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Unformatted text preview: 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 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??? (stomach acid) 3) Acid indicators MJ Bojan Chem 112 2 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 ARRHENIUS Acids and Bases DefiniBons 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 What is happening on a molecular level Proton Transfer Hydronium ion MJ Bojan Chem 112 5 Ammonia Fountain H2O + phenolphthalein (pink in base) MJ Bojan Chem 112 6 BRNSTED - LOWRY ACIDS AND BASES Brnsted ACID: Example: Brnsted Base: Example: MJ Bojan Chem 112 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 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 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: ! The stronger the acid, the weaker its conjugate base. ! The weaker the acid, the stronger its conjugate base. MJ Bojan Chem 112 10 Auto-ionization of water: Water electrolyzes slightly to produce H+ and OH- reversibly, auto-ionizaBon. What is the equilibrium constant expression for this process at 25C? What is the [H+] of pure water at 25C? MJ Bojan Chem 112 11 NOTE: Kw is constant even when [H+] and [OH-] are not equal. Kw is the Ion Product Constant for water Autoioniza_on constant EXAMPLE: Calculate [H+] in a 0.05 M Ca(OH)2 soluBon at 25C. MJ Bojan Chem 112 12 The pH scale pH is defined as the negaBve logarithm (base-10: log) of the concentraBon of hydronium ion. pH= pOH= Therefore, in pure water at 25C: MJ Bojan Chem 112 13 Some useful things to know At 25C The pH scale An has a higher [H3O+] than pure water, so its pH is <7. A has a lower [H3O+] than pure water, so its pH is >7. MJ Bojan 14 Chem 112 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 Most accurate method to measure pH is to use a pH meter. MJ Bojan 15 Chem 112 pH of common substances MJ Bojan 16 Chem 112 Useful InformaBon Kw = [H+][OH-] = 1.0 10-14 at 25C
pH = - log10[H+] low pH = acidic high pH = basic pOH= - log10[OH-] pKw = - log10 Kw = 14 pH + pOH = - log10[H+] + - log10[OH-] = 14
MJ Bojan Chem 112 17 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 solu_on. CONDUCTIVITY: ability to conduct electricity Which Bulbs Light Up? 2.5
Distilled H2O Tap H2O NaCl(aq) 0.1M HCl(aq) 0.1M CH3COOH sugar(aq) CH3OH Bulb Wattage 7.5 25 40 MJ Bojan Chem 112 18 STRONG ACIDS
Strong Acids dissociate completely when dissolved in water to form H+ and the corresponding Brnsted conjugate base. Strong acids are strong electrolytes: COMPLETE dissocia_on into ions *** If the analy_cal concentra_on, CHA, is less than 10-6M, then the autoioniza_on of water must be considered. Which one of the following is not a strong acids: 1. HNO3 5. HCl 2. HF 6. HBr 3. HClO3 7. HI 4. HClO4 8. H2SO4 MJ Bojan Chem 112 19 WEAK ACIDS
When dissolved in water weak acids only partially dissociate to form H+ and the corresponding Brnsted conjugate base. What is the expression for the equilibrium constant? Weak acids are weak electrolytes: PARTIAL dissociation into ions Examples: MJ Bojan Chem 112 20 DissociaBon Constants The greater the value of Ka, ! MJ Bojan 21 Chem 112 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 Keq. 3. Set up a data table: (may need algebraic unknowns) -initial conditions -changes in concentrations -equilibrium concentrations 4. Substitute equilibrium concentrations into the expression for Keq and solve.
MJ Bojan Chem 112 22 Weak acid equilibrium Calcula_on of [H+] for weak acids What is the [H+] of 0.10 M aceBc acid? Ka = 1.8 x 10-5 MJ Bojan Chem 112 23 CalculaBons ConBnued CHA = 0.10 M acetic acid: CH3COOH What is the pH? What is the % dissociation? Compare to the [H+] of 0.10 M HI. Bojan MJ Chem 112 24 POLYPROTIC ACIDS Polypro_c acids are capable of dona_ng more than one proton. Contain more than one ionizable proton. The Ka always gets smaller with each ioniza_on: what does this tell you? Examples:
H2CO3(aq) HCO3-(aq) H+ (aq) + HCO3-(aq) H+(aq) + CO32-(aq) Ka1 = 4.3 10-7 Ka2 = 5.6 10-11 MJ Bojan Chem 112 25 EXAMPLE: What are the concentra_ons of H+, HCO3-, and CO32- in 1 x 10-3 M H2CO3? What is the pH? MJ Bojan Chem 112 26 EXAMPLE: In H2CO3(aq) solu_on, what are the conjugate acids and bases present? Rank them in order of: a) increasing acid strength. b) increasing base strength. MJ Bojan Chem 112 27 POLYPROTIC ACIDS Phosphoric acid is a tripro_c acid. H3PO4 (aq) H2PO4-(aq) HPO42-(aq) H+(aq) + H2 PO4-(aq) H+(aq) + HPO42-(aq) H+(aq) + PO43-(aq) Ka1 = 7.5 10-3 Ka2 = 6.2 10-8 Ka3 = 4.2 10-13 MJ Bojan Chem 112 28 DistribuBon curve for phosphoric acid NaOH is added to a solution of H3PO4 until the pH reaches 12. Which species are present in the solution? H3PO4 = triproBc acid. The y-axis represents the fracBon of total phosphate in each form (H3PO4, H2PO4- , etc.) at a given pH. MJ Bojan Chem 112 29 STRONG BASES Completely dissociate into ions in aqueous soluBons. Strong bases are strong electrolytes. Examples: Group I and II hydroxides (except Mg and Be). Arrhenius bases: donate OH-. Bronsted bases: accept H+ MJ Bojan Chem 112 30 Strong Bases Examples What is the pH of a 0.028 M solution of NaOH? What is the [H+] concentration of a solution of Ca(OH)2 with pH 11.68? MJ Bojan Chem 112 31 WEAK BASES When dissolved in water weak bases only par_ally react to form OH- and the corresponding Brnsted acid. Weak bases are weak electrolytes React REVERSIBLY with water to form OH- What is the equilibrium constant for base hydrolysis? Chem 112 32 MJ Bojan WEAK BASES Weak bases can be neutral molecules Example: NH3, amines NH3(aq) + H2O() NH4+(aq) + OH-(aq) Or Anions (any ion derived from a weak acid) Example: F-, NO2-, CH3COO- F-(aq) + H2O() HF(aq) + OH-(aq) MJ Bojan Chem 112 33 Weak Bases Kb can be used to find [OH-] and, through it, pH. MJ Bojan 34 Chem 112 Base Hydrolysis What is the pH of 0.15 M NH3 (Kb = 1.8 x 10-5)? MJ Bojan Chem 112 35 Ka and Kb Ka and Kb are related in this way: Therefore, if you know one of them, you can calculate the other. MJ Bojan 36 Chem 112 Things to KNOW Acid Dissociation HA + H2O(l) pKa = -log Ka H3O+(aq) + A-(aq) Equilibrium constant = Ka [H + ][A- ] Ka = [HA] Base Hydrolysis + B + H2O(l) HB (aq) + OH-(aq) Equilibrium constant = Kb pKb = -log Kb [HB + ][OH - ] Kb = [B] YOU SHOULD KNOW pH [H+], [OH-], pOH [H+] or [OH-] pH List of pKa's or Ka's Weaker /Stronger Ka or pKa and [HX] pH, [H+], [OH-] pH and [HX] Ka Recall that a small Ka high pKa, and both mean weak acid and not much dissociaBon. MJ Bojan Chem 112 38 GIVEN FIND ...
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