

50.0 mL HIO ×
1.00 g
mL

= 50.0 g HIO

Therefore, the volume of the solution in mL is equal to the mass of the solution in grams,
𝑚LMN. Putting all the equations together, the heat of the reaction is equal to:

𝑞&67 = −𝑞”#$%&'()*)& = −(𝑚;%$) × (𝐶;,=#*)&) × ∆𝑇 (𝟒)

In this experiment, three processes involving heat transfer will be studied: Heat of
Neutralization, Enthalpy of Solution of Salts, and Specific Heat of a Metal.

A. Heat of Neutralization
The transfer of heat that results from an acid/base neutralization reaction carried out at
constant pressure is called the enthalpy of neutralization, ΔH

neutralization
, and is expressed in

units of kcal/mol or kJ/mol. The reaction to be studied is:

HCl(𝑎𝑞) + NaOH(𝑎𝑞) → NaCl(𝑎𝑞) + HIO(𝑙) (𝟓)

Since HCl and NaOH are strong electrolytes, this net ionic equation associated with this
molecular equation is:

HY(𝑎𝑞) + OHZ(𝑎𝑞) → HIO(𝑙) (𝟔)

As with any chemical reaction, the extent of the reaction is dependent on the amount of
limiting reactant present. Given the moles of limiting reactant undergoing reaction and the
measured heat of the reaction, ΔH

neutralization
can be determined as shown below, keeping in

mind that
q

rxn
= − q

calorimeter
.

∆𝐻7)]*&#

moles reacted
(𝟕)

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B. Enthalpy of Solution of Salts
When a salt dissolves in water at constant pressure, there is a transfer of heat associated
with the reaction called the enthalpy of solution, ΔH

solution.
It is expressed in units of

kcal/mol or kJ/mol of salt.

∆𝑯𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 =
𝒒𝒓𝒙𝒏

𝒎𝒐𝒍𝒆𝒔 𝒐𝒇 𝒔𝒂𝒍𝒕
(𝟖)

The solution process can be written as follows:

NHvNOw(𝑠) → NHvY(𝑎𝑞) + NOwZ(𝑎𝑞) (𝟗)

Heat may be given off or absorbed by the salt as it dissolves as ions in water.

C. Specific Heat of a Metal

You will find the specific heat of a metal by equating the heat lost by the metal (at high
temperature) to the heat gained by the water reservoir at a lower temperature when they are
mixed in the calorimeter. The metal must first be heated, and its temperature measured,
T

initial
(metal). The temperature of the water reservoir is measured prior to, T

initial
(water),

and after, T
final

(water), adding the solid to it. The heat transferred to the water is the
opposite sign of the heat lost by the metal.

𝑞()*#$ = −𝑞=#*)& (𝟏𝟎)

The formula for q given by Equation (4) can then be substituted on each side
of the equation to give:

(𝐶()*#$)(𝑚()*#$) × ∆𝑇()*#$ = −(𝐶=#*)&)(𝑚=#*)&) × ∆𝑇=#*)& (𝟏𝟏)

Rearranging this equation to solve for the specific heat of the metal, results in an
experimentally determined specific heat of the metal that can be compared to the actual
value of the specific heat of the metal.

𝐶()*#$ =
−(𝐶=#*)&)(𝑚=#*)&) × ∆𝑇=#*)&

(𝑚()*#$) × ∆𝑇()*#$
(𝟏𝟐)

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Equipment
• Coffee cup

calorimeter
• 1 cardboard lid • Thermometer

• 50.0-mL graduate
cylinder

• 150.0-mL beaker

Chemicals
• Metal: Zinc,

Aluminum, steel
• 1.00 M HCl

solution
• NH4NO3

• 1.00 M NaOH
solution

• MgSO4 • DI Water

Procedure

A. Heat of Neutralization
1. Obtain 2 Styrofoam cups and a piece of cardboard from the supply table. Nest the

cups and insert the thermometer through the small hole in the center of the
cardboard.

2. Carefully measure, using a graduate cylinder, 50.0 mL of 1.00 M HCl solution and
transfer it to the inner cup.[1] Record the temperature of the HCl to the nearest
0.1oC.[2]

3. Rinse the graduated cylinder with DI water, then rinse it with small amount of
NaOH solution, then carefully measure 50.0 mL of 1.00 M NaOH solution.[3] The
temperature of the NaOH is assumed to be the same as the HCl solution.

4. Add the 50.0 mL of the NaOH solution to the HCl in the cup, swirl gently, and
record the temperature to the nearest 0.1oC, when the temperature reaches a
maximum.[4] Assuming the liquid in the cup has the same density as water
(1.00 {

|}

), find the mass of the sample containing both solutions. With this mass

and the heat capacity (1.00 ~�N
{ ∙ ℃

) calculate the number of calories that were
evolved by your reaction.[5]

5. Calculate the number of moles of H+(aq) (or HCl(aq)) that were neutralized.[6]
6. Calculate the number of calories liberated per mole of H+(aq) neutralized.[7]
7. Repeat everything for a second trial. Record the average of the calories liberated

per mole of H+(aq) neutralized on your report sheet, DHneutralization.[8]

B. Enthalpy of Solution of Salts
1. Weigh out approximately 2.000 grams of NH4NO3

and record the exact amount to

the nearest 0.001 g [1].
2. Measure 50 mL of DI water and place it in the clean calorimeter, after recording

the exact volume [2]. Measure the initial temperature of the DI water as you did
in Part A, and record this to the nearest 0.1oC in your report sheet.[3]

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3.

|}

Remove the lid from the coffee cup calorimeter and quickly add the salt to the
calorimeter and begin swirling while holding the thermometer about 3 cm from
the bottom of the cup. Continue to stir for at least 1 minute to ensure all the salt
has dissolved and to avoid faulty temperature readings. Monitor the temperature
and record the values on your report sheet [4]. Note that in calculating the mass of
the reservoir needed for calculation of qrxn, you must add the mass of the salt to the
mass of water.[5] Density of water (1.00 { ). Calculate the DHsolution.[6]

Dispose of the solution down the sink and rinse the calorimeter with tap
water and then DI water.

C. Specific Heat of a Metal.
1. Prepare a hot water bath by filling a 400-mL beaker with tap water, add about 10

boiling chips to ensure smooth boiling or use your stirring rod, placing it on a hot
plate, and bringing it to a boil.

2. Measure about 20 grams of the metal assigned by your Instructor. Record the exact
mass and identity of the metal used in your report sheet.[1]

3. Measure 50 mL of de-ionized water and place it in the clean calorimeter. Record
the exact volume in your report sheet.[2]

4. Transfer the metal to a large test tube and place it in the boiling water bath to raise
the metal temperature to about that of the bath. This should take about fifteen
minutes. Measure and record the temperature of the metal by placing the
thermometer probe into the test tube in contact with the metal and wait for the
temperature reading to stabilize (less than 1oC change in two minutes). This will
serve as the initial temperature of the metal Tinitial (metal) [3]. Keep the metal in
the test tube in the hot water bath until just before mixing.

5. Cool the temperature probe to room temperature by placing it in a cold-water bath
and then measure and record the initial temperature of the water, Tinitial (water), in
the calorimeter, again make sure that the temperature is stabilized and record this
value in your report sheet. [4]

6. Using a test tube holder, carefully remove the hot test tube containing the metal
from the bath. Quickly, dry the outside of the test tube and pour the metal out of
the test tube into the calorimeter. Immediately begin swirling the calorimeter
contents. Monitor the temperature and record the values on your report sheet.[5]
Density of water (1.00 {

|}
). Calculate the specific heat of the metal Cmetal.[6]

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4. Clean the calorimeter and repeat this procedure with MgSO4.

Practice Problems

1. When 200.0 mL of 0.862 M HCl is mixed with 200.0 mL of 0.431 M Ba(OH)2 in a
coffee cup calorimeter, the temperature of the solution increases from 21.35°C to
27.05°C. Given the specific heat of solution of 4.18 J g-1 °C-1 and assuming the
density of each solution to be 1.00 g/mL, what is the heat of reaction in kJ? What is
the heat of reaction per mole of HCl in kJ/mol? The neutralization reaction is shown
below. Answer: -55.4kJ/mol HCl

2 HCl(aq) + Ba(OH)2(aq) → BaCl2(aq) + 2 H2O(l)

2. Calculate the heat required to raise the temperature of a 1.25 kg of iron from 15°C to
95°C. The specific heat of iron is equal to 0.444 J/g·°C. What is the heat capacity of
iron? What is the molar heat capacity of iron? Answer: 25 J/mol oC

3. A 9.96 g-sample of lithium chloride was dissolved in 100.0 mL of water at 22.2°C.
When the salt was dissolved, the temperature of the solution was 41.3°C. Given the
specific heat of solution of 4.18 J g-1 °C-1 calculate the molar enthalpy of solution of
lithium chloride. The density of water is 1.0 g/mL. Answer: -37.4 kJ/mol

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Name: _________________________________________________ Section: _________

Laboratory Instructor: _____________________________________ Date: ___/___/___

Report Sheet: Constant Pressure Calorimeter

A. Heat of Neutralization Sample 1 Sample 2

[1] Volume of HCl __________

__________

[2] Temperature of HCl __________ __________

[3] Volume of NaOH __________ __________

[4] Temperature of mixture after reaction __________ __________

Temperature difference __________ __________

[5] Number of calories evolved __________ __________

Calculations

[6] Moles of H+ that were neutralized __________ __________

Calculations

[7] Calories evolved per mole of H+ __________ __________

Calculations
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Report Sheet

[8]. Average of the two trials of calories
evolved per mole of H+ __________ __________

Calculations

Unknown grade

B. Enthalpy of Solution of Salts NH4NO3 MgSO4

[1] Mass of salt __________ __________

[2] Volume of DI water __________ __________

Mass of DI water __________ __________

[3] Temperature of DI water __________ __________

[4] Temperature of mixture after
dissolution __________ __________

Temperature difference __________ __________

[5] Total mass in reaction __________ __________

[6] Enthalpy of solution DHsolution __________ __________

Calculations
Unknown grade
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Report Sheet

C. Specific Heat of a Metal

Identity of metal ________________________

[1] Mass of metal __________

Mass of DI water __________

[2] Volume of DI water __________

__________

[3] Tinitial (metal) __________

[4] Tinitial (water) __________

[5] Temperature of mixture after addition of
metal __________

Temperature difference __________

Specific heat of metal Cmetal __________

Calculations
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^#*’%7 =
𝑞&67

moles reacted
(𝟕)
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B. Enthalpy of Solution of Salts
When a salt dissolves in water at constant pressure, there is a transfer of heat associated
with the reaction called the enthalpy of solution, ΔH
solution.
It is expressed in units of
kcal/mol or kJ/mol of salt.
∆𝑯𝒔𝒐𝒍𝒖𝒕𝒊𝒐𝒏 =
𝒒𝒓𝒙𝒏
𝒎𝒐𝒍𝒆𝒔 𝒐𝒇 𝒔𝒂𝒍𝒕
(𝟖)
The solution process can be written as follows:
NHvNOw(𝑠) → NHvY(𝑎𝑞) + NOwZ(𝑎𝑞) (𝟗)
Heat may be given off or absorbed by the salt as it dissolves as ions in water.
C. Specific Heat of a Metal
You will find the specific heat of a metal by equating the heat lost by the metal (at high
temperature) to the heat gained by the water reservoir at a lower temperature when they are
mixed in the calorimeter. The metal must first be heated, and its temperature measured,
T
initial
(metal). The temperature of the water reservoir is measured prior to, T
initial
(water),
and after, T
final
(water), adding the solid to it. The heat transferred to the water is the
opposite sign of the heat lost by the metal.
𝑞()*#$ = −𝑞=#*)& (𝟏𝟎)
The formula for q given by Equation (4) can then be substituted on each side
of the equation to give:
(𝐶()*#$)(𝑚()*#$) × ∆𝑇()*#$ = −(𝐶=#*)&)(𝑚=#*)&) × ∆𝑇=#*)& (𝟏𝟏)
Rearranging this equation to solve for the specific heat of the metal, results in an
experimentally determined specific heat of the metal that can be compared to the actual
value of the specific heat of the metal.
𝐶()*#$ =
−(𝐶=#*)&)(𝑚=#*)&) × ∆𝑇=#*)&
(𝑚()*#$) × ∆𝑇()*#$
(𝟏𝟐)
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Equipment
• Coffee cup
calorimeter
• 1 cardboard lid • Thermometer
• 50.0-mL graduate
cylinder
• 150.0-mL beaker
Chemicals
• Metal: Zinc,
Aluminum, steel
• 1.00 M HCl
solution
• NH4NO3
• 1.00 M NaOH
solution
• MgSO4 • DI Water
Procedure
A. Heat of Neutralization
1. Obtain 2 Styrofoam cups and a piece of cardboard from the supply table. Nest the
cups and insert the thermometer through the small hole in the center of the
cardboard.
2. Carefully measure, using a graduate cylinder, 50.0 mL of 1.00 M HCl solution and
transfer it to the inner cup.[1] Record the temperature of the HCl to the nearest
0.1oC.[2]
3. Rinse the graduated cylinder with DI water, then rinse it with small amount of
NaOH solution, then carefully measure 50.0 mL of 1.00 M NaOH solution.[3] The
temperature of the NaOH is assumed to be the same as the HCl solution.
4. Add the 50.0 mL of the NaOH solution to the HCl in the cup, swirl gently, and
record the temperature to the nearest 0.1oC, when the temperature reaches a
maximum.[4] Assuming the liquid in the cup has the same density as water
(1.00 {
|}
), find the mass of the sample containing both solutions. With this mass
and the heat capacity (1.00 ~�N
{ ∙ ℃
) calculate the number of calories that were
evolved by your reaction.[5]
5. Calculate the number of moles of H+(aq) (or HCl(aq)) that were neutralized.[6]
6. Calculate the number of calories liberated per mole of H+(aq) neutralized.[7]
7. Repeat everything for a second trial. Record the average of the calories liberated
per mole of H+(aq) neutralized on your report sheet, DHneutralization.[8]
B. Enthalpy of Solution of Salts
1. Weigh out approximately 2.000 grams of NH4NO3

and record the exact amount to
the nearest 0.001 g [1].
2. Measure 50 mL of DI water and place it in the clean calorimeter, after recording
the exact volume [2]. Measure the initial temperature of the DI water as you did
in Part A, and record this to the nearest 0.1oC in your report sheet.[3]
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3.
|}
Remove the lid from the coffee cup calorimeter and quickly add the salt to the
calorimeter and begin swirling while holding the thermometer about 3 cm from
the bottom of the cup. Continue to stir for at least 1 minute to ensure all the salt
has dissolved and to avoid faulty temperature readings. Monitor the temperature
and record the values on your report sheet [4]. Note that in calculating the mass of
the reservoir needed for calculation of qrxn, you must add the mass of the salt to the
mass of water.[5] Density of water (1.00 { ). Calculate the DHsolution.[6]
Dispose of the solution down the sink and rinse the calorimeter with tap
water and then DI water.
C. Specific Heat of a Metal.
1. Prepare a hot water bath by filling a 400-mL beaker with tap water, add about 10
boiling chips to ensure smooth boiling or use your stirring rod, placing it on a hot
plate, and bringing it to a boil.
2. Measure about 20 grams of the metal assigned by your Instructor. Record the exact
mass and identity of the metal used in your report sheet.[1]
3. Measure 50 mL of de-ionized water and place it in the clean calorimeter. Record
the exact volume in your report sheet.[2]
4. Transfer the metal to a large test tube and place it in the boiling water bath to raise
the metal temperature to about that of the bath. This should take about fifteen
minutes. Measure and record the temperature of the metal by placing the
thermometer probe into the test tube in contact with the metal and wait for the
temperature reading to stabilize (less than 1oC change in two minutes). This will
serve as the initial temperature of the metal Tinitial (metal) [3]. Keep the metal in
the test tube in the hot water bath until just before mixing.
5. Cool the temperature probe to room temperature by placing it in a cold-water bath
and then measure and record the initial temperature of the water, Tinitial (water), in
the calorimeter, again make sure that the temperature is stabilized and record this
value in your report sheet. [4]
6. Using a test tube holder, carefully remove the hot test tube containing the metal
from the bath. Quickly, dry the outside of the test tube and pour the metal out of
the test tube into the calorimeter. Immediately begin swirling the calorimeter
contents. Monitor the temperature and record the values on your report sheet.[5]
Density of water (1.00 {
|}
). Calculate the specific heat of the metal Cmetal.[6]
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4. Clean the calorimeter and repeat this procedure with MgSO4.

Practice Problems
1. When 200.0 mL of 0.862 M HCl is mixed with 200.0 mL of 0.431 M Ba(OH)2 in a
coffee cup calorimeter, the temperature of the solution increases from 21.35°C to
27.05°C. Given the specific heat of solution of 4.18 J g-1 °C-1 and assuming the
density of each solution to be 1.00 g/mL, what is the heat of reaction in kJ? What is
the heat of reaction per mole of HCl in kJ/mol? The neutralization reaction is shown
below. Answer: -55.4kJ/mol HCl
2 HCl(aq) + Ba(OH)2(aq) → BaCl2(aq) + 2 H2O(l)
2. Calculate the heat required to raise the temperature of a 1.25 kg of iron from 15°C to
95°C. The specific heat of iron is equal to 0.444 J/g·°C. What is the heat capacity of
iron? What is the molar heat capacity of iron? Answer: 25 J/mol oC
3. A 9.96 g-sample of lithium chloride was dissolved in 100.0 mL of water at 22.2°C.
When the salt was dissolved, the temperature of the solution was 41.3°C. Given the
specific heat of solution of 4.18 J g-1 °C-1 calculate the molar enthalpy of solution of
lithium chloride. The density of water is 1.0 g/mL. Answer: -37.4 kJ/mol
Copyright Catalyst Education 2019

Name: _________________________________________________ Section: _________
Laboratory Instructor: _____________________________________ Date: ___/___/___
Report Sheet: Constant Pressure Calorimeter
A. Heat of Neutralization Sample 1 Sample 2
[1] Volume of HCl __________ __________
[2] Temperature of HCl __________ __________
[3] Volume of NaOH __________ __________
[4] Temperature of mixture after reaction __________ __________
Temperature difference __________ __________
[5] Number of calories evolved __________ __________
Calculations
[6] Moles of H+ that were neutralized __________ __________
Calculations
[7] Calories evolved per mole of H+ __________ __________
Calculations
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Report Sheet
[8]. Average of the two trials of calories
evolved per mole of H+ __________ __________
Calculations
Unknown grade
B. Enthalpy of Solution of Salts NH4NO3 MgSO4
[1] Mass of salt __________ __________
[2] Volume of DI water __________ __________
Mass of DI water __________ __________
[3] Temperature of DI water __________ __________
[4] Temperature of mixture after
dissolution __________ __________
Temperature difference __________ __________
[5] Total mass in reaction __________ __________
[6] Enthalpy of solution DHsolution __________ __________
Calculations
Unknown grade
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Report Sheet
C. Specific Heat of a Metal
Identity of metal ________________________
[1] Mass of metal __________
Mass of DI water __________
[2] Volume of DI water __________
__________
[3] Tinitial (metal) __________
[4] Tinitial (water) __________
[5] Temperature of mixture after addition of
metal __________
Temperature difference __________
Specific heat of metal Cmetal __________
Calculations
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