Determining the enthalpy of a chemical reaction lab report answers

Determining the Enthalpy of a Chemical Reaction

Introduction

Thermochemistry is the study of energy and its transformations. As chemical bonds break and

form in a chemical reaction, energy in the form of heat (qrxn) is either released or absorbed by the

reaction (the system). At constant pressure (coffee cup calorimetry), the heat released or absorbed

is equal to the enthalpy change (ΔHrxn) of the reaction. The heat exchanged between the system

and surroundings can be measured if the reaction is performed in a container, known as a

calorimeter which insulates the reaction from its surroundings. If heat is released, the reaction is

exothermic and ΔH will be negative. If heat is absorbed, the reaction is endothermic and ΔH will

be positive.

In this experiment, you will determine enthalpy changes (ΔHrxn) of chemical reactions by coffee

cup calorimetry. It is difficult to directly measure heat exchange between reactants and products

(the system) directly. Therefore, we measure the heat change that occurs in the surroundings by

monitoring temperature changes at constant pressure. If we conduct a reaction between two

substances in aqueous solution, then the heat gained or lost by the solution can be calculated with

the following equation:

qsoln = Cp x m x ∆T

qsoln represents the heat that is gained or lost by the solution; Cp is the specific heat of water (4.18

J / g ºC); m is the mass of water, and ∆T is the temperature change of the solution.

The Styrofoam cups used to carry out the reactions are good insulators; however, they still absorb

some of the heat exchange involved in the reaction. Therefore, the heat of the calorimeter (qcal)

must also be taken into consideration to accurately calculate qrxn. Therefore, the heat gained/ lost

by the solution and the calorimeter is equal to the heat lost/ gained by the reaction:

(qcal + qsoln) = - qrxn qrxn = ΔHrxn (at constant pressure)

In this experiment, you will determine the calorimeter constant (Ccal) of two Styrofoam cup

calorimeters. You will also measure ΔHrxn for a series of reactions and use Hess’s Law to compare

calculated ΔHrxn values to experimental values.

This is a 2 week lab. Part A will be completed during week 1 and Part B will be completed

during week 2.

Week 1

Part A: You will be given two Styrofoam cups and some aluminum foil to make two calorimeters.

You will need to determine the heat capacity of your calorimeter (Ccal). By definition, Ccal is

defined as the amount of heat required to raise the temperature of the calorimeter 1 oC. To

determine Ccal you will measure the temperature change associated with mixing warm water with

room temperature water. The heat lost by the warm water is equal to the heat gained by both the

room temperature water and the calorimeter apparatus. The heat gained by the room temperature

water and the calorimeter is equal to the heat lost by the warm water. ΔT will be (Tfinal – Tinitial).

Therefore,

qlost = (specific heat of water) x (grams of warm water) x (ΔTlost)

qgain = (specific heat of water) x (grams of room temperature water) x (ΔTgain)

The heat gained by the calorimeter, qcal, is the difference between the heat lost by the warm water

and the heat gained by the room temperature water. Since qlost is negative, qcal is

qcal = - (qlost + qgain)

Therefore, to calculate the Ccal, the heat gained by the calorimeter (qcal) is divided by the change

in the temperature of the calorimeter. Since the room temperature water was in the calorimeter to

start and the temperature of the calorimeter increased, the change in temperature of the calorimeter

will be equal to ΔTgain.

Ccal = qcal / ΔTgain

You will need to take the heat capacity of the calorimeter into consideration for each of your

enthalpy calculations in this experiment.

Objectives

In this experiment, you will

• Determine the heat capacity of two Styrofoam cup calorimeters.

• Determine ΔHrxn for three neutralization reactions

• Use Hess’s law to determine the enthalpy change of an acid-base reaction and compare your

calculated value to your experimental results.

• Determine the heat of solution for dissolving a salt in water.

• Determine the heat of reaction for a redox reaction.

Equipment, Chemicals and Supplies

You will need a 50mL graduated cylinder from your drawer. The stockroom will provide all other

materials for weeks 1 and 2. You will need to keep the Styrofoam cups used as calorimeters in the

experiment between weeks 1 and 2. Make sure they are clearly labeled. YOU MUST RECORD

YOUR BOX NUMBER AND MAKE SURE YOU GET THE SAME BOX NUMBER NEXT WEEK.

Vernier Lab Quest Ring stand

Temperature probe Utility clamp

4 Styrofoam cups Aluminum foil/or cup lid

Stir plate Small stir bar

Safety

Wear goggles and lab coat throughout the experiment.

Procedure Week 1:

Part A: Determining the Heat Capacity of the Calorimeter

Your team of four will split into two teams of two to perform this part of the experiment.

You will complete 3 trials with your partner, then exchange data with the other two members

of the group to get two sets of calorimeter constants. Make sure the calorimeters are clearly

labeled as 1 and 2.

1. Nest two Styrofoam cups inside of each other to make your calorimeter. Add the magnetic

stir bar to the calorimeter (inner cup).

2. Cover the opening of the calorimeter with the aluminum foil /or cup lid. With a pencil tip,

pierce a small hole close to the edge of the cup (Figure 1). The hole should be small to

avoid heat loss.

Figure 1 Figure 2

3. Assemble the ring stand, utility clamp, and stir plate as seen in Figure 2. Place your

calorimeter inside the ring holder and rest it on the stir plate.

4. Measure 25 mL of room temperature water with a graduated cylinder. Remove the

aluminum foil and pour the water into the calorimeter.

5. Tightly cover the top of the calorimeter with the aluminum foil/or cup lid.

6. Carefully insert the temperature probe through the punctured hole in the aluminum foil

(Figure 1). Secure the temperature probe with the utility clamp (Figure 2). Be careful not

to puncture the bottom of the calorimeter.

7. Connect the temperature probe to Channel 1 of the Vernier LabQuest. Plug in the LabQuest,

turn it on, and set the data collection time to 180 seconds (3 minutes).

8. Turn on the stir plate. **The temperature probe is placed carefully to the side of the cup

to avoid collisions with the magnetic stir bar, carefully adjust probe if this happens**

9. Stir the room temperature water in the calorimeter for 3 minutes to allow the apparatus to

reach thermal equilibrium. After 3 minutes, record the temperature as the initial

temperature of the room temperature water on your data sheet.

10. Use a graduated cylinder to measure 25 mL of warm water. Remove the temperature probe

from the calorimeter and measure the temperature of the warm water. The initial reading

of the warm water should be over 70 ºC. Record the temperature of the warm water on your

data before mixing.

11. Place the temperature probe back in the calorimeter which contains the room temperature

water.

12. Start the LabQuest.

13. Lift the side of the aluminum foil and add the warm water into the calorimeter that contains

the room temperature water. Tightly re-cover the opening of the calorimeter with the

aluminum foil and allow to stir for 3 minutes.

14. After 3 minutes, record the maximum temperature observed on the LabQuest unit on your

data sheet (Tfinal).

15. Empty your calorimeter, dry all parts completely, and allow all equipment to return to room

temperature.

16. Repeat steps 4-15 to complete the remaining 2 trails for the calorimeter.

17. Calculate the calorimeter constant (Ccal) using the values for the heat gained or lost by the

calorimeter and the change in temperature.

18. For your calculations in Part B, you should use the average Ccal value for each of your

calorimeters.

Week 2

Part B: In Part B of this experiment you will experimentally determine the enthalpy change of

five reactions. The first three reactions are the acid-base reactions listed below.

1. NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

2. NaOH(aq) + NH4Cl(aq) → NaCl(aq) + NH3(aq) + H2O(l)

3. HCl(aq) + NH3(aq) → NH4Cl(aq)

In addition to experimentally determining the ΔHrxn for each of these reaction, you can use Hess’s

Law to determine the enthalpy change of reaction 3 above by manipulating reactions 1 and 2. You

will compare your calculated value of ΔHrxn for 3 to your experimentally determined value.

You will also determine the heat of reaction for two additional reactions: (1) dissolving ammonium

nitrate in water and (2) the redox reaction that results from dissolving magnesium powder in

hydrochloric acid.

Equipment, Chemicals and Supplies

Vernier LabQuest 2.0 M hydrochloric acid solution

Fume hood 2.0 M sodium hydroxide solution

Temperature Probe 2.0 M ammonium chloride solution

2 Styrofoam cup calorimeter 2.0 M ammonium hydroxide solution

400 mL beaker Ammonium nitrate salt

50 mL graduated cylinder Magnesium powder

Thermometer aluminum foil/or cup lid

Stir plate and small magnetic stir bar Electronic balance

Ring stand Utility clamp

Safety : ALL REACTIONS SHOULD BE PERFORMED IN THE HOOD

● Goggles and lab coat must be worn throughout the experiment.

● These reactions will be performed in the fume hoods to avoid inhaling hazardous gases.

● Handle all reagents with care to avoid coming in contact with the skin. They may cause

painful burns.

● If any chemicals get on your skin, rinse thoroughly for at least 15 minutes.

Procedure Week 2: ALL REACTIONS SHOULD BE PERFORMED IN THE HOOD

Failure to perform these reactions in the fume hood will result in dismissal from lab and a

zero for this experiment.

Part B: Determining Heat of Reactions

You must collect all reaction solutions in a waste beaker at your bench and discard in waste

container A at the end of the experiment.

Reaction between HCl and NaOH

NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

1. Add the magnetic stir bar to the calorimeters (inner cup) used in Part A.

2. Cover the opening of the calorimeter with the aluminum foil. With a pencil tip, pierce a

small hole close to the edge of the cup (Figure 1). The hole should be small to avoid heat

loss.

3. Assemble the ring stand, utility clamp, and stir plate as seen in Figure 2. Place your

calorimeter inside the ring holder and rest it on the stir plate.

4. Measure 25 mL of 2.0 M HCl solution with a graduated cylinder. Remove the aluminum

foil/or cup lid and pour the HCl solution into the calorimeter.

5. Tightly cover the top of the calorimeter with the aluminum foil/or cup lid.

6. Carefully insert the temperature probe through the punctured hole in the aluminum foil/or

cup lid (Figure 1). Secure the temperature probe with the utility clamp (Figure 2). Be careful

not to puncture the bottom of the calorimeter.

7. Connect the temperature probe to Channel 1 of the LabQuest unit. Plug in the LabQuest,

turn it on, and set the data collection time to 180 seconds (3 minutes).

8. Turn on the stir plate. **The temperature probe is placed carefully to the side of the cup

to avoid collisions with the magnetic stir bar, carefully adjust probe if this happens**

9. Stir the HCl solution in the calorimeter for 3 minutes to allow the apparatus to reach thermal

equilibrium.

10. While stirring the HCl solution, measure 25 mL of 2.0 M NaOH solution with a clean

graduated cylinder. Do not add it to the HCl solution yet.

11. After the HCl solution has been stirred for 3 minutes, record the temperature as the initial

temperature of the HCl solution on your data sheet.

12. Start the LabQuest. Lift the side of the aluminum foil and add the NaOH solution to the

calorimeter. Tightly re-cover the opening of the calorimeter with the aluminum foil/or cup

lid and allow to stir for 3 minutes.

13. After 3 minutes, record the maximum temperature observed on the LabQuest on your data

sheet.

14. Rinse and dry the temperature probe, calorimeter, and the stir bar. Dispose of the solution

in your waste beaker.

15. Calculate ΔHrxn.

Reaction between NaOH and NH4Cl

NaOH(aq) + NH4Cl(aq) → NaCl(aq) + NH3(aq) + H2O(l)

1. Add the magnetic stir bar to of the calorimeters (inner cup) used in Part A.

2. Measure 25 mL of 2.0 M NaOH solution with a graduated cylinder. Remove the aluminum

foil/or cup lid and pour the NaOH solution into the calorimeter.

3. Tightly cover the top of the calorimeter with the aluminum foil/or cup lid.

4. Carefully insert the temperature probe through the punctured hole in the aluminum foil/or

cup lid (Figure 1). Secure the temperature probe with the utility clamp (Figure 2). Be

careful not to puncture the bottom of the calorimeter.

5. Connect the temperature probe to Channel 1 of the LabQuest unit. Plug in the LabQuest,

turn it on, and set the data collection time to 180 seconds (3 minutes).