1. Acids and Bases

2. What are acids? Acids are molecular substances that ionize when added to water. On a related matter, what is the relationship between ionization and conductivity? We can speak about strong acids, that are completely ionized, and weak acids, that are only slightly ionized. Strong acids are strong electrolytes, weak acids are weak electrolytes.

3. What are acids? Acids react with chemically active metals to produce hydrogen gas. We saw this last semester when we studied the reaction of zinc and hydrochloric acid: Zn(s) + 2HCl(aq) ↔ ZnCl 2 (aq) + H 2 (g) Na, Mg, Al, are other examples of metals that do this. What are some of the unreactive metals?

4. What are acids? Acids affect the colors of acid base indicators. Indicators are very useful chemical dyes that have one color in acid solutions and another in basic solutions. Acids neutralize bases. During neutralization, acids react with bases to form salts. We will cover this in detail later.

5. What are acids? Dilute acids taste sour. Acids are an important part of our diet. Citrus fruit – citric acid Wine – tartaric acid Any product with vinegar – acetic acid Sour milk

6. What are bases? Bases, like acids, are electrolytes – they ionize in water. Consider sodium hydroxide (lye): NaOH(s) → Na + (aq) + OH - (aq) Just as with acids, bases that ionize completely are strong electrolytes, bases that don’t are much weaker electrolytes.

7. What are bases? Bases cause indicators to change color. As with acids, bases affect the color of many dyes. We will use a number of indicators in future labs. Bases neutralize acids, forming salts. For example, NaOH(aq) + HCl(aq) → NaCl(aq) + H 2 O(aq)

8. What are bases? Basic solutions feel slippery and have a bitter taste. However, basic solutions should not be tasted – many are corrosive and poisonous. Concentrated solutions of bases can cause blindness!

9. Common Acids and Bases

10. Which substance is an electrolyte? C 2 H 5 OH C 6 H 12 O 6 C 12 H 22 O 11 CH 3 COOH

11. An example of a nonelectrolyte is C 6 H 12 O 6 (aq) K 2 SO 4 (aq) NaCl(aq) HCl(aq)

12. Ionization of Water Now that we have covered some of the properties of acids and bases, let us go back to consider some of the properties of water, specifically, its ability to ionize.

13. This process is called self-ionization, and occurs naturally even if no acid or base is present. This can also be written as a dissociation:

14. Self-Ionization of Water The continuous motion of water molecules insures that this reaction takes place occasionally. Why? water molecules are highly polar, so oxygen atoms are constantly attracting other hydrogens (H-bonding) some water molecules have sufficient kinetic energy for hydrogen ion transfer

15. How common is the self ionization of water? Not very. At room temperature, approximately 1 molecule in 10 million ionizes at any given moment! And of course, at equilibrium, for every molecule that ionizes, another is being formed from hydronium and hydroxide ions.

16. Self-Ionization of Water For pure water at 25ºC, [H + ]=[OH - ]=1x10 -7 mol/liter for pure water, hydroxide ion and hydrogen ion concentrations must be equal. Any aqueous solution where hydroxide ion and hydrogen ion concentrations are equal are called neutral solutions. [H + ] and [OH - ] are interdependent. If one increases, the other decreases – Le Châtelier’s principle at work!

17. pH Measurement of acidity and alkalinity of solutions is so ubiquitous that a shorthand method is used to express hydrogen ion concentration instead of molarity. [H + ] is measured using the pH scale, first proposed in 1909 by Søren Sørenson: pH=-log[H + ] Consider the pH of a neutral solution where [H = ]=1x10 =7 mol/L: pH=-log(1x10 -7 mol/L)=-(log 1 + log 10 -7 )=-(0+(-7))=7

18. pH14131211109876543210 [H + ]10 -14 10 -13 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 According to the chart, neutral solutions have pH around 7 acidic solutions have pH less than 7 basic solutions have pH greater than 7 it is possible to have negative pH values for very strongly acidic solutions

19. Here are the pH for a number of substances you should be familiar with: SubstancepH Stomach acid1.6-1.8 Lemon Juice, Vinegar2.3-3.0 Soda3.8 Coffee5.0 Milk6.3-6.6 Blood7.4 Seawater, NaHCO 3 8.3 Ammonia cleanser11.5

20. An acidic solution could have a pH of 7 10 3 14

21. Which of these 1 M solutions will have the highest pH? NaOH CH 3 OH HCl NaCl

22. pH is a log scale! Solution A has a pH of 3 and solution Z has a pH of 6. How many times greater is the hydronium ion concentration in solution A than the hydronium ion concentration in solution Z? 100 2 3 1000

23. Indicator Lab Most indicators are complicated compounds produced by plants. They are weak acids that exist mainly as molecules with a small degree of ionization. For simplicity, the symbol HIn will be used to represent the molecular form of the indicator and In - to represent the negative ion of all indicators.

24. How Indicators Work The ionization of indicator molecules can be written as an equilibrium equation: The addition of acid will shift the equilibrium to the left according to LeChatelier’s Principle, revealing the acid color of the indicator. The addition of base (OH - ) will remove hydrogen ion (H + ) from the system.

25. The equilibrium will shift to the right to replace it, revealing the color of the indicator In -. Each acid indicator will shift below pH 7 and a basic indicator will shift above pH 7. Some indicators have an acid color, a basic color, and a transition color between the two. You will first determine if the indicators been provided to you are acid indicators or base indicators. With your new-found knowledge, you will test four products that are well known to you to determine their approximate pH.

26. Indicator Reference Table

27. Which indicator is yellow in a solution with a pH of 9.8? methyl orange bromthymol blue bromcresol green thymol blue

28. Which solution would turn blue litmus red? HCl(aq) NaCl(aq) K 2 CO 3 (aq) NaOH(aq)

29. Which solution will change litmus from blue to red? NaOH(aq) NH 4 OH(aq) CH 3 OH(aq) CH 3 COOH(aq)

30. Procedure Put 3-5 drops of each pH solution into consecutive well plates. Add a few drops of universal indicator, indicate the change in color, if any, on data table 1. Choose 3 others from the following: bromothymol blue, bromocresol green, litmus, thymol blue methyl orange, phenolphthalein Each lab station should have someone test all of these indicators and compare results. Look carefully at your results so far. Determine whether your indicators are acid indicators, base indicators, or both (universal) and write indicate this at the bottom of data table 1. Test the four household products by placing a few drops of each in a well, followed by the indicator. Estimate the pH of the product and fill in data table 2.

31. pHI1I2I3I4I5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Acid/Base Indicator?

32. ProductIndicator Used, color pH estimate Vinegar Lemon Juice Ammonia Windex

33. Answer these questions on a separate sheet of paper using several complete sentences. What are indicators? How do indicators work? Why is it helpful to use more than one indicator when trying to determine the pH of an unknown solution? What are some possible sources of error for this experiment?

34. When phenolphthalein indicator is added to a colorless solution with a pH of 10, a student observes and concludes that the tested solution remains colorless and is basic remains colorless and is acidic turns pink and is basic turns pink and is acidic