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Acid-Base Equilibria. Acid/Base Definitions Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions.

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Presentation on theme: "Acid-Base Equilibria. Acid/Base Definitions Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions."— Presentation transcript:

1 Acid-Base Equilibria

2 Acid/Base Definitions Arrhenius Model Acids produce hydrogen ions in aqueous solutions Bases produce hydroxide ions in aqueous solutions Bronsted-Lowry Model Acids are proton donors Bases are proton acceptors Lewis Acid Model Acids are electron pair acceptors Bases are electron pair donors

3 Acid Dissociation HA H + + A - Acid Proton Conjugate base Alternately, H + may be written in its hydrated form, H 3 O + (hydronium ion)

4 Dissociation of Strong Acids Strong acids are assumed to dissociate completely in solution. Large K a or small K a ? Reactant favored or product favored?

5 Dissociation Constants: Strong Acids AcidFormula Conjugate Base KaKa Perchloric HClO 4 ClO 4 - Very large Hydriodic HI I - Very large Hydrobromic HBr Br - Very large Hydrochloric HCl Cl - Very large Nitric HNO 3 NO 3 - Very large Sulfuric H 2 SO 4 HSO 4 - Very large Hydronium ion H 3 O + H 2 O 1.0

6 Dissociation of Weak Acids Weak acids are assumed to dissociate only slightly (less than 5%) in solution. Large K a or small K a ? Reactant favored or product favored?

7 Dissociation Constants: Weak Acids AcidFormula Conjugate Base KaKa Iodic HIO 3 IO x Oxalic H 2 C 2 O 4 HC 2 O x Sulfurous H 2 SO 3 HSO x Phosphoric H 3 PO 4 H 2 PO x Citric H 3 C 6 H 5 O 7 H 2 C 6 H 5 O x Nitrous HNO 2 NO x Hydrofluoric HF F x Formic HCOOH HCOO x Benzoic C 6 H 5 COOH C 6 H 5 COO x Acetic CH 3 COOH CH 3 COO x Carbonic H 2 CO 3 HCO x Hypochlorous HClO ClO x Hydrocyanic HCN CN x

8 Self-Ionization of Water H 2 O + H 2 O H 3 O + + OH - At 25, [H 3 O + ] = [OH - ] = 1 x K w is a constant at 25 C: K w = [H 3 O + ][OH - ] K w = (1 x )(1 x ) = 1 x

9 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

10 pH and pOH Calculations

11 pH Scale

12 A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #1: Write the dissociation equation HC 2 H 3 O 2 C 2 H 3 O H +

13 A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #2: ICE it! HC 2 H 3 O 2 C 2 H 3 O H + I C E x +x x xx

14 A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #3: Set up the law of mass action HC 2 H 3 O 2 C 2 H 3 O H xxx E

15 A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #4: Solve for x, which is also [H + ] HC 2 H 3 O 2 C 2 H 3 O H xxx E [H + ] = 3.0 x M

16 A Weak Acid Equilibrium Problem What is the pH of a 0.50 M solution of acetic acid, HC 2 H 3 O 2, K a = 1.8 x ? Step #5: Convert [H + ] to pH HC 2 H 3 O 2 C 2 H 3 O H xxx E

17 Dissociation of Strong Bases Strong bases are metallic hydroxides Group I hydroxides (NaOH, KOH) are very soluble Group II hydroxides (Ca, Ba, Mg, Sr) are less soluble pH of strong bases is calculated directly from the concentration of the base in solution MOH(s) M + (aq) + OH - (aq)

18 Reaction of Weak Bases with Water The base reacts with water, producing its conjugate acid and hydroxide ion: CH 3 NH 2 + H 2 O CH 3 NH OH - K b = 4.38 x 10 -4

19 K b for Some Common Weak Bases BaseFormula Conjugate Acid KbKb Ammonia NH 3 NH x Methylamine CH 3 NH 2 CH 3 NH x Ethylamine C 2 H 5 NH 2 C 2 H 5 NH x Diethylamine (C 2 H 5 ) 2 NH (C 2 H 5 ) 2 NH x Triethylamine (C 2 H 5 ) 3 N (C 2 H 5 ) 3 NH x Hydroxylamine HONH 2 HONH x HydrazineH 2 NNH 2 H 2 NNH x Aniline C 6 H 5 NH 2 C 6 H 5 NH x Pyridine C 5 H 5 N C 5 H 5 NH x Many students struggle with identifying weak bases and their conjugate acids.What patterns do you see that may help you?

20 Reaction of Weak Bases with Water The generic reaction for a base reacting with water, producing its conjugate acid and hydroxide ion: B + H 2 O BH + + OH - (Yes, all weak bases do this – DO NOT endeavor to make this complicated!)

21 A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #1: Write the equation for the reaction NH 3 + H 2 O NH OH -

22 A Weak Base Equilibrium Problem What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? Step #2: ICE it! I C E x +x x xx NH 3 + H 2 O NH OH -

23 A Weak Base Equilibrium Problem Step #3: Set up the law of mass action xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O NH OH -

24 A Weak Base Equilibrium Problem Step #4: Solve for x, which is also [OH - ] xxx E [OH - ] = 3.0 x M NH 3 + H 2 O NH OH - What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ?

25 A Weak Base Equilibrium Problem Step #5: Convert [OH - ] to pH xxx E What is the pH of a 0.50 M solution of ammonia, NH 3, K b = 1.8 x ? NH 3 + H 2 O NH OH -

26 Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is from a strong base, anion from a strong acid KCl, KNO 3 NaCl NaNO 3 Both ions are neutral Neutral These salts simply dissociate in water: KCl(s) K + (aq) + Cl - (aq)

27 Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is from a strong base, anion from a weak acid NaC 2 H 3 O 2 KCN, NaF Cation is neutral, Anion is basic Basic 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:

28 Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is the conjugate acid of a weak base, anion is from a strong acid NH 4 Cl, NH 4 NO 3 Cation is acidic, Anion is neutral Acidic NH 4 + (aq) NH 3 (aq) + H + (aq) Acid Conjugate Proton base The acidic cation can act as a proton donor:

29 Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is the conjugate acid of a weak base, anion is conjugate base of a weak acid NH 4 C 2 H 3 O 2 NH 4 CN Cation is acidic, Anion is basic See below 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

30 Acid-Base Properties of Salts Type of SaltExamplesCommentpH of solution Cation is a highly charged metal ion; Anion is from strong acid Al(NO 3 ) 2 FeCl 3 Hydrated cation acts as an acid; Anion is neutral Acidic 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

31 Effect of Structure on Acid-Base Properties


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