Chemistry Lab Define Acids and Bases

Transcription

1 Introduction Chemistry Lab Define Acids and Bases By the 1500 s chemists recognized that certain substances shared a common property a sour taste. These substances possessed other characteristic properties as well. They were given the collective name of acids from the Latin word acidus, meaning sour. Another group of substances, called alkalis (or bases), was prepared from the ashes of wood. Bases had a slippery feel and were discovered to be effective cleaners. When defining acids or bases, it is important to realize that there are two types of definitions operational (or laboratory) definitions and conceptual definitions: A An operational definition is a description of expected test results from a laboratory situation (what do you observe or measure, like color changes, bubbles etc.). B A conceptual definition attempts to explain the operational definition (what defines each on a submicroscopic level, or definitions you would find in the book) Tests for acidity and alkalinity are common today in such fields as gardening and swimming pool maintenance. For example, specialized acid-base test kits are available to home gardeners who wish to monitor the acidity or alkalinity of their soil. One component of such kits is a chemical indicator solution, which will show a characteristic color, depending on the conditions of acidity. In Part I of this experiment, you will first test several unknown solutions of acids and bases and note some of their properties. The similarities of properties should enable you to write some operational definitions. Next, the identities of the unknown solutions will be revealed and you will attempt to write conceptual definitions of acids and bases. In the past, chemists proposed several conceptual definitions of acids and bases. Svante Arrhenius suggested what is certainly the most fundamental definition in the late 1800 s. Arrhenius, one of Sweden s most famous chemists, was awarded the Nobel Prize in chemistry in 1903 for his work with ionic solutions. It is quite likely that your conceptual definitions of acids and bases in this experiment will be similar to those of Arrhenius. Part II of this experiment involves acid-base tests of household products. From the results you obtain and the information provided, you will be asked to classify the household products according to your operational and conceptual definitions of acids and bases. Objectives 1. to become familiar with a variety of typical laboratory tests for acids and bases 2. to develop operational definitions of acids and bases 3. to develop conceptual definitions of acids and bases 4. to test a variety of household products and classify them as acids or bases Supplies 2 x 6 dropping pipets in two cups with solutions to be tested (to be filled at the teacher s desk, microwell plate and paper in sheet protector, safety goggles, scissors and tweezers for Mg and toothpicks for stirring Chemical Reagents set of 6 unknown solutions (acids or bases) labeled A F chemical indicators: phenolphthalein solution in a dropper bottle methyl orange solution in a dropper bottle red litmus paper blue litmus paper piece of magnesium ribbon, set of 6 household products (vinegar, oven cleaner, household ammonia, lemon juice, 7-up, milk of magnesia) 1

2 Safety Precaution: Remember, A to F are unknowns. Whether they are hazardous or not, it is always a good practice to minimize your contact with unknown chemicals. Some of these chemicals are corrosive to skin, eyes, and clothing. Wash away any spills or splashes with plenty of water. The phenolphthalein indicator used in this experiment is toxic and flammable. Make sure there are no open flames in the vicinity. PROCEDURE Part I: Tests of Unknown Solutions 1. Put on your lab apron and safety goggles. 2. Place 2 drops of each solution of the six unknown acids or bases the appropriate squares on your protected sheet and add 1 or 2 drops of phenolphthalein solution. Record your results in Table 1 below. 3. Repeat Step 2 using methyl orange solution instead of phenolphthalein. 4. Repeat Step 2, but tear off and add a small piece of red litmus paper instead of the indicator solution. 5. Repeat Step 2, but tear off and add a small piece of blue litmus paper instead of the indicator solution. 6. Repeat Step 2, but use 12-well plate instead and add a small (about 0.5 cm) strip of magnesium. Part II: Tests of Common Products Repeat Part I for the household products provided by your teacher. Record the observations in Table 2 in your notebook. Reagent disposal Use tweezers to remove unreacted magnesium and the litmus paper first to avoid plugging the drain and place them in the designated waste container. Rinse all chemicals from the microwells and protective sheet with plenty of water (away from the sheet opening to avoid getting water inside. Before leaving the laboratory, wash your hands thoroughly with soap and water. POST LAB CONSIDERATIONS You should be able to place each of the six unknown solutions into one of two groups, on the basis of Table 1. One property of acids is that they react with active metals such as magnesium. Knowing this, you will be able to determine which group is the acid group and which is the base group. In Part II, you may have noticed that in some instances some of the indicators did not react as you might have expected. For now, you can think of the indicators as having different sensitivities to acids and bases, the effect being that they will not always give positive results. As long as one of the tests showed a positive result and provided that you did not get any conflicting results, you can classify a solution on the basis of that one result. In order to develop conceptual definitions, you will require more information about the solutions tested. Your instructor will provide relevant information such as chemical formulas and electrical conductivity at an appropriate time. 2

3 Chemistry Lab Define Acids and Bases Objective: Develop operational and conceptual definitions of acids and bases [date] Experimental Results Table 1: Part I, Tests of Unknown Solutions Unknown solution A B C D E F Table 2: Part II, Tests of Common Products Products Vinegar Oven Cleaner Ammonia 7-up Lemon Juice Milk of Magnesia ANALYSIS OF RESULTS 1. Examine the data in Table 1 and form groups of solutions on the basis of similar properties. Classify one of these groups as acids and the other as bases. (A hint was provided in the Post Lab Considerations.) [Acids are ] [Bases are ] 2. Write operational definitions for acids and bases based on the results of Part I of this experiment. (how you can distinguish acids and bases solely from observations in the lab, color changes, bubbles etc.) [Acids : ] [Bases: ] 3

4 3. Using your results from Part II and your operational definition from Part I, classify the six household products as acids or bases. [acids are,, ] [bases are,, ] 4. Where are acids and bases typically found in household products? [cleaning, food, etc] [a) Acids are often used.. b) Bases are often ] Extension 5. The chemical formulas of your unknown acid and base solutions are listed below (but not in order!!). Sort them into two list classifying them as a) acids or b) bases. HNO 3, H 2 SO 4, NH 3, HCl, NaOH, Ca(OH) 2 6. Write conceptual definitions of acids and bases. ( hydronium ions etc. ) [Acids: ] [Bases: ] 7. The chemical formulas of the active ingredients in your household products are listed below (not in order). Group them as acids or bases. HC 2 H 3 O 2, H 3 C 6 H 5 O 7, NH 3, H 2 CO 3, NaOH, Mg(OH) 2 [Acids:,, ] [Bases:,, ] 8. Acid rain contains varying amounts of sulfuric acid and/or nitric acid. a) Predict the test results you might expect if you were to test a sample of acid rain in this experiment. Acid rain Sulfuric acid Nitric acid b) What have the chemical formulas of the acids HNO 3 and H 2 SO 4 in common? [ ] c) Which formula (HNO 3 or H 2 SO 4 ) is sulfuric acid and which is nitric acid? [The formula for sulfuric acid is ] [The formula for nitric acid is ] 4

5 Use 2 drops of solution to be tested and one drop of indicator or a small piece of indicator paper (Litmus) Record your results in the data tables of your lab worksheets To clean: Remove pieces of Litmus paper with tweezers first. Then rinse protective sheet with water without getting water or solutions inside!!! Keep the paper dry!!! Solution to be tested A B C D E F Vinegar Oven Cleaner Easy-Off Ammonia 7-up Lemon Juice Milk of Magnesia 5