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The Thermochemistry of Reactions of Potassium Hydroxide And Hydrochloric Acid Reading Assignment:

Tro, section 6.2, 6.3, and 6.7 (and any other parts of the chapter you need to refresh).

Challenge: From measurements of the temperature changes observed when various

materials are combined with water or with one another, to learn some things about energy

relationships in chemical reactions.

In addition to the usual items (name, date, title, partner, purpose, hypothesis, safety, etc.) your

notebook should include answers to these Pre-Lab Questions:

1) There is a chemical equation at the beginning of each of the first 3 parts of the experiment. Write

out balanced Net Ionic Equations for each of those three reactions, showing only the specific

changes that occur, leaving out any species that are present but not involved in the process under

consideration. Comment on any relationships you see among these reactions.

2) Look up (and write in your notebook) definitions of the following terms, using the texts glossary

or other sources (cite any other than the text):

Heat:

Calorimeter:

Heat Capacity:

3) The reaction of Part I of this experiment will be run in a calorimeter that is a fancy version of the

one in Figure 6.9 of the text. Identify specifically what constitutes the System in Part I, and what

specific portions of the universe constitute the relevant Surroundings for this experiment.

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(aq)2

(s) KOHOH

KOH

The Thermochemistry of Reactions of

Potassium Hydroxide And Hydrochloric Acid

This exercise affords you the opportunity to explore the energy changes in a set of related simple physical and chemical changes specifically making a solution of a solid compound and running an acid/base reaction. The energy changes will be determined by taking temperature measurements. Since these chemical processes are relatively fast, but not instantaneous, you will need to measure the temperature of the system at intervals over a several-minute time period, and pull out of the resulting data the temperature change actually due to the reactions. SAFETY PRECAUTIONS: Potassium Hydroxide (KOH), either as a solid or in solution, and Hydrochloric Acid (HCl) solutions are both corrosive to metals, to some clothing fibers, and to human flesh. Avoid handling KOH pellets with your fingers use a clean spatula. If small amounts of either reagent are spilled, immediately wipe up the spill, wash the area with water and wipe it dry. In the case of KOH solutions, you might wish to spread a small amount of solid citric acid from the base spills bottle onto the spill to neutralize the basic material. Similarly, if you spill HCl solutions, it is useful to spread some potassium carbonate from the acid spills bottle on the surface to neutralize the acid before wiping it up. If the chemicals get on your person, wash the affected area immediately under a stream of water. If large spills occur, consult the instructor. Solid KOH pellets are quite hygroscopic that is, they rapidly pick up water vapor from the air, gaining weight and becoming wet. For this reason, weigh out the amounts you need quickly, make sure you put the top back on the supply bottle immediately, and dont leave your weighed samples standing in open containers on your bench more than a minute or two before using them.

Weigh all of todays materials, liquid or solid, in beakers, not directly on balance pans or on paper.

Part I. A. In your laboratory notebook prepare a blank table like the one below (leave room for at least

20-25 time and temperature data pairs):

mass of beaker (grams)_____________________ mass of beaker + KOH _____________________ mass of KOH ____________________________ moles of KOH ___________________________ Elapsed Temp Time ______ ______ ______ ______ etc. T for the reaction________________

B. Using a clean graduated cylinder, obtain 100.0 mL of distilled water from the large plastic

container on the bench near the windows.

Suspend a 0.1 C thermometer so that it hangs into a calorimeter, with its bulb no more than 1/2 inch from the bottom. Add the water from your graduated cylinder to the calorimeter, start recording time, stir, and record the temperature at 30-second intervals. Read your

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thermometer as precisely as you can. Your results are likely to be better if you can estimate temperatures to the nearest 0.01 C or 0.02 C rather than just settling for the nearest 0.1 C.

While one partner continues those measurements, the other should weigh (analytical balance) between 2.5 and 5 grams of KOH pellets into a small, clean, dry beaker. (The table youve prepared has places for recording the relevant measurements.) [NOTE: Your lab instructor may suggest specific amounts to different lab teams, so that the class can explore whether or how the quantities we measure depend on sample size.]

No less than 3 minutes after you began collecting temperature data on the water, note the time (dont restart the clock here all times need to be recorded from the same starting point) and add the sample of solid KOH to the calorimeter. (This probably wont be at exactly the time you take a temperature measurement, but rather between two measurements. Thats OK, but do note when the addition happened.) Continue to stir vigorously and record temperature data; if the temperature seems to change rapidly you may wish to collect data at shorter time intervals, but make sure you have at least one data point every 30 seconds. Continue recording Temperature data for at least 5 minutes after adding the solid, and longer if the temperature hasnt settled into a pattern by that time.

When data-taking is complete, pour the entire KOH solution youve made into a clean, dry labeled beaker and cover it with a watch glass to prevent contamination. Some of this solution will be used in part III.

C. Construct a graph of your Temperature versus Time data. The horizontal scale should show time since the measurements started, and should clearly indicate the point at which the solid was added to the water. From the graph, determine your best estimate of the temperature change, T, due to the process of dissolving the KOH. Explain in your notebook how you did this. Record the T value in your notebook. NOTE: Your volume and mass data for this part of the experiment are quite precise at least 3 or 4 significant figures. As you construct your graph, think about how the scale and other features of the graph might affect the uncertainty of your T value.

Part II. KOH(s) + HCl(aq) H2O(liq) + KCl(aq)

A. In your notebook, prepare another table with space for the following sorts of data:

HCl Concentration __________ mass of beaker ___________________________ mass of beaker + KOH_____________________ mass of KOH ____________________________ moles of KOH ___________________________ # moles HCl in 100 mL_____________________ Time Temp _____ ______ _____ ______ etc. pH test result ____________________ T for the reaction _______________

B. Rinse and dry your calorimeter, and set it up with a thermometer as before. In a clean dry beaker obtain approximately 175 mL of the HCl solution provided (some of this will be used in part III). Note its concentration. Transfer 100.0 mL of that solution into a graduated cylinder, then add that to the calorimeter. Start timing and begin taking temperature measurements as in Part I.

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While one partner continues to make these temperature measurements, the other should weigh out another sample of KOH, similar in mass to that used in Part I. Use the analytical balance again.

No less than 3 minutes after you began collecting temperature data on the HCl solution, note the time and add the solid KOH to the calorimeter. Collect data as before, choosing appropriate time intervals (not more than 30 seconds apart), until you have collected data for at least 5 minutes after the addition of the solid and the temperature behavior has established a pattern again. Test the final solution with pH paper.

C. Graph these data as you did those from Part I and determine T for the reaction. In your notebook, briefly comment on your results, and explain how you decided on a value for T if your data treatment differed from that in the previous part.

Was there a limiting reagent in this reaction? What was it, and how do you know?

Part III: KOH(aq) + HCl(aq) H2O + KCl(aq)

A. In your notebook, prepare yet another table for the following sorts of data: HCl Concentration __________ # grams KOH in 50 mL___________________ # moles KOH in 50 mL___________________ # moles HCl of 50 mL ___________________ temp. of KOH solution at time of mixing ______ temp. of HCl solution at time of mixing ______ Time Temp _____ _____ _____ _____ etc. pH test result ____________________ T for the reaction _______________

B. Rinse and dry your calorimeter. Transfer 50.0 mL of the HCl stock solution you obtained at the beginning of Part II into the polystyrene cup, using your 100 mL graduated cylinder. Start timing once again, and take temperature measurements in the beaker containing the KOH solution you prepared in Part I for at least 3 minutes (at 30 second intervals). Keeping the clock running, wipe off the thermometer bulb, mount the thermometer in the calorimeter as before, and start collecting temperature data on the HCl solution (with the time still running) for a minimum of 3 minutes. Rinse and dry the graduated cylinder, and measure 50.0 mL of the KOH solution from Part I whose temperature you recently measured. Note the time once again, add this KOH solution to the HCl solution in the calorimeter, stir, and continue your temperature measurements as in the earlier parts of the experiment. As in Part II, test the final solution with pH paper.

C. Graph these data and determine T for this process. (NOTE: The temperatures of the two

starting solutions probably differed at the time of mixing; how can you decide what the temperature after mixing would be if no other reaction had happened?) Comment in your notebook on your results, explaining any differences in how you handle the data in this part of the experiment.

Was there a limiting reagent in the reaction? How did parts II and III compare in this respect?

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Part IV. Data Analysis

A. Describe the similarities and any differences among the time versus temperature graphs for the three reactions. Offer an explanation for any differences.

B. Generate a table like that below in your notebook. Be sensitive to the precision of your

measurements. (The least precise numbers will probably be the T values.)

Show in your notebook your calculations of the quantity of heat measured (Q), which is the energy transferred to the water in the styrofoam cup as a result of the chemical reaction taking place. This value is found by solving the equation:

Q = C x M x T.

In this calculation, assume that C = 4.18 joules/(g C). That is, we assume that the heat capacity of the calorimeter is equal to the value for pure water.

For M, the mass of material whose temperature is being changed, use the total mass of water or solution in the Styrofoam cup; assume a density of 1.0 g/mL, or 1.0 g of H2O for every 1.0 mL of solution. These two approximations assume (reasonably correctly) that the small amounts of dissolved KOH, HCl, and KCl do not contribute significantly to the heat capacity of the solutions. If the temperature increases, Q for the solution will be a positive number.

Convert the value of Q into kJ, to get a number of more convenient magnitude. Calculate the value of Q/moles of KOH, with units kJ/mol. Here, Q represents the energy generated by the process or reaction, which will have an opposite sign to the energy absorbed by the water. Since the data were obtained at constant external pressure, this value is also the molar enthalpy change (H) for the process or processes taking place in each case.

Summary Data Table

Part I Part II Part III mass of KOH(g) ________ ________ ________ moles of KOH ________ ________ ________ moles of HCl ________ ________ ________ mL of final solution ________ ________ ________ pH test ________ ________ ________ limiting reagent ________ ________ ________ T (C) ________ ________ ________ heat Q (kJ) ________ ________ ________ Q / moles KOH (kJ/mol) ________ ________ ________

C. When you have completed your calculations, report your results for the six quantities listed in

boldface in the table above to the instructor (enter them on the board, on a computer spreadsheet, or whatever is asked of you), and compare them to results reported by other groups (a class data summary will be provided on Moodle or on the course web site within a day or two). You may use the department laptop computers or other computers near the lab if you wish to construct spreadsheets to do some of the computations and comparisons. When you have class data, it would be worth your while to calculate averages and standard deviations for the values the various class teams obtained for the heat per mole of KOH to find out if there is reasonable agreement in the class.

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D. In your results section, briefly explain how you conducted the experiment, and provide a

verbal summary of your own results. In particular, mention the values you obtained for heat per mole of KOH in the three experiments.

E. In your discussion section, discuss your results and those of the class as a whole. In particular, consider (and number so that we can find them):

1. How do the data you collected on the individual reactions compare with the data collected by other students?

2. Are there patterns that you see in the data for the separate parts of the experiment? For example, does T depend on the amount of solid KOH used in Part I or in Part II? What about Q, or Q/mole KOH?

3. Is there a relationship among your results, or those of the class as a whole, for the three parts of the experiment?

4. What relationship is there among the three chemical equations in this lab? What process or processes take place in each part of the experiment? Is there a similar relationship in any of the quantitative data?

F. Include a conclusion section that summarizes the report and suggests further implications. That is, why was this experiment relevant beyond an introduction to thermochemistry and the KOH/HCl reaction?

G. In your error analysis section, assess the accuracy of your measured values, and discuss what affect those random errors in measurement would have on your results.

Close your report, as usual, with acknowledgement of the contributions of your partner(s), the sources of any data or information not supplied in this handout, acknowledgement of any other consultation with students or faculty about the experiment or its interpretation, and a signed reaffirmation of the Lawrence Honor Pledge. This experiment is based on calorimetry experiments reported or published in a variety of sources, and was particularly influenced by the approach of Michael R. Abraham and Michael J. Pavelich in Inquiries Into Chemistry, 2nd Ed., Waveland Press, (1991).