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7. STOICHIOMETRY IN SOLUTIONS

IntroductionTitration is a method of volumetric analysis where volume measurements are used to calculate the concentration of an unknown analyte solution. In acid-base chemistry, titrations are often used to calculate the amount of acid or base in a solution. The neutralization reaction between an acid and base is stoichiometric and produces water as a product. In this lab, the stoichiometry of the reaction between an acid and base will be determined using temperature and conductivity.

Concepts Stoichiometry Titration Acid-base reactions Thermochemistry Net ionic equation Limiting reactant Green chemistry

BackgroundOne of the most important concepts of chemistry is stoichiometry. Stoichiometry is the study of the quantities of reactants and products in a chemical reaction. The word is derived from the Greek words: stoikheion (element) and metron (measure). It is sometimes referred to by another name: mass relations. Mass relations are based on three important laws. Law of Conservation of Mass: the mass of products equals the mass of reactants Law of Multiple Proportions: the mass of one element combines with a fixed mass of

another element in a ratio of whole numbers Law of Constant Composition: all samples of any given compound have the same

elemental compositionMany of the most important materials of the modern world are products of the application of chemistry and chemical reactions. Gasoline, oil, pharmaceuticals, plastics, solvents and fertilizers are a few of the countless products of chemistry that impact our lives every day. The cost-efficiency of producing these materials is fundamentally based on maximizing the utilization of reactants and creation of products in chemical processes and minimizing the amount of waste. In other words, it’s all about stoichiometry! Over the years, however, some processes used to create these products were shown to be harmful to people and the environment. To address this, various groups in academia and industry have begun to implement Green Chemistry practices in research, chemical processes and the production and use of chemicals. Green Chemistry is guided by the following principles:Preventing wasteAtom economyNo hazardous chemical productionGreen products that work as well as non-green products

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7. STOICHIOMETRY IN SOLUTIONS / STUDENT HANDOUT

Removal of non-essential additivesEnergy efficiency during productionUse of renewable materialsMinimization of production stepsOptimization of steps to speed productionUse biodegradable materialsMonitor processes for pollution preventionSafety firstPrinciple 2 is called the “atom economy principle” meaning the less left over, the better. It is the keystone of stoichiometry: maximize product, minimize waste. In this lab, you will use titration to demonstrate the atom economy principle and quantitate the stoichiometric masses of reactants in an acid-base neutralization reaction. Titration is a powerful analytical technique that uses a substance with a known concentration to determine the concentration of a solution with an unknown concentration. The neutralization of acid and base produces water as a product, thus operating within the principles of green chemistry. Acid-base titrations are often performed with indicators or a pH sensor. In this lab, the progression of the chemical reaction is monitored using conductivity and temperature. Matter is not the only aspect of a chemical reaction that is conserved. Energy is conserved as well. The amount of heat taken in or given off in a chemical reaction is proportional to the molar amounts of reacting species. During an acid-base neutralization, energy, in the form of heat, is given off as chemical bonds are rearranged to produce water. Temperature of the system is expected to rise until the equivalence point is achieved. Conductivity of the system is also expected to change during neutralization. While unreacted ions of a strong acid or base exist in solution, they will continue to conduct a current. Thus, conductivity is expected to decrease in an acid-base reaction until the equivalence point is reached.

Pre-Lab Questions

1. Identify each of the following substances as an acid or a base.

HCl:

H2SO4:

KOH:

NaOH:

HNO3:

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2. In the beakers below, draw particle-level representations showing a strong acid and a strong base dissolved in water.

3. Describe the procedure you would use to make 100 mL of 0.85 M HCl from a 2.0 M HCl solution. Assume you have a 100-mL volumetric flask, distilled water and a 50-mL graduated cylinder available.

4. List three pieces of evidence that indicate a chemical reaction has taken place.

Materials and EquipmentUse the following materials to complete the initial investigation. For conducting an experiment of your own design, check with your teacher to see what materials and equipment are available.

Data collection system Micro stir bar Wireless conductivity sensor Wireless temperature sensor

Ring stand Phenolphthalein

Wireless drop counter 1.0 M Hydrochloric acid (HCl), 50 mL1 Beaker, 250-mL Beakers, 150-mL Graduated cylinder, 50-mL Volumetric pipette, 25-mL Pipet bulb Magnetic stirrer

1.5 M Hydrochloric acid (HCl), 50 mL1 2.0 M Sodium hydroxide (NaOH), 120 mL1 Wash bottle Distilled water Calibration solutions for sensors (Appendix A)

SafetyFollow these important safety precautions in addition to your regular classroom procedures: Wear safety goggles and gloves at all times. This lab uses strong acids and bases. In case of contact with your skin, wash off the solution with a large amount of water. Excess acid or base may be flushed down the sink with plenty of water. Wash hands thoroughly with soap and water before leaving laboratory. Review chemical handling and disposal instructions as directed by Material Safety Data

Sheet.

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DisposalIf your drain system is connected to a sanitary sewer system, the following instructions apply. Dilute acid and base solutions may be rinsed down the drain with an excess of water

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Initial Investigation

Equivalence Point and End Point Determination in Acid-Base Titration

1. Start a new experiment on the data collection system from your Chromebook, computer or mobile device.

2. Connect a wireless conductivity sensor, a wireless temperature sensor, and a wireless drop counter to the data collection system. Open lab file "07 Stoichiometry in Solutions" or create a graph of Conductivity and Temperature vs. Volume (mL).

3. Set up titration apparatus using the illustration as a guide.

4. Rinse the drop dispenser syringe:

a. Place a 250-mL beaker under the drop dispenser and open both stopcocks.

b. Rinse the drop dispenser syringe and stopcock three times with approximately 20 mL of distilled water. This will remove any residue.

c. Rinse the drop dispenser three times with 20 mL of the 2.0 M NaOH. This removes remaining water that would dilute the NaOH solution.

d. Discard the rinse solution as directed by your teacher.

5. Calibrate the drop counter using instructions in Appendix A.NOTE: Do not disconnect the drop counter from the data collection system or it will need to be calibrated again.

6. Use the top stopcock to adjust the flow rate to approximately 1 drop per second. Close the bottom stopcock and fill the syringe to the top mark with the 2.0 M NaOH solution. NOTE: The top valve controls the flow rate and the bottom valve turns the flow on and off.

7. Assemble the rest of the apparatus, using the steps below and the illustration as a guide.

a. Position the magnetic stir plate on the base of the ring stand.

b. Position the drop counter over the magnetic stir plate.

c. Place the temperature sensor though the small hole in the drop counter.

d. Place the conductivity sensor, with the micro stir bar attached, through a large hole in the drop counter.

8. Transfer 25.0 mL of 1.0 M HCl solution to a clean, dry 150-mL beaker. Record the molarity and volume in Table 1.

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Table 1 — Volumes and Concentrations of Reactants

Parameter ValueConcentration of HCl (M)

Volume of 1.0 M HCl solution (mL)

Concentration of NaOH used (M)

Volume of NaOH added to change the color of the solution (mL)

a. Calculate the number of moles of acid added to the beaker in Initial Investigation.

b. Calculate the molarity of the solution after the 50.0 mL of distilled water is added.

c. Calculate the number of moles of acid after the 50.0 ml of distilled water is added.

9. Add 50.0 mL of distilled water to the beaker.

10. Put 3 drops of phenolphthalein indicator into the beaker with the HCl solution.

11. Place the 150-mL beaker with the hydrochloric acid solution under the drop dispenser. The sensors should be immersed in the solution. Turn on the magnetic stirrer at a slow and steady rate.

12. Start recording data.

13. Open the bottom stopcock on the drop dispenser to begin the flow of the 2.0 M NaOH into the HCl solution.

14. Record the volume of titrant used when the phenolphthalein indicator changes color in the table below.

15. Continue until approximately 20 mL of NaOH solution has been added to the beaker.

16. Stop recording data.

17. Save your experiment and dispose of the used solutions according to your teacher's instructions.

18. Graph Temperature and Conductivity vs. Volume of HCl in the space provided or attach graph generated with the lab file.

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19. The point where the solution changes color is called the end point. What was the volume of 2.0 M NaOH required to reach the end point of the reaction? Label this point on the graph and record data in Table 2.

Table 2: Volumes and Molarities at End Point

Solution Molarity (M)

Volumes(mL)

End Point Amount (mol)

HCl (analyte)NaOH (titrant)

20. In this procedure the color change end point is also the equivalence point. What is equal at the equivalence point?

21. Write the word “energy” on the appropriate side of the balanced equation. Explain your answer.

HCl + NaOH → NaCl + H2O

22. The neutralization reaction between HCl and NaOH occurs in stoichiometric proportions.

Once the reaction is past the equivalence point, will excess reactant continue to have an effect on the change in temperature? Explain your response.

b. Describe the region of the graph where NaOH is the limiting reactant in the reaction.

c. Describe the region of the graph where HCl is the limiting reactant in the reaction.

23. Looking at the graphical data above, explain why there is a change in the temperature curve around the equivalence point.

24. Looking at the graphical data above, explain why there is a change in the conductivity curve around the equivalence point.

25. Draw a particulate level representation of the substances in the beaker before the experiment, at the equivalence point, and at the end of the experiment.

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Advanced Investigation

Predicting equivalence point and end point

1. Set up the equipment as detailed in the Initial Investigation.

3. Rinse and fill the drop dispenser with 2.0 M NaOH.

4. If the drop counter has been disconnected from the data collection system since it was last calibrated, calibrate it using the procedure in Reference Guide 013-16026A at pasco.com.

5. A sample of HCl to react with the 2.0 M NaOH will be provided by the instructor. Record its concentration.

6. Transfer 25.0 mL of the HCl solution into a clean, dry 150-mL beaker. Record the molarity and volume in the table below.

7. Add 50.0 mL of distilled water to the beaker.

8. Add 3 drops of phenolphthalein to the acid.

9. Draw your predicted graphical results below. Indicate the point on the graph where the predicted equivalence point will occur.

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10. Place the 150-mL beaker with the hydrochloric acid solution under the drop dispenser. Turn on the magnetic stirrer at a slow and steady rate.

11. Start recording data.

12. Turn the drop dispenser stopcock carefully, allowing the titrant (2.0 M NaOH) to drip slowly (1 to 2 drops per second) into the HCl solution.

13. Record the volume when the phenolphthalein indicator changes color in Table 2 below.

14. Add NaOH until the equivalence point has been reached and exceeded.

15. Stop recording data.

16. Save your experiment and dispose of the used solutions according to your teacher's instructions.

Table 2: Volumes and Concentrations of Reactants

Parameter Value

Concentration of HCl sample (M)

Volume of 1.0 M HCl solution (mL)

Concentration of NaOH used (M)

Volume of NaOH added to change the color of the solution (mL)

17. Graph the experimental temperature and conductivity data versus volume of HCl.

18. How did your predicted graph compare to the experimental graph?

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Extended Inquiry Investigation

How Much Acid Can an Antacid Tablet Neutralize?

In order to solve problems requiring an HCl solution with known concentration, an HCl solution must first be standardized. This means its exact concentration must be determined using titration with a sodium hydroxide standard.Design a protocol to use titration to determine the calcium carbonate (the acid-neutralizing component of antacids) content of a stomach-acid neutralizing pill using the standardized HCl solution.

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Synthesis Questions

1. During a titration experiment with HCl and NaOH, the phenolphthalein color-change end point did not occur at the same volume of NaOH as the change in the temperature graph and the conductivity graph? Provide some reasons for the discrepancy.

2. Phenolphthalein, a temperature sensor, and a conductivity sensor were all used to determine when the reaction was complete. For the following reactions, which would work best? Explain your reasoning.

a. A reaction in which a precipitate is formed.b. A reaction that is endothermic.c. A reaction in which one of the products has a dark color.

AP® Chemistry Review Question

1. Potassium sorbate, KC6H7O2 (molar mass 150 g/mol) is commonly added to diet soft drinks as a preservative. A stock solution of KC6H7O2(aq) of known concentration must be prepared. A student titrates 45.00 mL of the stock solution with 1.25 M HCl(aq) using both an indicator and a pH meter. The value of Ka for sorbic acid, HC6H7O2, is 1.7 x 10-5.

a. Write the net ionic equation for the reaction between KC6H7O2(aq) and HCl(aq).

b. A total of 29.95 mL of 1.25 M HCl(aq) is required to reach the equivalence point. Calculate [KC6H7O2] in the stock solution.

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