Toxins 4/27/2010. Acids and Bases Lab. IV-17 to IV-22

Transcription

1 Toxins IV-17 to IV-22 Countless products are advertised on TV with the promise of reducing acid indigestion. a.what is acid indigestion? b.what does acid have to do with your stomach? c.how do you think antacids work? Acids and Bases Lab Prepare for the Lab Indicator: An indicator is a molecular substance that changes color when it comes into contact with an acid or a base. 1. You will test one set of each solution with cabbage juice indicator and the universal indicator. Acids and bases are corrosive. Do not get solutions on skin. In case of a spill, rinse thoroughly with water. Wear safety goggles at all times. 2. In Part 2 test the solutions with solid calcium carbonate (base) Fill Out Class Performance Sheet ph Scale Acids and bases respond to indicators by changing color. 1

2 ph numbers are associated with specific indicator colors. Acids have ph values between 0 and 7 Bases have ph values between 7 and 14. Neutral solutions have ph of about 7. An unknown substance is purple with cabbage juice and does not react with sodium bicarbonate (NaHCO 3 ). Is it an acid, base or neutral substance? Explain your reasoning. Which of these four solutions conduct electricity: 0.10 M HCl (hydrochloric acid), 0.10 M CH 3 COOH (acetic acid), 0.10 M NaCl (sodium chloride), 0.10 M C 12 H 22 O 11 (sugar)? Explain. Acid-Base Solution Cards CHOOH Formic Acid These are all aqueous solutions, however water is not shown. Acids are substances that add H+ to solution. Bases are substances that add OH- to solution. Which substances do you expect will conduct electricity: hydrocyanic acid, HCN; magnesium hydroxide, Mg(OH) 2 ; methanol, CH 3 OH? Explain your thinking. Neutral substances do not add H+ or OH- to solution. 2

3 The definitions of acids and bases have changed over time. Arrhenius Definition of Acids and Bases: An acid is any substance that adds hydrogen ion (H+) to solution. A base is any substance that adds hydroxide ion (OH-) to solution. Important: Not all compounds that have OH in them are bases For example: CH 3 COOH is acetic acid (vinegar) Because acids and bases add ions to water, they are both electrolytes and will conduct electricity. Brønsted-Lowry Definition of Acids and Bases: An acid is a proton donor. A base is a proton acceptor. Example of a base as a proton acceptor: NH 3 + H 2 0 NH 4+ + OH - Acids and bases that break apart (dissociate) completely in solution are called strong acids and strong bases. Acids and bases that do not dissociate completely in solution are called weak acids and weak bases. What does ph measure? Pure water has an H + concentration of 1.0 x 10 7 M and an OH of 1.0 X 10 7 M. The ph of a solution is determined by: The molarity of the solution AND The identity of the solution (acid, base or neutral) What does this mean? What patterns do you see in the data? Molarity [H + ] [H + ] ph poh Hydrochloric acid, HCl acid, base, neutral? 0.10 M HCl 0.10 mol/l 1 x 10-1 mol/l 1 13 acid M HCl mol/l 1 x 10-2 mol/l 2 12 acid M HCl mol/l 1 x 10-3 mol/l 3 11 acid Salt, NaCl 1.0 M NaCl mol/l 1 x 10-7 mol/l 7 7 neutral 0.10 M NaCl mol/l 1 x 10-7 mol/l 7 7 neutral Sodium hydroxide, NaOH NaOH mol/l 1 x mol/l 11 3 base You can think about ph as the power of hydrogen. ph measures the concentration of H+ ions in the solution, [H+] ph = [H+] M NaOH mol/l 1 x mol/l 12 2 base 0.10 M NaOH mol/l 1 x mol/l 13 1 base 3

4 poh is the concentration of OH- ions in solution. poh = [OH-] The ph of water is 7. Water is considered neutral because the number of H+ is equal to the number of OH-. [H+] = [OH-] ph + poh = 14 Water is neutral. Neutral: [H+] = [OH-] The ph of water is 7, so [H+] =7 and [OH-] =7. ph plus poh adds to 14. Water is neutral. Neutral: [H+] = [OH-] Water has: 1.0 x 10 7 moles of H+ ions per liter of water or moles of H+ ions per liter of water and 1.0 x 10 7 moles/l of OH- or moles OH- per liter of water. Acids tend to dissociate into H+ ions in solution. Example: HCl (aq) H + + Cl Bases tend to dissociate into OH- ions in solution. Example: NaOH (aq) Na + + OH 4

5 The ph number of a solution tells you the concentration of H+ ions in solution. A ph of 1 corresponds to a H+ concentration of 0.1 moles/l. [ph of 1= 0.1 M H+] Solutions that are more acidic have more H+ ions. Solutions that are more basic have less H+ ions. Neutral ph = 7 poh =7 [H+] = 1.0 x 10 7 moles/l = M [OH-] = 1.0 x 10 7 moles/l = M ph plus poh adds to 14 ph + poh = 14 pure water: ph=7.0 poh=7.0 H+ OH - Acidic ph = 2 poh =12 [H+] = 1.0 x 10 2 moles/l = M [OH-] = 1.0 x moles/l = M Flask 6: M HCl ph = 2 poh=12 Basic Example: M NaOH ph = 12 [H+] =? poh =? [OH-] =? Flask 6: M NaOH ph= H+ Cl HCl Answer: M NaOH ph = 12 [H+] = 1.0 X moles/l or M poh = 2 [OH-] = 1.0 X 10-2 moles/l or 0.01 M 6 Flask 6: M NaOH ph=12 H + concentration OH concentration 1.0 X 10 0 M 1.0 X M X 10 1 M 1.0 X M X 10 2 M 1.0 X M X 10 3 M 1.0 X M X 10 4 M 1.0 X M X 10 5 M 1.0 X 10 9 M X 10 6 M 1.0 X 10 8 M X 10 7 M 1.0 X 10 7 M X 10 8 M 1.0 X 10 6 M X 10 9 M 1.0 X 10 5 M 9 ph 1.0 X M 1.0 X 10 0 M 14 More acidic neutral More basic 5

6 If you know the ph of a solution, what else do you know? IV-21: Neutral Territory Neutralization Reactions Milk of Magnesia, Mg(OH) 2, can reduce excess stomach acid, HCl. 1) What do you think happens when you add an acid together with a base? 2) What products do you think are produced when Mg(OH) 2 and HCl are mixed? Write the chemical equation. Prepare for the Lab Wear safety goggles at all times. Acids and bases are corrosive. Do not get any on skin or near eyes. In case of a spill, rinse with large amounts of water. Neutralization reaction: Acid + Base salt + water (neutral) An acid and a base form an ionic salt and water When acids and bases are mixed, the ph of the product approaches 7. HNO 3 (aq) + NaOH (aq) NaNO 3 (aq) + H 2 O (l) HNO 3 (aq) + NaOH (aq) NaNO 3 (aq) + H 2 O (l) 6

7 A neutralization reaction is a reaction in which an acid and a base react in aqueous solution to produce a salt and water. Acid + Base salt + water (neutral) Sulfuric acid, H 2 SO 4, reacts with magnesium hydroxide, Mg(OH) 2. Write a balanced equation for the reaction that occurs. HNO 3 (aq) + NaOH (aq) NaNO 3 (aq) + H 2 O (l) A student mixes 100 ml of 0.10 M HCl with different volumes of 0.10 M NaOH. 1: 100 ml of 0.10 M HCl + 50 ml of 0.10 M NaOH 2: 100 ml of 0.10 M HCl ml of 0.10 M NaOH 3: 100 ml of 0.10 M HCl ml of 0.10 M NaOH a. Which solution is the least toxic? b. Are the final solutions acid, basic, or neutral? How much base do you need to neutralize an acid? A titration is a procedure in which a neutralization reaction is monitored with an indicator allowing you to calculate the unknown concentration of an acid or base. When the endpoint is reached in a titration, the number of moles of H+ is equal to the number of moles of OH-. moles H+ = moles OH- How much base do you need to neutralize an acid? When the endpoint is reached in a titration, the number of moles of H+ is equal to the number of moles of OH-. The equation that assists you in completing titration calculations is MV acid = MV base moles acid (H+) = moles base (OH-) moles H+ = moles OH- Don t forget how to find moles: M = moles / V Or MV = moles 7

8 How much base do you need to neutralize an acid? to neutralize ml of sodium hydroxide, NaOH, what is the molarity of the NaOH? Practice Problem: Acid Base Acid? Volume? Molarity? Moles? In liters? Base? Volume? Molarity? Moles? In liters? Practice Problem: to neutralize ml of sodium hydroxide, NaOH, what is How much base do you need to neutralize an acid? Take 2: Acid Base moles = MV Acid? Volume? Molarity? Moles? In liters? Base? Volume? Molarity? Moles? In liters? M H+ = 0.50 moles/liter V H+ = L moles HNO 3 =? How much base do you need to neutralize an acid? Take 2: moles = MV M H+ = 0.50 moles/liter V H+ = L moles HNO 3 =? Take 2: to neutralize ml of sodium hydroxide, NaOH, what is moles = MV M H+ = 0.50 M V H+ = L mole H+ = moles HNO 3 At the endpoint, the moles of acid = the moles of the base M OH- =? V OH- = L moles NaOH = Answer: O.0125 moles nitric acid 8

9 ACID M = 0.50 M V = L mol= mol BASE M = 0.20 M L = L mol= mol acid mol = mol M x V = mol base mol = mol M x V = mol mol acid = mol base MV acid = MV base Practice Problem: Practice Problem: Answer: MV = MV (0.5 M HNO 3 )(25 ml) = (? M NaOH)(62 ml) Answer: MV = MV (0.5 M HNO 3 )(25 ml) = (? M NaOH)(62 ml) 0.2 M NaOH = M Practice Problem 3: You place 30 drops of HCl in a beaker with 1 drop of indicator. After adding 50 drops of 0.50 M NaOH the color is faint pink. What is the concentration of HCl? Show your work and explain your reasoning. Practice Problem 3: You place 30 drops of HCl in a beaker with 1 drop of indicator. After adding 50 drops of 0.50 M NaOH the color is faint pink. What is the concentration of HCl? Show your work and explain your reasoning. Answer: MV = MV (?M) (30 drops) = (0.50 M)(50 drops) M=0.83 M HCl 9

10 You place 50 ml of HCL in a beaker with 1 drop of indicator. After adding 100 ml of 0.10 M NaOH the color is faint pink. What is the concentration of HCl? Show your work. 10