Acids and Bases 4

1. Acids and Bases 4 Boon Chemistry January 22& 23, 2013

2. Catalyst • Take out your homework please. Apples and other produce contain an enzyme (called polyphenoloxidase) that reacts with oxygen and iron-containing phenols that are also found in the apple. The oxidation reaction forms a sort of rust on the surface of the fruit. You see the browning when the fruit is cut because these actions damage the cells in the fruit, allowing oxygen in the air to react with the enzyme. The reaction can be slowed or prevented by inactivating the enzyme with heat, reducing the pH on the surface of the fruit, reducing the amount of available oxygen, or by adding certain preservative chemicals. Question:How does reducing the pH of an apple’s surface affect the rate at which it turns brown? • Objectives • I can calculate pH from hydronium ion or hydroxide ion concentration. • I can work cooperatively and efficiently in a group. • Agenda • Catalyst andHomework Review • Acid/Base Stations • ExitSlip: acid/base calculations

3. Exit Slip: Answers Standard 5c • How can two acids, 1M sulfuric acid and 1M citric acid, exhibit different electrical conductivities even though they have the same concentration? • (a) Sulfate ions are smaller, so they carry more electrical charge. • (b) Citric acid dissociates only partially, so there are fewer ions in solution. • (c) Citric acid does not dissolve completely in water. • (d) Citric acid is a nonionic compound, so it has low conductivity. • Why? If the concentrations are the same, one must dissociate less producing less ions to conduct electricity. • When rubidium hydroxide (RbOH) is added to water, it almost completely dissociates into Rb+ and OH– ions. What type of substance is rubidium hydroxide? • (a) weak acid (c) weak base • (b) strong acid(d) strong base • Why? Bases form hydroxide ions (OH-). So it must be a base. “Almost completely dissociates” infers its strong. Students who picked “C” got ½ credit because the word “almost” is misleading.

4. Exit Slip answers continued What is a difference between a strong base and a weak base? (a) A weak base dissociates less completely than a strong base. (b) A weak base dissociates more completely than a strong base. (c) A weak base produces hydrogen ions, while a strong base produces hydroxide ions. (d) A weak base produces hydroxide ions, while a strong base produces hydrogen ions. Why? Strong bases dissociate completely while weak bases do not. All bases produce hydroxide ions and accept a proton. What evidence best indicates that sulfuric acid (H2SO4) is a strong acid? (a) Sulfuric acid completely dissociates by donating hydrogen ions (b) Sulfuric acid partially dissociates by donating hydrogen ions (c) Sulfuric acid completely dissociates by forming hydroxide ions (d) Sulfuric acid partially dissociates by forming hydroxide ions Why? All acids donate a hydrogen ion. They do not produce hydroxide. Strong acids dissociate completely while weak acids dissociate partially.

5. Exit Slip Review Standard 5d • A solution that is mildly acidic would have a pH of approximately • (a) 2. (c) 4. • (b) 6. (d) 8. • Why? “mildly” means “a little bit” or “slightly”. pH of 6 is the closest to neutral. • The label on a bottle indicates that the substance inside has a pH of 13. This tells you that the substance is • (a) neutral. (c) strongly acidic. • (b) mildly basic. (d) strongly basic. • Why? pH 13 is very basic. As pH increases, the solution becomes more basic.

6. Exit Slip Review Continued • A solution has a pOH of 4. What is the pH of the solution? • (a) 0 (c) 4 • (b) 10 (d) 14 • Why? pH + pOH = 14. If pOH is 4, pH must be 10. • A solution with a pH of 8 is classified as… • (a) strong acid (c) strong base • (b) weak base (d) weak acid • Why? pH above 7 is basic and 8 is very close to 7. The best answer is “weak base”. • A solution with a pH of 3 has 100or 102times the hydronium ions [H3O+] as a solution with a pH of 5. • Why? 0.001 is 100 times larger than 0.00001. Students got ½ credit for 10-2.

7. Homework Review page 547 #1-5, pp. 567 # 24, 25, 26 1) The relationship between the hydronium ion concentration and hydroxide ion concentration in a solution is that as one increases the other decreases proportionally. This is due to the fact that [H3O+][OH-] = kw= 1.0 x 10-14 at 25 degrees Celsius. 2) pH measures the acidity of a solution. It is defined as: pH = -log[H3O+]. Therefore, acidic solutions have low pH and basic solutions have high pH. 3) A neutral solution has a pH of 7. The concentration of hydronium ions and hydroxide ions in a neutral solution are equal. 4) [H3O+][OH-] = kw= 1.0 x 10-14 at 25 degrees Celsius and pH = -log[H3O+] 5) Pancreatic juice has a pH of 7.9. This makes it slightly basic because substances with a pH over 7 are basic. • 24. The self-ionization of water: • H2O + H2O çèH3O+ + OH- • 25. pH = 3 is acidic; pH= 7 is neutral; pH = 11 is basic • 26. The concentration of OH- changes by a factor of 1000 when pH increases by 3, by a factor of 100 when pH increases by 2, by a factor of 10 when pH increases by 1, and by a factor of 3.16 when pH increases by 0.5. This is because pH is a logarithmic scale.

8. Acid/Base Stations • Expectations: • You must work record all your work on your own sheet of paper. • You must cooperate with your table group mates to share materials and work quickly but carefully. • You may speak quietly with your group members about your work. • You will have about 20 minutes at each station. If you finish early, work on any acid/base worksheet, HW, or DWO challenge material. • The Stations: • Station 1: Antacids • Station 2: Acid/Base Calculations • Station 3: Chemistry and Cooking • Station 4: Independent Work Station/Vocabulary Review

9. Exit Slip • Expectations: • You will work silently and independently. • When you are done, cover your exit slip with your handouts. • Tools: • You may use all your notes, worksheets, and flash cards. • You may use your own calculator. • What do I turn in? • You will turn in your exit slip only. • Homework: • Read pp. 539-547 pp. 544 #1-4 and pp. 545 #1-4 • Complete any worksheets or article questions that you have not finished.

10. Homework • Due Next Class: Read pp. 539-547 pp. 544 #1-4 and pp. 545 #1-4 • Due Thursday/Friday: Complete any worksheets or article questions that you have not finished. • Practice your vocabulary with flashcards! • Exam next class. • Bring more index cards for Unit 8 flashcards.