Hess Law Heat of Reaction of KOH and HCL - Experiment 4 | CHEM 140 | Lab Reports Chemistry

Experiment #4

Hess’ Law Heats of Reaction of KOH and HCl

Introduction

In this lab, potassium hydroxide will undergo several different exothermic reactions. In each

reaction a characteristic amount of heat is given off that can be quantified by measuring the change in

temperature, ∆T. In order to measure the amount of heat produced a baseline temperature

will be established prior to the addition of KOH and then the temperature of the solution monitored as

the reaction occurs. How will you determine the ∆T for each reaction?

The Problem

An important thermodynamic relationship exists among the reactions performed in this

experiment and it is your responsibility to determine the nature of this relationship.

Procedure

CAUTION: Potassium hydroxide (KOH) is a strong base and hydrochloric acid (HCl) is a

strong acid so both are extremely corrosive chemicals. Wear your safety goggles at all times. If

small quantities of these chemicals are spilled on you, wash the chemicals off for several minutes with a

stream of water from the sink. In the case of a small spill on a countertop, wipe up the chemicals, and

rinse the area with water. In the case of a larger spill, seek the assistance of your instructor. The

thermometers employed in this experiment contain mercury, a volatile toxic substance. If you break a

thermometer, notify the instructor immediately.

Reaction 1: KOH(s) à KOH(aq)

Using a buret clamp and a ring stand, suspend a 0.1°C thermometer in a polystyrene (Styrofoam)

cup that has been placed on a magnetic stirrer. Adjust the height of the thermometer so that it is

approximately an inch from the bottom. Put a magnetic stirbar in the cup. Obtain 100.0 mL of distilled

water in a graduated cylinder, pour it into the cup and stir. Establish a baseline temperature by recording

the temperature (to the nearest tenth of a degree!) of the solution every 30 s for approximately three

minutes (set up a table with Time and Temperature columns). Meanwhile, your partner should weigh

between 2.5 and 5 g of KOH into a second styrofoam cup, being certain to record the amount of KOH

weighed. Continue uninterrupted timing and pour the water and stirbar into the cup with the KOH and

stir, making sure that the solid dissolves completely. Continue to record temperatures every 30 s for at

least five minutes after the addition. What would you observe if you continued to take temperature

readings for an additional hour?

Use pH paper to determine whether the solution is acidic or basic after the reaction. After the

experiment has been completed, save the KOH solution in a clean, labeled 150 mL beaker for storage

until required in Reaction 3.

Reaction 2: KOH(s) + HCl(aq) à H2O(l) + KCl(aq)

Use a graduated cylinder to obtain 100.0 mL of 1.5 M HCl solution and pour it into a cup. Add a

stirbar, stir and record the temperature every 30 s for 3 minutes. Again, your partner should weigh

between 2.5 and 5 g of KOH into another cup being certain to record the amount of KOH.

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Experiment

Hess’ Law Heats of Reaction of KOH and HCl

Introduction In this lab, potassium hydroxide will undergo several different exothermic reactions. In each reaction a characteristic amount of heat is given off that can be quantified by measuring the change in temperature, ∆T. In order to measure the amount of heat produced a baseline temperature will be established prior to the addition of KOH and then the temperature of the solution monitored as the reaction occurs. How will you determine the ∆T for each reaction?

The Problem An important thermodynamic relationship exists among the reactions performed in this experiment and it is your responsibility to determine the nature of this relationship.

Procedure CAUTION: Potassium hydroxide (KOH) is a strong base and hydrochloric acid (HCl) is a strong acid so both are extremely corrosive chemicals. Wear your safety goggles at all times. If small quantities of these chemicals are spilled on you, wash the chemicals off for several minutes with a stream of water from the sink. In the case of a small spill on a countertop, wipe up the chemicals, and rinse the area with water. In the case of a larger spill, seek the assistance of your instructor. The thermometers employed in this experiment contain mercury, a volatile toxic substance. If you break a thermometer, notify the instructor immediately.

Reaction 1: KOH(s) ‡ KOH(aq)

Using a buret clamp and a ring stand, suspend a 0.1°C thermometer in a polystyrene (Styrofoam) cup that has been placed on a magnetic stirrer. Adjust the height of the thermometer so that it is approximately an inch from the bottom. Put a magnetic stirbar in the cup. Obtain 100.0 mL of distilled water in a graduated cylinder, pour it into the cup and stir. Establish a baseline temperature by recording the temperature (to the nearest tenth of a degree!) of the solution every 30 s for approximately three minutes (set up a table with Time and Temperature columns). Meanwhile, your partner should weigh between 2.5 and 5 g of KOH into a second styrofoam cup, being certain to record the amount of KOH weighed. Continue uninterrupted timing and pour the water and stirbar into the cup with the KOH and stir, making sure that the solid dissolves completely. Continue to record temperatures every 30 s for at least five minutes after the addition. What would you observe if you continued to take temperature readings for an additional hour?

Use pH paper to determine whether the solution is acidic or basic after the reaction. After the experiment has been completed, save the KOH solution in a clean, labeled 150 mL beaker for storage until required in Reaction 3.

Reaction 2: KOH(s) + HCl(aq) ‡ H 2 O(l) + KCl(aq)

Use a graduated cylinder to obtain 100.0 mL of 1.5 M HCl solution and pour it into a cup. Add a stirbar, stir and record the temperature every 30 s for 3 minutes. Again, your partner should weigh between 2.5 and 5 g of KOH into another cup being certain to record the amount of KOH.

Continue timing and transfer the HCl solution and stirbar to the cup with the KOH, magnetically stir and continue recording temperature readings for 5 minutes using 30 s intervals. Use pH paper to determine whether the final solution is acidic or basic after the reaction. What do you learn about the excess/limiting reaction stoichiometry from this observation?

Reaction 3: KOH(aq) + HCl(aq) ‡ H 2 O(l) + KCl(aq)

Rinse your graduated cylinder with water, obtain 50 mL of 1.5 M HCl solution and pour this solution into a polystyrene cup. Rinse and dry the thermometer tip, place it in the HCl solution, measure the temperature of the solution and record it. After rinsing the graduated cylinder with distilled water again, obtain 50 mL of the KOH solution saved from Reaction 1 (not all of it only 50 mL) , and transfer it to a second polystyrene cup. Rinse and dry the thermometer tip and measure the temperature of the KOH solution every 30 s for 3 minutes while stirring. Continue timing and stirring and add the HCl solution to the KOH solution. Continue timing and recording the temperature for 5 minutes. Use pH paper to determine whether the final solution is acidic or basic after the reaction. What do you conclude from that? How will you determine ∆T for this reaction? (Remember, you mixed two solutions together to begin the reaction. How would you handle the situation in which the temperatures of the solutions are not identical?) Use this procedure to calculate ∆T. Was a limiting reagent present in this reaction? If so, what was it? What evidence do you have to verify this?

Waste Management Both HCl and NaOH are slightly toxic but the neutralized mixture of the two is just salt water. Place all of your waste material in a very large beaker in the hood and the instructor will neutralize the mixture at the end of the lab so it can be safely poured down the drain. CAUTION : The thermometers employed in this experiment contain mercury, a volatile toxic substance. If you break a thermometer, notify the instructor immediately.

Data Analysis

Prepare a scatter plot graph with points connected of the temperature versus time data from Reaction 1. Be sure to include all data collected over the eight minutes. Overlay the data from Reaction 2 on this graph. Finally, overlay the data from Reaction 3. Use the curves on the graph to help determine ∆T for each of these reactions.
Perform the calculations necessary to complete the SUMMARY DATA TABLE on the report sheet. Clearly record all your calculations IN YOUR LAB BOOK!!! Use the calorimetry equation to calculate the heat: q = mc ∆ Τ , where c is the specific heat of water with a value of 4.18 J/g⋅°C, m is the mass of water in each reaction and ∆Τ is the change in temperature during the reaction. Show an example of this calculation on the report sheet.
Calculate ∆H for each reaction (HINT: Note the units on ∆H in the SUMMARY DATA TABLE ). Show an example of this calculation on the report sheet.
Write the equations for the three chemical reactions in the write-up with their ∆H values. Use your results from reactions 1 and 2 to predict the expected results for reaction 3 (Hint: look at the sub-title for this experiment). Compare this predicted ∆H for reaction 3 with your actual results; do they agree? It is this thermodynamic relationship that is the important point of the whole experiment!

For the lab you need to turn in: Lab notebook (CALCULATIONS), graph , report sheet

Reaction #1 KOH (s) ‡ KOH (aq) ∆H 1 = ___________________

Reaction #2 KOH (s) + HCl (aq) ‡ H 2 O (l) + KCl (aq) ∆H 2 = ___________________

Use the reaction equations and your ∆H values for reactions #1 and #2 above in (4) to predict the ∆H

value for reaction #3 KOH (aq) + HCl (aq) ‡ H 2 O (l) + KCl (aq).

Clearly show all your work and your answer below.

Compare your ACTUAL experimental ∆H value for reaction #3 from your SUMMARY DATA TABLE with the value you PREDICTED above in (5).

Reaction #3 KOH (aq) + HCl (aq) ‡ H 2 O (l) + KCl (aq)

∆H 3 = ___________________ (from your summary data table)

∆H 3 = ___________________ (you predicted in 5 above)

Calculate the percent difference between your predicted and experimental ∆H3. Do you think they compare ve ry well?

Hess Law Heat of Reaction of KOH and HCL - Experiment 4 | CHEM 140, Lab Reports of Chemistry

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Download Hess Law Heat of Reaction of KOH and HCL - Experiment 4 | CHEM 140 and more Lab Reports Chemistry in PDF only on Docsity!

Experiment

Hess’ Law Heats of Reaction of KOH and HCl

Reaction 1: KOH(s) ‡ KOH(aq)

Reaction 2: KOH(s) + HCl(aq) ‡ H 2 O(l) + KCl(aq)

Reaction 3: KOH(aq) + HCl(aq) ‡ H 2 O(l) + KCl(aq)

Reaction #1 KOH (s) ‡ KOH (aq) ∆H 1 = ___________________

Reaction #2 KOH (s) + HCl (aq) ‡ H 2 O (l) + KCl (aq) ∆H 2 = ___________________

value for reaction #3 KOH (aq) + HCl (aq) ‡ H 2 O (l) + KCl (aq).

Reaction #3 KOH (aq) + HCl (aq) ‡ H 2 O (l) + KCl (aq)