Acid and Base Equilibria Chapter 16

Transcription

1 Acid and Base Equilibria Chapter 16! Review Chapter 4.1 (Electrolytes)! Review Chapter 4.3 (Acid- Base ReacBons)! Memorize 7 Strong Acids (Table 4.2)! Memorize the Strong Bases (Also Table 4.2)! Review Equilibrium (Ch. 15) MJ Bojan Chem 112 1

2 Acids and bases are everyday substances: What do you already know about ACIDS? 1) Acids are corrosive they dissolve compounds that are otherwise hard to dissolve. 2) Acids in our everyday life??? 3) Acid indicators MJ Bojan Chem 112 2

3 Acids and bases are everyday substances: What do you already know about BASES? 1) Bases Taste? 2) Bases dissolve oil and grease. 3) Bases are slippery to the touch. 4) Bases react with many metal ions to form precipitates. 5) Base indicators MJ Bojan Chem 112 3

4 We start with a simple definibon of Acids and Bases Arrhenius ACID: a compound that releases H + when dissolved in water Arrhenius BASE: a compound that releases OH when dissolved in water MJ Bojan Chem 112 4

5 Use the definibon of a base to explain this H 2 O + phenolphthalein (pink in base) MJ Bojan Chem 112 5

6 We cannot use the Arrhenius definibon to explain the behavior of all acids and bases. We need a more general definibon. BRØNSTED LOWRY ACIDS AND BASES BrØnsted ACID: Example: BrØnsted Base: Example: MJ Bojan Chem 112 6

7 CONJUGATE ACID BASE PAIRS differ only by the presence or absence of a proton (H + ). Conjugate Acid = Conjugate Base + H + Examples: MJ Bojan Chem 112 7

8 The stronger the acid, the weaker its conjugate base. The weaker the acid, the stronger its conjugate base. ***The conjugate of a weak acid is a weak base. The conjugate of a strong acid is a spectator ion Example: Cl is the conjugate base of HCl. The conjugate acid of OH (strong base) is water. MJ Bojan Chem 112 8

9 FYI: Conjugate Acids and Bases The term conjugate comes from the LaBn word conjugare, meaning to join together. ReacBons between acids and bases always yield their conjugate bases and acids. Note: Ø The stronger the acid, the weaker its conjugate base. Ø The weaker the acid, the stronger its conjugate base. MJ Bojan Chem 112 9

10 What is happening on a molecular level Proton Transfer Hydronium ion MJ Bojan Chem

11 Proton transfer: H X è H + + X Why does proton transfer occur? Can we predict the factors that will affect this process? 1. Strength of the bond: 2. Polarity of the bond: 3. Stability of the conjugate base. MJ Bojan 11 Chem 112

12 Water electrolyzes slightly to produce H + and OH reversibly This process is called auto- ionizabon. What is the equilibrium constant expression for this process at 25 C? What is the [H + ] of pure water at 25 C? MJ Bojan Chem

13 K w is the Ion Product Constant for water AutoionizaYon constant NOTE: K w is constant even when [H + ] and [OH ] are not equal. EXAMPLE: Calculate [H + ] in a 0.05 M Ca(OH) 2 solubon at 25 C. MJ Bojan Chem

14 ph is defined as the negabve logarithm (base- 10: log) of the concentrabon of hydronium ion. ph= poh= Some useful things to know At 25 C Therefore, in pure water at 25 C: MJ Bojan Chem

15 An ACID has a higher [H 3 O + ] than pure water A BASE has a lower [H 3 O + ] than pure water SoluYon [H + ] [OH ] ph value Acid Neutral Base MJ Bojan 15 Chem 112

16 How do we measure ph? Indicators: certain dyes change color as ph changes. These can be used as acid- base indicators. HIn = H + + In Many indicators do not have a sharp color change as a funcbon of ph. Indicators are less precise than ph meters. ph meter is a calibrated voltmeter; voltage is proporbonal to ph ph meter is the most accurate method to measure ph. MJ Bojan 16 Chem 112

17 ph of common substances MJ Bojan 17 Chem 112

18 Useful InformaBon K w = [H + ][OH ] = at 25 C ph = log 10 [H + ] low ph = acidic high ph = basic poh= log 10 [OH ] pk w = log 10 K w = 14 ph + poh = log 10 [H + ] + log 10 [OH ] = 14 MJ Bojan Chem

19 What is an electrolyte? A substance that conducts electricity when dissolved in solution For bulb to light, some minimum amount of current is needed. amount of current is related to the CONDUCTIVITY of the soluyon. CONDUCTIVITY: ability to conduct electricity Which Bulbs Light Up? Distilled H 2 O Tap H 2 O NaCl(aq) 0.1M HCl(aq) 0.1M CH 3 COOH sugar(aq) Bulb Wattage CH 3 OH MJ Bojan Chem

20 STRONG ACIDS Strong Acids dissociate completely when dissolved in water to form H + and the corresponding BrØnsted conjugate base. Strong acids are strong electrolytes: COMPLETE dissociayon into ions *** If the analyycal concentrayon, C HA, is less than 10 6 M, then the autoionizayon of water must be considered. Which one of the following is not a strong acids: 1. HNO 3 5. HCl 2. HF 6. HBr 3. HClO 3 7. HI 4. HClO 4 8. H 2 SO 4 MJ Bojan Chem

21 WEAK ACIDS When dissolved in water, weak acids only partially dissociate to form H + and the corresponding BrØnsted conjugate base. What is the expression for the equilibrium constant? Weak acids are weak electrolytes: PARTIAL dissociation into ions Examples: MJ Bojan Chem

22 K a is the acid dissociabon constant The value of K a is related to acid strength MJ Bojan 22 Chem 112

23 How do you find [H + ] for a weak acid? It is an equilibrium problem! General Approach to Equilibrium Constant Problems 1. Write the balanced reaction. 2. Write the expression for K eq. 3. Set up a data table: (may need algebraic unknowns) I initial conditions C changes in concentrations E equilibrium concentrations 4. Substitute equilibrium concentrations into the expression for K eq and solve. MJ Bojan Chem

24 Example of weak acid equilibrium calculabon of [H + ] for weak acids. What is the [H + ] of 0.10 M acebc acid? K a = MJ Bojan Chem

25 What is the ph? CalculaBons ConBnued C HA = 0.10 M acetic acid: CH 3 COOH What is the % dissociation? Compare this to the [H + ] of 0.10 M HI. MJ Bojan Chem

26 POLYPROTIC ACIDS PolyproYc acids are capable of donayng more than one proton. They contain more than one ionizable proton. The K a always gets smaller with each ionizayon: what does this tell you? Examples: H 2 CO 3 (aq) H + (aq) + HCO 3 (aq) K a1 = HCO 3 (aq) H + (aq) + CO 3 2 (aq) K a2 = MJ Bojan Chem

27 K a for some polyprobc acids Acid Formula K a1 K a2 K a3 Carbonic H 2 CO phosphoric H 3 PO Sulfurous H 2 SO Sulfuric H 2 SO 4 large Oxalic H 2 C 2 O Ascorbic H 2 C 6 H 6 O Citric H 3 C 6 H 5 O MJ Bojan Chem

28 EXAMPLE: What are the concentrabons of H +, HCO 3, and CO 3 2 in M H 2 CO 3? What is the ph? (Note: K a values are in the table on previous page.) MJ Bojan Chem

29 EXAMPLE: In H 2 CO 3 (aq) soluyon, what are the conjugate acids and bases present? Rank them in order of: a) increasing acid strength. b) increasing base strength. MJ Bojan Chem

30 POLYPROTIC ACIDS Phosphoric acid is a triproyc acid. H 3 PO 4 (aq) H + (aq) + H 2 PO 4 (aq) H 2 PO 4 (aq) H + (aq) + HPO 4 2 (aq) HPO 4 2 (aq) H+ (aq) + PO 4 3 (aq) K a1 = K a2 = K a3 = MJ Bojan Chem

31 DistribuBon curve for phosphoric acid NaOH is added to a solution of H 3 PO 4 until the ph reaches 12. Which species are present in the solution? fracbon [H 3 PO 4 ] [H 2 PO 4 ] [HPO 4 2 ] [PO 4 3 ] ph H 3 PO 4 = triprobc acid. The y- axis represents the fracbon of total phosphate in each form (H 3 PO 4, H 2 PO 4, etc.) at a given ph. MJ Bojan Chem

32 STRONG BASES: are strong electrolytes. Strong bases completely dissociate into ions in aqueous solubons. Examples: Group I and II hydroxides (except Mg and Be). Arrhenius bases: donate OH. Bronsted bases: accept H + MJ Bojan Chem

33 Strong Bases Examples What is the ph of a M solution of NaOH? What is the [H + ] concentration of a solution of Ca(OH) 2 with ph 11.68? MJ Bojan Chem

34 Hydrolysis: when a cation or anion reacts with H 2 O to form H 3 O + (aq) or OH (aq) WEAK BASES are weak electrolytes When dissolved in water, weak bases only parbally react to form OH and the corresponding BrØnsted acid. They React REVERSIBLY with water to form OH. This is an example of a hydrolysis reaction. What is the equilibrium constant for base hydrolysis? MJ Bojan Chem

35 WEAK BASES Weak bases can be neutral molecules Example: NH 3, amines NH 3 (aq) + H 2 O( ) NH 4+ (aq) + OH (aq) Or Anions (any ion derived from a weak acid) Example: F, NO 2, CH 3 COO F (aq) + H 2 O( ) HF(aq) + OH (aq) MJ Bojan Chem

36 K b is the base hydrolysis constant The value of K b is related to base strength K b can be used to find [OH ] and ph. MJ Bojan 36 Chem 112

37 Sample problem: finding ph for Base Hydrolysis What is the ph of 0.15 M NH 3 (K b = 1.8 x 10 5 )? MJ Bojan Chem

38 Acid Dissociation Things to KNOW HA + H 2 O(l) H 3 O + (aq) + A (aq) Equilibrium constant = K a pk a = log K a K a = [H + ][A ] [HA] Base Hydrolysis B + H 2 O(l) HB + (aq) + OH (aq) Equilibrium constant = K b K b = [HB + ][OH ] pk b = log K b [B]

39 YOU SHOULD KNOW GIVEN FIND ph [H + ], [OH ], poh [H + ] or [OH ] ph List of pk a s or K a s Weaker /Stronger K a or pk a and [HX] ph, [H + ], [OH ] ph and [HX] K a Recall that a small K a è high pk a, and both mean weak acid and not much dissociabon. MJ Bojan Chem