CHEMISTRY 130 General Chemistry I. Acetic Acid in Vinegar

Transcription

1 CHEMISTRY 130 General Chemistry I Acetic Acid in Vinegar Burets, glassware most commonly used in a technique called titration, are filled with solutions of various colors. DEPARTMENT OF CHEMISTRY UNIVERSITY OF KANSAS

2 Acetic Acid in Vinegar Introduction What is vinegar? Vinegar principally consists of acetic acid (CH 3COOH) and water (H 2O). Because of its chemical properties, vinegar can be used in a variety of ways: as a cleanser, salad dressing, disinfectant, preservative, or cooking ingredient. As such, it is a very common household item. Vinegar is produced from fermentation of sugars. Fermentation processes have been used for food preservation since ancient times. Fermentation is a chemical change produced in organic substances by the action of enzymes. Fermentation can produce lactic acid, as in fermentation of milk to yogurt; or alcohol or acetic acid, as in fermentation of fruit. As Ben Selinger explains in Chemistry in the Marketplace, "Fermentation of food to produce acid is common to all cultures and cuisines, e.g., pickles, sauerkraut, coffee beans, kimchi, salami, cheese, sour dough bread, soy sauce." Bacteria, yeasts, and molds are used to produce lactic acid or acetic acid or both. Fermentation to alcohol produces an unstable product unless the concentration of alcohol is fairly high. Thus, wine turns to vinegar over time. Check the following link on fermented beverages: How can we determine the amount of acetic acid present in vinegar? Acids react with bases, and acetic acid is no exception. In fact, with a known amount of base, we can use this acidbase reaction to measure the amount of acetic acid contained in vinegar. Consider the reaction of acetic acid with the base sodium hydroxide. In water, both split apart into ions: (1) acetic acid dissolved in water: CH 3 COOH (aq) CH 3 COO (aq) + + H (aq) + (2) sodium hydroxide dissolved in water: NaOH (aq) Na (aq) + HO (aq) The H+ and HO- ions react with one another to form more water: + (3) H (aq) + HO (aq) H 2 O (l) It s worth mentioning that the CH 3COO- and Na+ ions are leftover, remaining dissolved in the surrounding water and not reacting with anything. In this week s experiment, you ll measure the amount of acetic acid that is present in vinegar using a technique called titration. In this version of a titration, you ll add just enough of a sodium hydroxide solution so that all of the acid is reacted to form water. Knowing the amount of base that s required and how many H+ ions each acid molecule can generate, it s possible to calculate the number of moles of acid, and ultimately, their initial concentration. For a discussion with images, see pages of your textbook. 2

3 Pre-lab Safety: Goggles must be worn at all times. Gloves are strongly required, as you will be handling solutions of a strong base. Caution! Acids and bases are hazardous if splashed on clothing, exposed skin or in the eyes. Prolonged exposure of the skin to even dilute solutions of acid and base can cause serious burns. If acids and bases splash on skin or clothes, remove the affected clothing and flush the affected areas thoroughly with cold water. Chemical waste disposal: All of the solutions used in this laboratory can be discarded down the drain after being neutralized. Stock solutions, which are not neutralized, should be put in the appropriately labeled waste container. Talk to your TA if you have any questions about this. Ingestion: Never taste anything in the laboratory, and do not bring food or drink into the laboratory. Never put objects, such as pens, into your mouth. It is necessary that you wash your hands thoroughly when you leave the laboratory to ensure that you do not ingest anything that might be on your hands. In case of a spill: For this experiment, use a damp sponge to collect the spilled solution and wash the area with water. Rinse the sponge thoroughly, and wash your hands thoroughly. Pre-lab Assignment: Please write out the following in your lab notebook. This assignment must be completed before the beginning of lab. You will not be allowed to start the experiment until this assignment has been completed and accepted by your TA. 1) List all of the chemicals you will use for this week's experiment. Then, for each chemical, list specific safety precaution(s) that must be followed. In order to find specific safety information, please obtain a Materials Safety Data Sheet (MSDS) on the chemical of interest. MSDSs can be found through an internet search (e.g., Google) or from the following website: 2) In this experiment, you will need to understand the concept of concentration and the unit called molar. To play with this idea, go to the following website: and click Play. Then click the button labeled show values. Answer the following: a) The solute is the chemical that gets dissolved, and it should be set to Drink Mix. Adjust the volume to 1.0 L and the solute amount to 1.0 mole. What is the molar concentration ( Molarity )? b) Decrease the volume to 0.2 L. What solute amount is required to form a 1.0 M solution? c) What is the molarity (in M) of the most concentrated solution you can make? What solute amount (in moles) and volume (in L) did you use? d) What is the molarity (in M) for the least concentrated solution you can make? What solute amount (in moles) and volume (in L) did you use? e) What is the relationship between M, mol, and L? f) The amount of material that can be dissolved has a maximum value. We say that a solution that can t dissolve any more solute is saturated. Change the solute to Copper (II) sulfate. What is the maximum concentration of copper(ii) sulfate in water? 3

4 3) In your notebook, write the balanced chemical equations for the dissociation of acetic acid in water, the dissociation of sodium hydroxide in water, and the combination of hydroxide ion and hydrogen ion to form water. 4) Based on your balanced chemical equations from problem 3), fill in the blanks in the following calculation. Perform the calculation. Your answer should include the correct units and a chemical formula. mol HO mol NaOH ( ) ( 1 mol H+ ) 1 mol NaOH mol HO (1 mol CH 3COOH ) = mol H+ Procedure Part 1 Titration Procedure In this part of the experiment, you will perform a titration to determine the amount of acetic acid present in vinegar. If you have questions, be sure to ask your TA. Your TA will discuss or review the topics of acids and bases, concentration, and titration. Based on this information, discuss the following with your group members before your being your experiment. (Doing so will make things go much more smoothly!) What key things do you need to know before starting the experiment? What chemical reaction will be taking place during the titration? How will you calculate the acetic acid concentration from your titration data? How will you know whether your results are accurate? Do you wish to compare your results with those of other lab groups who may be studying different brands or different types of vinegar? In performing a titration, the relative concentrations of the acid and base need to be considered. In this experiment the NaOH concentration is about 0.15 M. As you perform the experiment, consider what problems might arise if, for example, 1 M or 0.01 M NaOH was employed instead. In this experiment only 5.00 ml of vinegar will be titrated. It can be difficult to see the color of the indicator in this small volume. Thus, deionized water will be added to increase the volume, rendering the endpoint to visually easier to observe. Titration setup: 1a. Obtain ml of vinegar in a small beaker. Weigh a 100-mL beaker. Using a pipet, transfer 5.00 ml of vinegar into a 100-mL beaker. Reweigh the beaker and calculate the mass of vinegar that was added. Add approximately 20 ml deionized water. Why is it necessary to add a precise volume of vinegar? Why is it unnecessary to add a precise volume of H 2O? 4

5 1b. Add a magnetic stir bar to the beaker and place the beaker atop a magnetic stir plate. (Continuous stirring during the titration will homogenize the sample as drops are added from the buret..) 1c. Add approximately 3 drops of phenolphthalein ph indicator to the beaker. (While titrating with the base, the phenolphthalein will exhibit a faint pink color that disappears as the solution is mixed.) 1d. Obtain about 50 ml of NaOH solution for titrating your vinegar sample. Note that the specific molarity (M) is shown on the reagent bottle. Should you use this molarity value, or the nominal value of 0.15 M, in your calculations? 1e. To prepare the buret for use, rinse the buret with distilled water and then with a small quantity of the NaOH solution. Allow the some of the solution to run out the tip; then slowly rotate the buret while tipping it to a horizontal position to rinse the walls of the buret. Dispose of this wash solution. This process removes leftover rinse water from the buret, so that the NaOH is not diluted when the buret is filled. Fill the buret with the NaOH solution until the level of the solution is around the 1-mL mark. 1f. Using a piece of paper with a blackened area behind and below the meniscus (to reflect off the meniscus), read the initial buret volume. How many decimal places should this reading contain? Test titration: 2a. Add NaOH solution in 1 ml at a time. As the color of the indicator takes longer and longer to fade, add smaller and smaller volumes of NaOH. Adding progressively smaller volumes should result in adding single drops of NaOH at a time. (After you become comfortable with the procedure, you can focus on the accuracy of your titration. It is possible to add half-drops. This is done by opening the buret just enough to allow some liquid out of the buret, but not so much that a full drop forms and falls from the buret tip. The tip of the buret can then be touched to the side of the beaker. This half-drop on the side of the beaker then can be washed into the main solution using a squirt bottle filled with deionized water.) 2b. The equivalence point, at which the amount of base added is equal to the amount of acid present, occurs when the very faintest pink color persists. Have your TA check your faint pink color. Make special note of the buret volume at this visually determined equivalence point. 2c. Continue titrating dropwise beyond the equivalence point to assess how the color changes beyond the equivalence point. This will help you know if you have gone too far during subsequent titrations. How much more NaOH does it take to generate a strong color? Does the color continue to deepen as you continue to add NaOH? 2d. Discard your test solution according to instructions from your TA. Do not discard the solution in your buret. In fact, you can top off the buret, filling it again to around the 1 ml mark for your next titration. Why can you do this? 5

6 Now you have an estimate of the volume required to reach the equivalence point. With this information, you will perform at least 3 good titrations. Titration for data collection: 3. Having conducted at least one test titration, perform three titrations. For each titration, repeat steps 1a 1c. Subtract 2-3 ml from your estimated volume for reaching the equivalence point. Add this volume of NaOH solution to the sample. Then begin slowly adding NaOH, as in 2b. Repeat 2c, and record the volume of solution you added to reach the equivalence point. If you go beyond the equivalence point, discard the result and repeat this procedure. In the end, you want three good volume measurements. Lab clean-up: Carefully rinse your burets with deionized water. Then place them in the stand upside down. This allows excess water to drain from the buret and prepares the burets for the next class. All other glassware should be cleaned and returned to the lab drawer. Neutralized solutions can go in the sink. Unused stock solutions of NaOH, which are not neutralized, should not be poured in the sink. There will be a separate waste container for stock solutions. Ask your TA if you have questions or need assistance. Data and Results: You will use your data to calculate the concentration of acetic acid in vinegar. The procedure is the same as the stoichiometry calculations you will be learning in lecture this week and that are outlined in Chapter 4 of your book. The calculation contains a lot of information. Be sure you understand the calculation before leaving the laboratory. A summary of the calculation is provided in the Appendix to this laboratory experiment. Record your data in your notebook in a table similar to the one below: After completing the table, calculate average values for the mass percent of acetic acid in vinegar, as well as the molarity of acetic acid in vinegar 6

7 Post-lab Questions Please answer the following in your notebook: 1. Assume that carbonic acid dissociates in water by the following chemical equation: + H 2 CO 3 (aq) 2 H (aq) 2 + CO 3(aq) (The actual process is a bit more complicated.) Using a titration, 7.81 ml of a 0.98 M solution of sodium hydroxide were used to react with all of the acid. What was the mass of carbonic acid originally present in the sample? 2. You accidentally added deionized water to the buret containing the stock solution of NaOH. Would you results be lower or higher than they should be? Reference(s) 1. Accessed Feb. 11, Selinger, Ben. Chemistry in the Marketplace, 5 th ed,; Allen & Unwin: Crows Nest, Australia,

8 Glossary Acid; HA For this experiment: a molecular compound that is able to donate one or more H + ions (protons) when dissolved in water, thereby increasing the concentration of H + ions and decreasing the concentration of OH- (hydroxide) ions; additional important definitions of acid exist. Molarity; (M) A means of expression solution concentration as the number of moles of solute per liter of solution. Solute A minority component of a solution. Acid-base titration (neutralization reaction) a laboratory procedure in which a basic (or acidic) solution of unknown concentration is reacted with an acidic (or basic) solution of known concentration, in order to determine the concentration of the unknown Solution A homogeneous mixture of two or more substances. Solvent The majority component of a solution. Base; B For this experiment: a molecular compound that is able to release one or more OH- (hydroxide) ions when dissolved in water, thereby increasing the concentration of OH- ions and decreasing the concentration of H + ions; additional important definitions of base exist. Concentration The amount of a substance contained in a specified volume or mass. Mass percent In this experiment: a substance s percentage of the total mass of a solution containing the substance. 8

9 Appendix Acids versus Bases Acids are proton (i.e., H+) donors. Accordingly, chemists often use the symbol H+ to represent the ion produced by dissociation of an acid molecule. However, in aqueous solutions, "bare" H+ ions do not exist. Instead, they reside on water molecules--that is, the H+ is "donated" to the water molecule by the acid. The symbol H3O+, often called the hydronium ion, thus provides a better representation of reality than H+. Nonetheless, for describing aqueous solutions, H+ and H3O+ are used interchangeably by scientists. The stronger the acid, the greater the number of H+ (H3O+) ions found in a solution of a given concentration. Bases are proton acceptors and will accept protons from acids. When combined, aqueous acids and bases react to neutralize each other, forming water and a salt. A solution resulting from a mixture of a simple strong acid and strong base will be neutral if exactly the same amounts (a stoichiometric ratio) of each are mixed. If an excess of one or the other is present, the solution will exhibit either acidic or basic character. Overview: Calculating the Concentration of Acetic Acid in Vinegar The titration calculation is outlined in a few steps below. V initial V NaOH = V final V initial (1) Based on the volume of sodium hydroxide you used (shown left) and the concentration of sodium hydroxide, how many moles of sodium hydroxide were reacted? buret V final (2) Similarly to calculations in your book and prelab exercises, use the coefficients in the chemical equations to convert from moles of NaOH to moles of CH 3COOH. The chemical equations for this lab are shown right. CH3COOH(aq) CH3COO-(aq) + H + (aq) NaOH(aq) Na + (aq) + OH - (aq) H + (aq) + OH - (aq) H2O(l) (3) Convert your moles of acetic acid to a mass of acetic acid. Use moles of acetic acid and volume of vinegar to arrive at the molarity of acetic acid in vinegar. Use the mass of acetic acid and mass of vinegar to calculate mass percent.