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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:

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)]*&#$'^#*'%7 = 𝑞&67

moles reacted (𝟕)

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)

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]

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]

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

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

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

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