1 CHAPTER 13 Acids & Bases

2 Properties of Aqueous Solutions of Acids & Bases n Acidic properties  taste sour  change the colors of indicators  turn litmus red  react with metals to generate H 2(g)  react with metal oxides and hydroxides to form salts and water  aqueous solutions conduct electricity

3 Properties of Aqueous Solutions of Acids & Bases n Basic properties  taste bitter  feel slippery  change colors of indicators  turn litmus blue  react with acids to form salts and water  aqueous solutions conduct electricity

4 Strong Electrolytes n Strong electrolytes ionize or dissociate completely n Three classes of strong electrolytes 1 Strong Acids

5 Strong Electrolytes 2 Strong Soluble Bases

6 Strong Electrolytes 3 Most Soluble Salts

7 Strong Electrolytes n Calculation of concentrations of ions in solution of strong electrolytes is easy n Example: Calculate the concentrations of ions in M nitric acid, HNO 3.

8 Strong Electrolytes n Calculation of concentrations of ions in solution of strong electrolytes is easy n Example: Calculate the concentrations of ions in M nitric acid, HNO 3.

9 Arrhenius Theory n Svante Augustus Arrhenius n acids generate H + in aqueous solutions

10 Arrhenius Theory n bases generate OH - in aqueous solutions

11 Arrhenius Theory n neutralization - combination of H + (or H 3 O + ) with OH - n strong acids - ionize 100% in water HCl, HBr, HI, H 2 SO 4, HNO 3, HClO 4, HClO 3 n strong bases - ionize 100% in water LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH) 2, Sr(OH) 2, Ba(OH) 2

12 Arrhenius Theory n total ionic equation for strong acid with strong base n net ionic equation for strong acid with strong base

13 Bronsted-Lowry Acid-Base Theory n acids - proton (H + ) donor

14 Bronsted-Lowry Acid-Base Theory n bases - proton (H + ) acceptor

15 Bronsted-Lowry Acid-Base Theory n acid-base reactions are proton transfer reactions –note that we are often making coordinate covalent bonds

16 Bronsted-Lowry Acid-Base Theory n conjugate acid-base pairs –species that differ by a proton HNO 3 + H 2 O  H 3 O + + NO 3 - HNO 3 - acid 1 NO base 1 H 2 O - base 2 H 3 O + - acid 2 HF + H 2 O  H 3 O + + F - HF - acid 1 F - - base 1 H 2 O - base 2 H 3 O + - acid 2

17 Bronsted-Lowry Acid-Base Theory n differences between Arrhenius & Bronsted- Lowry theories  reaction does not have to occur in an aqueous solution  bases do not have to be hydroxides n for example- ammonia is not a hydroxide

18 Bronsted-Lowry Acid-Base Theory n weak acids have strong conjugate bases n weak bases have strong conjugate acids n primary reason they are weak acids or bases n strong conjugates recombine to form the original species

19 Bronsted-Lowry Acid-Base Theory NH 4 + must be a strong acid-it gives up H + to reform NH 3 NaOH  Na + (aq) + OH - (aq) Na + must be a weak acid or it would recombine to form NaOH remember NaOH ionizes 100%

20 Bronsted-Lowry Acid-Base Theory n amines are weak bases that behave like ammonia n amines have organic groups attached to -NH 2 group

21 Bronsted-Lowry Acid-Base Theory n water can be either an acid or base in Bronsted-Lowry theory n amphoteric - species that can be either an acid or base n amphiprotic - proton transfer reactions that species behave as either an acid or base

22 The Auto-Ionization of Water n Pure water ionizes very slightly –less than one-millionth molar

23 The Auto-Ionization of Water n Because the activity of pure water is 1, the equilibrium constant for this reaction is

24 The Auto-Ionization of Water n Experimental measurements have determined that the concentration of each ion is 1.0 x M at 25 0 C

25 The Auto-Ionization of Water n This particular equilibrium constant is called the ion-product for water, K w.

26 The pH and pOH scales n A convenient way to express acidity and basicity n pH is defined as

27 The pH and pOH scales n In general, a lower case p before a symbol is read as ‘negative logarithm of” the symbol

28 The pH and pOH scales n If we know either [H 3 O + ] or [OH - ], then we can calculate pH and pOH. n Example: Calculate the pH of a solution in which the [H 3 O + ] =0.030M.

29 The pH and pOH scales n If we know either [H 3 O + ] or [OH - ], then we can calculate pH and pOH. n Example: Calculate the pH of a solution in which the [H 3 O + ] =0.030M.

30 The pH and pOH scales n A convenient relationship between pH and pOH may be derived for all dilute aqueous solutions at 25 0 C.

31 The pH and pOH scales Remember these two expressions!!

32 The pH and pOH scales n The usual range for the pH scale is n and for pOH the scale is

33 The pH and pOH scales

34 Strengths of Acids n Binary Acids n acid strength increases with decreasing bond strength n hydrogen halides n bond strength HF>>HCl>HBr>HI n acid strength HF<<HCl<HBr<HI

35 Strengths of Acids

36 Strengths of Acids n VIA hydrides n bond strength H 2 O>> H 2 S> H 2 Se> H 2 Te n acid strength H 2 O<< H 2 S< H 2 Se< H 2 Te

37 Strengths of Acids n for HBr in water HBr + H 2 O  H 3 O + + Br - HBr + H 2 O  H 3 O + + Br - essentially 100% n can only distinguish acid strength differences of strong acids in nonaqueous solutions like acetic acid

38 Strengths of Acids n Acid Conjugate Base n Strongest acid Weakest base n HClO 4 ClO 4 - n H + (H 3 O + ) -H + H 2 O CH 3 CO 2 H  CH 3 CO 2 - CH 3 CO 2 H  CH 3 CO 2 - n H 2 O +H + OH - n NH 3 NH 2 - n Weakest acid Strongest base

39 Strengths of Acids n strongest acid in water is H 3 O + HCl + H 2 O  H 3 O + + Cl - HCl is so strong it forces water to accept H + n strongest base in water is OH - NH H 2 O  NH 3 + OH - NH 2 - is strong enough to remove H + from water n because water is amphiprotic

40 Strengths of Bases n Strong Bases are strong electrolytes n Dissociate completely in solution n Bases do not need to contain OH - ion O 2- (aq) + H 2 O (l)  2OH - (aq) H - (aq) + H 2 O(l)  H 2 (g) + OH - (aq) N 3 - (aq) + H 2 O(l)  NH 3 (aq) + 3OH - (aq)

41 Ionization Constants for Weak Monoprotic Acids and Bases n Let’s look at the dissolution of acetic acid, a weak acid, in water as an example. n The equation for the ionization of acetic acid is:

42 Ionization Constants for Weak Monoprotic Acids and Bases n The equilibrium constant for this ionization is expressed as:

43 Ionization Constants for Weak Monoprotic Acids and Bases n The water concentration in dilute aqueous solutions is very high. n 1 L of water contains 55.5 moles of water. n Thus in dilute aqueous solutions:

44 Ionization Constants for Weak Monoprotic Acids and Bases n The water concentration is many orders of magnitude greater than the ion concentrations. n Thus the water concentration is essentially constant.

45 Ionization Constants for Weak Monoprotic Acids and Bases n Since a constant multiplied by a constant is a new constant - let’s give this new constant its own name and symbol n K a = ionization constant

46 Ionization Constants for Weak Monoprotic Acids and Bases n In simplified form the equation and expression are written as:

47 Ionization Constants for Weak Monoprotic Acids and Bases n Values for several ionization constants

48 Ionization Constants for Weak Monoprotic Acids and Bases n From the above table we see that the order of increasing acid strength for these weak acids is: n The larger K a The stronger the acid n If K a >> 1 then the acid is completely ionized and the acid is a strong acid.

49 Ionization Constants for Weak Monoprotic Acids and Bases n The order of increasing base strength of the anions (conjugate bases) of these acids is:

50 Ionization Constants for Weak Monoprotic Acids and Bases n n Using K a, the concentration of H + (and hence the pH) can be calculated. – –Write the balanced chemical equation clearly showing the equilibrium. – –Write the equilibrium expression. Find the value for K a. – –Write down the initial and equilibrium concentrations for everything except pure water. We usually assume that the change in concentration of H + is x. n n Substitute into the equilibrium constant expression and solve. Remember to turn x into pH if necessary. n n A sample problem can be found at end of the slides.

51 Ionization Constants for Weak Monoprotic Acids and Bases n n Percent Ionization HA(aq) + H 2 O(l) ↔ H 3 O + (aq) + A - (aq) % ionization = [H + ] equ x 100 [HA] Relates equilibrium H + concentration to the initial HA concentration

52 Ionization Constants for Weak Monoprotic Acids and Bases n n Percent Ionization The higher the percent ionization The stronger the acid For weak acids Percent ionization decreases as the molarity of the solution decreases acetic acid0.05M2.0% ionized 0.15 M1.0% ionized

53 Ionization Constants for Weak Monoprotic Acids and Bases

54 Polyprotic Acids n Many weak acids contain two or more acidic hydrogens. –polyprotic acids ionize stepwise –ionization constant for each step n Consider arsenic acid, H 3 AsO 4, which has three ionization constants 1K 1 =2.5 x K 2 =5.6 x K 3 =3.0 x

55 Polyprotic Acids n The first ionization step is

56 Polyprotic Acids n The second ionization step is

57 Polyprotic Acids n The third ionization step is

58 Polyprotic Acids n Notice that the ionization constants vary in the following fashion: n This is a general relationship. It is always easier to remove the first proton.

59 Polyprotic Acids

60 Weak Bases n Remove protons from substances n Equilibrium established Weak base + H 2 O ↔ Conjugate acid + OH - Calculate K b base dissociation constant

61 Weak Bases

62 Weak Bases n Generally have lone pair or negative charge n Neutral weak bases contain N n Anions of weak acids are weak bases

63 Relationship between K a and K b n When two reactions are added to give a third, the equilibrium constant for the third reaction is the product of the equilibrium constants for the first two. n Reaction 1 + Reaction 2 = Reaction 3 K 1 x K 2 = K 3

64 Relationship between K a and K b n For a conjugate acid base pair: K a x K b = K w pK a x pK b = pK w The larger the K a, the smaller the K b The stronger the acid, the weaker the conjugate base.

65 Calculation of Ionization Constants n Example: In 0.12 M solution, a weak monoprotic acid, HY, is 5.0% ionized. Calculate the ionization constant for the weak acid.

66 Calculation of Ionization Constants n Example: In 0.12 M solution, a weak monoprotic acid, HY, is 5.0% ionized. Calculate the ionization constant for the weak acid.

67 Calculation of Ionization Constants n Since the weak acid is 5.0% ionized, it is also 95% unionized. n Calculate the concentrations of all species in solution.

68 Calculation of Ionization Constants n Substitute into the ionization constant expression to get the value of K a

69 Calculations Based on Ionization Constants n Example: Calculate the concentrations of the various species in 0.15 M acetic acid, CH 3 COOH, solution. n It is always a good idea to write down the ionization reaction and the ionization constant expression.

70 Calculations Based on Ionization Constants n Next we combine the basic chemical concepts with some algebra to solve the problem

71 Calculations Based on Ionization Constants n Next we combine the basic chemical concepts with some algebra to solve the problem

72 Calculations Based on Ionization Constants n Next we combine the basic chemical concepts with some algebra to solve the problem

73 n Substitute these algebraic quantities into the ionization expression. Calculations Based on Ionization Constants

74 n Solve the algebraic equation, using simplifying assumption. Calculations Based on Ionization Constants

75 n Solve the algebraic equation, using simplifying assumption. Calculations Based on Ionization Constants

76 n Complete the algebra and solve for concentrations. Calculations Based on Ionization Constants

77 n Note that the properly applied simplifying assumption gives the same result as solving the quadratic equation does. Calculations Based on Ionization Constants

78 Calculations Based on Ionization Constants

79 Synthesis Question n One method of increasing the solubility and the absorption of a medication is to convert weakly acidic drugs into sodium salts before making the pills that will be ingested. How does this preparation method enhance the drug’s solubility in the stomach?

80 Synthesis Question n The sodium salt of a weakly acidic compound is a strong conjugate base. In the presence of stomach fluids, 1.0 M HCl, the conjugate base readily reacts with the HCl generating the active and soluble form of the medication.

81 Group Question n Medicines that are weakly basic are not absorbed well into the bloodstream. One method to increase their absorption is to take an antacid at the same time that the medicine is administered. How does this method increase the absorption?