UNC CHEM 101 Exp2 - Post-lab assignment on Aqueous Reactions (Part 1) PDF

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Post-lab assignment on Aqueous Reactions (Part 1)...

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ONYEN

Experiment 2: Aqueous Reactions Experiment 2: In-Lab Data Collection RESULTS (34 pts for data, correct calculations, and scatter plots; 8 pts for correct significant figures; 8 pts for specific, concise observations) Record your general measurements, recorded data and observations directly into this section during the lab. Data can be transcribed directly into the tables provided below. Add titles and captions to all tables and figures for full credit. Remove and replace any highlighted text.     

Present your data in the figures and tables below. Summarize your observations during the progress of the experiment. Observations should not include numbers (measurements). Tables should be accompanied by a caption which helps the reader understand what the data means and what conclusions can be drawn from the data in the table. Always fill in units in tables - in parentheses below. To avoid confusion, do not add units to numbers, but instead to data headers or labels (such as what was done in the Excel tutorial) Add observations between data tables to log what you see during the experiment (1-2 sentences is usually sufficient for any major steps - i.e. the volume increased noticeably when...)

Observations

1: Foaming bubbles formed almost immediately and persisted throughout the 4 minutes, overall temperature decreased quickly and then evened out. 2: No visible changes to the solution, temperature increased quickly before leveling out. 3: Foaming bubbles formed, loud enough to make a noise, temperature decreased before leveling out 4: Precipitate forming (solution getting cloudy), bubbles surrounding magnesium strip, strip appears to be dissolving, temperature increase before leveling out.

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CHEM 101L Post-Lab 2

Fall 2019

A. HEAT OF SOLUTION DATA COLLECTION Observations The condition of our calorimeter was the same before both the reactions occurred-- there were no observable abnormalities in our water quality and everything seemed to rotate properly using our spin plate. After adding ammonium chloride, we immediately observed the formation of bubbles and fizziness in our solution. The temperature decreased relatively swiftly before reaching its minimum and flattening back out. My initial reaction was that this process would be considered endothermic because the overall temperature of the solution decreased. After adding the sodium carbonate, we experienced dissimilar observations. There were no bubbles or other visible changes to the solution, and the temperature increased relatively swiftly before reaching a maximum and flattening back out. My initial reaction was that this process would be considered exothermic because the overall temperature of the solution increased. Table 1. Heat of Ammonium Chloride & Sodium Carbonate Solutions Ammonium Chloride Sodium Carbonate Mass (g) 1.000 1.007 22.7 22.3 Initial Temperature ( oC) o Final Temperature ( C) 20.7 24.2 Moles (mol) 0.01869 0.009501 2.7 1.9 Change in Temp. ( oC) DT/mol (oC/mol) 140 200

Figure 1. This graph shows the relationship between temperature and time for our ammonium chloride and sodium carbonate solutions. It shows a positive relationship between temperature and time for the sodium carbonate solution and a negative relationship between temperature and time for the ammonium chloride solution.

2

CHEM 101L Post-Lab 2

Fall 2019

B. HEAT OF REACTION: SODIUM BICARBONATE WITH CITRIC ACID Observations Before adding the sodium bicarbonate to the citric acid in our calorimeter, I observed that the initial temperature of the citric acid was relatively greater than that of the water used as a solvent in the previous trials with ammonium chloride and sodium carbonate. After adding the sodium bicarbonate, we noticed foam and bubbles forming aggressively enough to be heard through the closed lid of the calorimeter. The temperature decreased relatively swiftly before hitting a minimum and leveling back out. My initial reaction was that this process would be considered endothermic because the overall temperature of the solution decreased. I observed that this bubbling also occurred in our reaction with ammonium chloride and water which also resulted in an overall temperature decrease of the solution.

Table 2. Data Analysis for Sodium Bicarbonate with Citric Acid Solution Equations

3NaHCO3 (s) + H3C6H5O7 (aq) Na3C6H5O7 (aq) + 3H2O (l) + 3CO2 (g)

Mass (g) Moles (mol) Final Temp. ( oC) Initial Temp. (oC) Temp. Change ( oC) DT/mol (units)

HCO3- (aq) + H3O+ (aq)  2H2O (l) + CO2 (g) 3.501 0.04168 14.1 23.8 9.7 230

Figure 2. This graph shows the relationship between temperature and time for a solution of sodium bicarbonate and citric acid. The graph shows a negative relationship between temperature and time following a reaction between these substances.

3

CHEM 101L Post-Lab 2

Fall 2019

C. HEAT OF REACTION: HYDROCHLORIC ACID AND MAGNESIUM Observations Before adding the strip of magnesium to our hydrochloric acid, I noted that the starting temperature of our hydrochloric acid was greater than that of the water used as a solvent for ammonium chloride and sodium carbonate. After adding the magnesium, we noticed a precipitate forming in our calorimeter, making the solution appear cloudy. After some more time, we noticed the temperature begin to increase (though not as swiftly compared to the previous three solutions we tested). Then, we noticed that the magnesium strip was developing bubbles on its surface and beginning to dissolve in the acid, causing more bubbles to form and the solution to become even cloudier. My first thought was that this solution would be considered exothermic because the overall temperature increased. Table 3. Data Analysis for Magnesium and Hydrochloric Acid Solution Mg (s) + 2HCl (aq)  MgCl2 (aq) + H2 (g)

Equations Mass (g) Moles (mol) Final Temp. ( oC) Initial Temp. (oC) Temp. Change ( oC) DT/mol (units)

Mg (s) + 2H+ (aq)  Mg+2 (aq) + H2 (g) 0.046 0.001893 29.7 23.5 6.2 3300

Figure 3. This graph shows the relationship between temperature and time for a solution of magnesium and hydrochloric acid. The graph shows a positive relationship between temperature and time following a reaction between these two substances.

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CHEM 101L Post-Lab 2

TA ONLY:

Fall 2019

Performance

Experiment 2: Post-Lab Analysis SAMPLE CALCULATIONS (6) Equations should be expressed first algebraically, with clear labels, and then with substituted numbers and units (and correct significant figures) for one example. Be clear which sample/trial is being used in the sample calculation.

-Moles: mass of solute used ÷ molar mass of solute  ex., ammonium chloride: 1.000g ÷ 53.491 g/mol = 0.01869 mol -Change in temperature: |final temperature - initial temperature|  ex., sodium carbonate: |24.2 oC - 22.3 oC| = 1.9 oC -Change in temperature per mole of reagent: change in temperature ÷ moles of solute  ex., sodium bicarbonate: 9.7 oC ÷ 0.04168 mol = 230 oC/mol

Post-Lab Discussion Questions Answer the following questions within the required sentence criteria. Practice using a professional, scientific voice in your analysis. This discussion is an analysis of the data you collected during the experiment. Reference each table and figure and point out the most important conclusion or evidence provided by each. Specifically relate your observations to the decisions you made when writing the net ionic equation, and in determining which reactions were exo- or endothermic. Always refer to specific data and examples from the experiment to support your reasoning. (5 pts) Consider your results and observations from the dissolution of ammonium chloride, answer the following series of questions using 3-5 sentences. Was this an endo, or exothermic process? (and how do you know - refer to your observations and data and reference the specific table from your results). What happened on a molecular level when the solid was dissolved into the liquid? Which species were present in solution? Report actual numbers to support your conclusion. Why did you calculate DT/mol rather than DT/g? Based on my results and observations from the dissolution of ammonium chloride in water, I conclude that the reaction was an endothermic process. I know this because the overall temperature of the solution decreased (22.7 oC  20.7 oC), showing evidence of heat energy being absorbed by the reaction from the breaking of bonds in ammonium chloride to form separate ions. Because breaking bonds requires energy, that energy is absorbed from the surroundings (in this case, the water in our Styrofoam cup) which causes the temperature of the entire solution to decrease. We calculated this decrease in temperature using DT/mol rather than DT/g because we did not want our result to be dependent on the amount of substance used, but on its molecular properties. 5

CHEM 101L Post-Lab 2

Fall 2019

(5 pts) Consider your results and observations from the reaction of citric acid with sodium bicarbonate, answer the following series of questions using 3-5 sentences. What physical observations revealed that a reaction was taking place? (Be specific and refer to chemicals and physical observations) Was this an endo, or exothermic process? (and how do you know - refer to your observations and data and reference the specific table from your results). After the reaction, which species were present in solution? Report actual numbers to support your conclusion.

We observed the formation of gas (CO2) in the form of foam and bubbles in our experiment involving sodium bicarbonate and citric acid, which revealed that a reaction was taking place. This reaction was an endothermic process because the overall temperature of the solution decreased greatly (23.8 o C  14.1 oC), indicating that heat was being absorbed from the surroundings in this system. After the reaction, we had water, sodium citrate, and carbon dioxide present in the solution. The breaking of the bonds in sodium bicarbonate and citric acid required energy, which it absorbed from the system in the form of heat which is why we saw an overall temperature decrease.

(5 pts) Consider your results and observations from the reaction of solid magnesium with hydrochloric acid, answer the following series of questions using 3-5 sentences. What physical observations revealed that a reaction was taking place? (Be specific and refer to chemicals and physical observations) Was this an endo, or exothermic process? (and how do you know - refer to your observations and data and reference the specific table from your results). After the reaction, which species were present in solution? Report actual numbers to support your conclusion.

In this experiment, it was obvious that a reaction was taking place because of the observed dissolving of our solid magnesium strip. It was evident that the strip was dissolving because bubbles formed around it, the solution became cloudy, and the strip appeared to have lost a significant portion of its size after a few minutes. It was also obvious that this reaction was exothermic, not only because of the overall temperature increase of the solution (23.5 oC  29.7 oC), but because of the formation of Magnesium Chloride in the products of our reaction. Because the forming of new bonds releases energy in the form of heat, the creation of magnesium chloride released that heat energy to its surroundings resulting in an overall temperature increase in the solution.

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CHEM 101L Post-Lab 2

Fall 2019

(5 pts) Error Analysis (short paragraph, 4-5 sentences). Highlight sources of error in these experiments, the extent to which these errors are random or systematic, whether these errors primarily affected accuracy or precision, and how these errors impacted the results (for example, were your numerical results too high or too low due to these errors? Were these errors significant? Did they change whether a reaction was exothermic or endothermic?). What is the function of the calorimeter in this experiment? What role do the Styrofoam cups, lids, and stir bar play in improving the measurement results? How could the experiment be improved? I do not believe there were any significant sources of error in this experiment for my group. Possible sources of systematic error include an improperly calibrated weigh boat or temperature sensor. Sources of random error could include temperature or pressure inconsistencies in the lab room, or human error like the misreading of a weight measurement or amount of solvent in a beaker. These types of errors could have impacted our results in any number of ways since we did not observe any errors that would have skewed our data in any particular direction. The function of the calorimeter in this experiment was to serve as a closed system in which our reactions could take place-- the insulation from the Styrofoam cups, lids, and stir bar all played a role in keeping this system as closed as possible so our results could be as accurate as possible and remain unaffected by conditions outside of the calorimeter. This experiment could be improved by having a temperature sensor inside of the Styrofoam cup so the lid could remain entirely sealed at all times instead of having a small opening through which the sensor was placed.

CONCLUSIONS (4) Were you able to accurately measure and investigate physical changes using this experimental setup? What did you learn about these reactions? (1-2 sentences) I believe we were able to accurately measure and investigate physical changes using this experimental setup. I learned that the reactions involving ammonium chloride and sodium bicarbonate as solutes were endothermic and that the reactions involving sodium bicarbonate and magnesium as solutes were exothermic.

TA ONLY:

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