Name: Block: Date: Student Notes

Transcription

1 Name: Block: Date: LCPS Core Experience Acids and Bases Student Notes OBJECTIVES Students will: recognize some acids and bases as common and familiar household chemicals. realize that acids and bases are not necessarily strong or dangerous. determine the ph of different chemical compounds and categorize them as acids or bases. investigate how the difference between acids and bases correlates to the difference in hydrogen ion concentration of solutions of the two classes of compounds. LINK 1. Are all acids and bases strong or dangerous? 2. What does chemically reactive mean? Materials Per Lab Group 1 test tube rack 8 15 ml falcon tubes ph paper 1 pair of forceps 5 ml acetic acid solution (vinegar) 5 ml 1M sulfuric acid (automobile battery acid) 5 ml 0.01 M hydrochloric acid (stomach acid) 5 ml citric acid (lemon juice) 5 ml carbonic acid solution (carbonated soft drink) 5 ml 0.1M sodium hydroxide solution (drain cleaner) 5 ml ammonium hydroxide solution (window cleaner) 5 ml magnesium hydroxide solution (antacid medication) 1 pair of goggles per student 1 lab apron per student Page 1

2 LCPS Core Experience Background Acids are a group of chemical compounds that produce hydrogen ions (H + ) when mixed with water. Ions are atoms or groups of atoms that have either a positive or a negative charge. Acids are compounds that produce hydrogen ions (H + ) when the acid is mixed with water. The chemical properties of acids result from the production of hydrogen ions (H + ) which are very reactive. For example, hydrochloric acid (HCl), the acid that helps food digestion in the stomach, produces hydrogen ions (H + ) according to the following equation: HCl H + + Cl Hydrochloric Hydrogen Chloride acid ion ion Notice that producing a hydrogen ion (H + ) also produces a negative ion, the chloride (Cl ) ion. The amount of anything dissolved in a solution is called its concentration. The concentration of hydrogen ions (H + ) depends on the amount of acid dissolved. A large amount of dissolved acid results in a large number of hydrogen ions (H + ) in the water. The concentration of hydrogen ions (H + ) also depends on the acid. Each molecule of the strong acid hydrochloric acid (HCl) separates into one hydrogen ion (H + ) and a negative chloride (Cl ) ion. Each molecule of acetic acid, the weak acid found in vinegar, does not completely separate into a hydrogen ion (H + ) and a negative ion like hydrochloric acid. This results in a lower concentration of hydrogen ions. (H + ) The ph scale allows a scientist to measure the concentration of hydrogen ions (H + ). This scale starts at zero which represents a high concentration of hydrogen ions (H + ). The scale extends to 14, which represents a low concentration of hydrogen ions (H + ). For example, a solution with a ph of 0 has a higher concentration of hydrogen ions (H + ) than a solution with a ph of 14. A solution with a ph of 4 has a lower concentration of hydrogen ions (H + ) than a solution with a ph of 3. Special indicator paper is used to measure the ph of a solution. ph paper has several different chemical compounds added to it that change color when the concentration of hydrogen ions (H + ) is low or high. Page 2

3 LCPS Core Experience High H + Ion Concentration Low Strong Acid Neutral ph Scale Strong Base STOP AND DISCUSS EXPERIMENT 1. In this lesson, you will determine the ph of several different solutions that contain acids. Lesson One: Observing the ph of Acidic Solutions 2. Obtain the test tube rack labeled Lesson One. 3. Using forceps grasp the top of a ph paper and dip it into the vinegar. Remove the ph paper strip. 4. Match the color(s) of the ph paper with the pictures on the box. Estimate the ph of the vinegar that most closely corresponds to the color(s) in the pictures. Estimate the ph to the nearest half of a ph unit, for example, 3.0, 3.5 or 4.0. Record the ph value in Table A. 5. Repeat steps 1 through 3 for each of the other acids. Table A: The ph of Various Solutions Solution Compound Compound Name Formula Vinegar Acetic acid H(C2H3O2) Automobile Battery Acid Sulfuric acid H2(SO4) Stomach Acid Hydrochloric acid HCl ph of Solution Lemon Juice Citric acid H2(C6H7O7) Carbonated Drink Carbonic acid H2(CO3) 6. Question: Which of the acids that were tested are familiar to you? Page 3

4 LCPS Core Experience 7. Question: Using the data from Table A, write the names of the acids in order from lowest to highest ph. Use the ph Scale from the Background. 8. Question: What does the order of the acids tell you about the concentration of H + ions in each solution of acid? (Hint: Use the ph scale for help). 9. Question: Which atom is present in the chemical formula of each acid? Background Bases are a second group of chemical compounds that produce ions when mixed with water. When a base is mixed with water, it produces hydroxide ions (OH ) from the paired OH atoms plus a positively charged ion. For example, sodium hydroxide (NaOH), the base in drain cleaner, produces hydroxide ions (OH ) according to the following equation: NaOH Na + + OH Sodium hydroxide Sodium ion Hydroxide ion When a base and an acid are mixed with water, hydroxide ions (OH ) react with hydrogen ions (H + ) to form water which removes hydrogen ions (H + ) from the solution. Sodium hydroxide (NaOH) reacts with hydrochloric acid (HCl) according to the following equation: Na + + OH + H + + Cl H2O + Na + + Cl Sodium Hydroxide Hydrogen Chloride Water Sodium Chloride ion ion ion ion molecule ion ion Hydrogen ions (H + ) are not a product of the reaction showing that they are removed from the solution and their concentration is decreased. Notice that the other products are water (H2O) and Na + and Cl ions. Page 4

5 LCPS Core Experience Background (continued) The concentration of hydrogen ions (H + ) also depends on which base is in solution. A strong base like sodium hydroxide always reacts with hydrogen ions (H + ). The weaker base, magnesium hydroxide, Mg(OH)2, that is found in antacid medications, does not always react with hydrogen ions (H + ) leaving some unreacted base. The ph scale can also be used to measure the concentration of hydrogen ions (H + ) in a solution that contains a base. A weak base cannot remove many hydrogen ions (H +) from a solution, so the concentration of hydrogen ions (H + ) will be higher and the ph will be lower. A strong base will remove many more hydrogen ions (H +) from a solution, so the concentration of hydrogen ions (H + ) will be lower and the ph will be higher. Refer to the ph scale earlier in the investigation. Lesson Two: Observing the ph of Basic Solutions 1. In this Lesson, you will determine the ph of several different solutions that contain bases. 2. Obtain a test tube rack labeled Lesson Two. STOP AND DISCUSS 3. Using forceps grasp the top of the ph paper strip and dip it in the drain cleaner. Remove the ph paper strip. 4. Match the color(s) of the ph paper strip with the pictures on the box. Estimate the ph of the drain cleaner that most closely corresponds to the color in the pictures. Estimate the ph to the nearest half of a ph unit, for example, 3.0, 3.5 or 4.0. Record the ph value in Table B. 5. Repeat steps 1 through 3 for each of the other bases. Table B: The ph of Various Solutions Solution Chemical Chemical Compound Formula Drain Cleaner Sodium hydroxide NaOH Window Cleaner Ammonium hydroxide NH4OH ph of Solution Antacid Medication Magnesium hydroxide Mg(OH)2 Page 5

6 LCPS Core Experience 6. Question: Which of the bases that were tested are familiar to you? 7. Question: Write the names of the bases in order from lowest to highest ph. Use the ph Scale from the Background. 8. Question: What does the order of the bases tell you about the concentration of hydrogen ions (H + ) in each solution of base? 9. Question: Which two elements are present in the chemical formula of each base? LEARNING REVIEW List three things you have learned about acids, bases, hydrogen ion concentration and ph from this investigation. a. b. c. STOP AND DISCUSS Page 6

7 LCPS Core Experience EVALUATION 1. Why are the ph values of the substances in Lesson One all in the acidic range? What happened to the H + ion concentration in this lesson? 2. Why are the ph values of the substances in Lesson Two all in the basic range? What happened to the H + ion concentration in this lesson? 3. What is the relationship between a low ph and the hydrogen ion (H + ) concentration? 4. What is the relationship between a high ph and the hydrogen ion (H + ) concentration? 5. What properties make acids and bases chemically reactive? Page 7

8 LCPS Core Experience 6. What is the relationship between the concentration of hydrogen ions (H + ) in a solution and the chemical reactivity of strong acids and weak acids? 7. You are a lifeguard at a community pool and take a ph test of the pool water. The ph reading is 8. Explain what you would add to the water so that the ph reading is neutral? Page 8