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Acids and Bases Chapter 14 Acids and Bases. Acids and Bases Some Definitions Arrhenius  Acid:Substance that, when dissolved in water, increases the concentration.

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Presentation on theme: "Acids and Bases Chapter 14 Acids and Bases. Acids and Bases Some Definitions Arrhenius  Acid:Substance that, when dissolved in water, increases the concentration."— Presentation transcript:

1 Acids and Bases Chapter 14 Acids and Bases

2 Acids and Bases Some Definitions Arrhenius  Acid:Substance that, when dissolved in water, increases the concentration of hydrogen ions.  Base:Substance that, when dissolved in water, increases the concentration of hydroxide ions.

3 Acids and Bases Some Definitions Brønsted–Lowry  Acid:Proton donor  Base:Proton acceptor

4 Acids and Bases A Brønsted–Lowry acid… …must have a removable (acidic) proton. A Brønsted–Lowry base… …must have a pair of nonbonding electrons.

5 Acids and Bases If it can be either…...it is amphiprotic. HCO 3 − HSO 4 − H2OH2O

6 Acids and Bases What Happens When an Acid Dissolves in Water? Water acts as a Brønsted–Lowry base and abstracts a proton (H + ) from the acid. As a result, the conjugate base of the acid and a hydronium ion are formed.

7 Acids and Bases Acid Dissociation (Ionization) Reactions Write the simple dissociation (ionization) reaction (omitting water) for each of the following acids:  a. Hydrochloric acid  b. Acetic acid  c. Ammonium ion  d. Anilinium ion (C 6 H 5 NH 3 )  e. Hydrated aluminum (III) ion [Al(H 2 O) 6 ] 3+

8 Acids and Bases Conjugate Acids and Bases: From the Latin word conjugare, meaning “to join together.” Reactions between acids and bases always yield their conjugate bases and acids.

9 Acids and Bases Acid and Base Strength Strong acids are completely dissociated in water.  Their conjugate bases are quite weak. Weak acids only dissociate partially in water.  Their conjugate bases are weak bases.

10 Acids and Bases Acid and Base Strength Substances with negligible acidity do not dissociate in water.  Their conjugate bases are exceedingly strong.

11 Acids and Bases Acid and Base Strength In any acid-base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base. HCl (aq) + H 2 O (l)  H 3 O + (aq) + Cl − (aq) H 2 O is a much stronger base than Cl −, so the equilibrium lies so far to the right K is not measured (K>>1).

12 Acids and Bases Acid and Base Strength Acetate is a stronger base than H 2 O, so the equilibrium favors the left side (K<1). C 2 H 3 O 2 (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 − (aq)

13 Acids and Bases Relative Base Strength Using the following Ka values, arrange the following species according to their strength as bases: H2OH2O F-F- Ka for HF = 7.2 x Cl - N/A NO 2 - Ka for HF = 4.0 x CN - Ka for HF = 6.2 x

14 Acids and Bases Autoionization of Water As we have seen, water is amphoteric. In pure water, a few molecules act as bases and a few act as acids. This is referred to as autoionization. H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH − (aq)

15 Acids and Bases Ion-Product Constant The equilibrium expression for this process is K c = [H 3 O + ] [OH − ] This special equilibrium constant is referred to as the ion-product constant for water, K w. At 25°C, K w = 1.0  10 −14

16 Acids and Bases Calculating [H + ] & [OH - ] Calculate [H + ] & [OH - ] as required for each of the following solutions at 25 0 C, & state whether the solution is neutral, acidic, or basic.  a. = 1.0 x M OH -  b. a. = 1.0 x M OH -  c M H +

17 Acids and Bases pH pH is defined as the negative base-10 logarithm of the hydronium ion concentration. pH = −log [H 3 O + ]

18 Acids and Bases pH In pure water, K w = [H 3 O + ] [OH − ] = 1.0  10 −14 Because in pure water [H 3 O + ] = [OH − ], [H 3 O + ] = (1.0  10 −14 ) 1/2 = 1.0  10 −7

19 Acids and Bases pH Therefore, in pure water, pH = −log (1.0  10 −7 ) = 7.00 An acid has a higher [H 3 O + ] than pure water, so its pH is <7 A base has a lower [H 3 O + ] than pure water, so its pH is >7.

20 Acids and Bases pH These are the pH values for several common substances.

21 Acids and Bases Other “p” Scales The “p” in pH tells us to take the negative log of the quantity (in this case, hydrogen ions). Some similar examples are  pOH −log [OH − ]  pK w −log K w

22 Acids and Bases Watch This! Because [H 3 O + ] [OH − ] = K w = 1.0  10 −14, we know that −log [H 3 O + ] + −log [OH − ] = −log K w = or, in other words, pH + pOH = pK w = 14.00

23 Acids and Bases Calculating pH, pOH pH = -log 10 (H 3 O + ) pOH = -log 10 (OH - ) Relationship between pH and pOH pH + pOH = 14 Finding [H 3 O + ], [OH - ] from pH, pOH [H 3 O + ] = 10 -pH [OH - ] = 10 -pOH

24 Acids and Bases Calculating pH & pOH Calculate pH & pOH for each of the following solutions at 25 0 C.  a. = 1.0 x M OH -  b. a. = 1.0 M H +

25 Acids and Bases Calculating pH The pH a sample of human blood was measured to be 7.41 at 25 0 C. Calculate pOH, [H + ], & [OH - ] for the sample.

26 Acids and Bases How Do We Measure pH? For less accurate measurements, one can use  Litmus paper “Red” paper turns blue above ~pH = 8 “Blue” paper turns red below ~pH = 5  An indicator

27 Acids and Bases How Do We Measure pH? For more accurate measurements, one uses a pH meter, which measures the voltage in the solution.

28 Acids and Bases Strong Acids You will recall that the seven strong acids are HCl, HBr, HI, HNO 3, H 2 SO 4, HClO 3, and HClO 4. These are, by definition, strong electrolytes and exist totally as ions in aqueous solution. For the monoprotic strong acids, [H 3 O + ] = [acid].

29 Acids and Bases pH of Strong Acids Calculate pH of 0.10 M HNO 3. Calculate pH of 1.0 x M HCl.

30 Acids and Bases Dissociation Constants For a generalized acid dissociation, the equilibrium expression would be This equilibrium constant is called the acid-dissociation constant, K a. [H 3 O + ] [A − ] [HA] K c = HA (aq) + H 2 O (l) A − (aq) + H 3 O + (aq)

31 Acids and Bases Dissociation Constants The greater the value of K a, the stronger the acid.

32 Acids and Bases Calculating K a from the pH The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is Calculate K a for formic acid at this temperature. We know that [H 3 O + ] [COO − ] [HCOOH] K a =

33 Acids and Bases Calculating K a from the pH The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is Calculate K a for formic acid at this temperature. To calculate K a, we need the equilibrium concentrations of all three things. We can find [H 3 O + ], which is the same as [HCOO − ], from the pH.

34 Acids and Bases Calculating K a from the pH pH = −log [H 3 O + ] 2.38 = −log [H 3 O + ] −2.38 = log [H 3 O + ] 10 −2.38 = 10 log [H 3 O + ] = [H 3 O + ] 4.2  10 −3 = [H 3 O + ] = [HCOO − ]

35 Acids and Bases Calculating K a from pH Now we can set up a table… [HCOOH], M[H 3 O + ], M[HCOO − ], M Initially Change −4.2    10 −3 At Equilibrium 0.10 − 4.2  10 −3 = =  10 −3

36 Acids and Bases Calculating K a from pH [4.2  10 −3 ] [0.10] K a = = 1.8  10 −4

37 Acids and Bases Solving Weak Acid Equilibrium Problems 1. List the major species in the solution. 2. Choose the species that can produce H + and write balanced equations for the reactions producing H Using the values of the equilibrium constants for the reactions you have written, decide which equilibrium will dominate in producing H Write the equilibrium expression for the dominant equilibrium 5. ICE the problem 6. Substitute the equilibrium [ ] into the equilibrium expression 7. Solve for x the “easy” way; that is, by assuming [HA] 0 -x  [HA] 0 8. Use the 5% rule to verify whether the approximation is valid 9. Calculate [H + ] and pH

38 Acids and Bases The pH of Weak Acids The hypochlorite ion (OCl - ) is a strong oxidizing agent often found in household bleaches & disinfectants. It is also the active ingredient that forms when swimming pool water is treated with chlorine. In addition to its oxidizing abilities, the hypochlorite ion has a relatively high affinity for protons (it is a much stronger base than Cl - for example) & forms the weakly acidic hypochlorous acid (HOCl, Ka = 3.5 x ). Calculate pH of M aqueous solution of hypochlorous acid.

39 Acids and Bases The pH of Weak Acids continued HOCl, Ka = 3.5 x Calculate pH of M aqueous solution of hypochlorous acid.

40 Acids and Bases The pH of Weak Acid Mixtures Calculate the pH of a solution that contains 1.00 M HCN (Ka = 6.2 x ) and 5.00 M HNO 2 (Ka = 4.0 x ). Also calculate the concentration of cyanide ion in this solution at equilibrium.

41 Acids and Bases Percent Dissociation In general, the more dilute the weak acid solution, the greater the percent dissociation of the weak acid. Percent Dissociation Acid Concentration H + Concentration

42 Acids and Bases Calculate Percent Dissociation Calculate the percent dissociation of acetic acid (K a = 1.8x ) in each of the following solutions. a M HC 2 H 3 O 2 b M HC 2 H 3 O 2

43 Acids and Bases Calculation Ka from Percent Dissociation Lactic acid (HC 3 H 5 O 3 ) is a waste product that accumulates in muscle tissue during exertion, leading to pain & feeling of fatigue. In a M aqueous solution, lactic acid is 3.7% dissociated. Calculate the value of Ka for this acid.

44 Acids and Bases Strong Bases Strong bases are the soluble hydroxides, which are the alkali metal and heavier alkaline earth metal hydroxides (Ca 2+, Sr 2+, and Ba 2+ ). Again, these substances dissociate completely in aqueous solution.

45 Acids and Bases The pH of strong bases Calculate the pH of a 5.0x M NaOH solution.

46 Acids and Bases Weak Bases Many types of proton acceptors (bases) do not contain hydroxide ions. When dissolved in water, they increase the concentration of hydroxide ions because of their reaction with water. Ex. Bases such as ammonia typically have at least one unshared pair of electrons that is capable of forming a bond with a proton.

47 Acids and Bases Weak Bases We will solve weak base problems in the same manner we solved weak acid problems (look back over the steps you were given We will use K b instead of K a and will find [OH - ] instead of [H + ]  Remember the process for “switching” from pOH to pH &/or from [OH - ] [H + ]

48 Acids and Bases pH of a Weak Base (I) Calculate the pH for a 15.0 M NH 3 solution (K b = 1.8x ).

49 Acids and Bases pH of a Weak Base (II) Calculate the pH for a 1.0 M methylamine solution (K b = 4.38x ).

50 Acids and Bases Polyprotic Acids Some acids furnish more than one acidic proton such as H 2 SO 4, H 3 PO 4.  Ex. H 2 CO 3

51 Acids and Bases pH of a polyprotic acid Calculate the pH of a 5.0 M H 3 PO 4 solution and the equilibrium concentrations of the species H 3 PO 4, H 2 PO 4 -, HPO 4 2-, & PO 4 3-.

52 Acids and Bases pH of sulfuric acid Calculate the pH of a 1.0 M H 2 SO 4 solution. Calculate the pH of a.0100 M H 2 SO 4 solution.

53 Acids and Bases Acid-Base Properties of Salts

54 Acids and Bases Acid-Base Properties of Salts These salts simply dissociate in water: KCl(s)  K + (aq) + Cl - (aq)

55 Acids and Bases Acid-Base Properties of Salts C 2 H 3 O H 2 O  HC 2 H 3 O 2 + OH- base acid acid base The basic anion can accept a proton from water:

56 Acids and Bases Acid-Base Properties of Salts NH 4 + (aq)  NH 3 (aq) + H + (aq) Acid Conjugate Proton base The acidic cation can act as a proton donor:

57 Acids and Bases Acid-Base Properties of Salts  IF K a for the acidic ion is greater than K b for the basic ion, the solution is acidic  IF K b for the basic ion is greater than K a for the acidic ion, the solution is basic  IF K b for the basic ion is equal to K a for the acidic ion, the solution is neutral

58 Acids and Bases Acid-Base Properties of Salts Step #1: AlCl 3 (s) + 6H 2 O  Al(H 2 O) 6 3+ (aq) + Cl - (aq) Salt water Complex ion anion Step #2: Al(H 2 O) 6 3+ (aq)  Al(OH)(H 2 O) 5 2+ (aq) + H + (aq) Acid Conjugate base Proton

59 Acids and Bases Salts as Weak Bases Calculate the pH of a 0.30 M NaF solution. The K a value for HF is 7.2 x 10 -4

60 Acids and Bases Salts as Weak Acids I Calculate the pH of a 0.10 M NH 4 Cl solution. The K b value for NH 3 is 1.8 x 10 -5

61 Acids and Bases Salts as Weak Acids II Calculate the pH of a M AlCl 3 solution. The K a value for Al(H 2 O) 6 3+ is 1.4 x 10 -5

62 Acids and Bases The Acid/Base Properties of Salts Predict whether an aqueous solution of each of the following salts will be acidic, basic, or neutral. a.NH 4 C 2 H 3 O 2 b.NH 4 CN c.Al 2 (SO 4 ) 3

63 Acids and Bases Effect of Structure on Acid-Base Properties Increasing Acidity HypochlorousacidChlorousacidChloricacidPerchloricacid

64 Acids and Bases The Lewis Acid-Base Model Lewis Acid: An electron pair acceptor Has an empty orbital to accept a pair of electrons Lewis Base: An electron pair donor Has a lone pair of electrons

65 Acids and Bases Lewis Acids & Bases For each reaction, identify the Lewis acid & base. a.Ni 2+ (aq) + 6NH 3(aq)  Ni(NH 3 ) 6 2+ (aq) b.H + (aq) + H 2 O (l)  H 3 O + (aq)


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