Acids and Bases What is the relationship between K a and K b ? When two reactions are added to give a third reaction, the equilibrium constant for the third reaction is equal to the products of the equilibrium constants for the two added reactions
Acids and Bases K a and K b K a and K b are related in this way: K a K b = K w (1.0 x10 -14 ) Therefore, if you know one of them, you can calculate the other.
Acids and Bases How does this relate to pK a and pK b ? Just like K a x K b = K w Just like pK a + pK b = pK w = 14
Acids and Bases Acid-Base Properties of Salt Solutions Hydrolysis is the reaction of dissolved ions with water to generate H + ions or OH - ions Example: C 2 H 3 O 2 − (aq) + H 2 O (l) HC 2 H 3 O 2(aq) + OH − (aq)
Acids and Bases Acid-Base Properties of Salt Solutions Why do all cations (except alkali metals heavier alkaline earth metals) act as weak acids? Cu + (aq) + H 2 O (l) CuOH (aq) + H + (aq) They are ABLE to hydrolyze with water creating H+ ions thus a weak acid solution.
Acids and Bases Acid-Base Properties of Salt Solutions The pH of a solution may be qualitatively predicted using the following guidelines: 1. Salts derived from a strong acid and strong base are neutral. Examples: NaCl [ NaOH + HCl NaCl + H 2 O ] Ca(NO 3 ) 2 [ Ca(OH) 2 + HNO 3 Ca(NO 3 ) 2 + 2H 2 O ] 2. Salts derived from a strong base and weak acid are basic. Examples: NaClO Ba(C 2 H 3 O 2 ) 2 strong base ion/weak acid ion strong base ion/weak acid ion [ NaOH + HClO NaClO + H 2 O ] [ BaOH + HC 2 H 3 O 2 Ba(C 2 H 3 O 2 ) 2 + H 2 O ]
Acids and Bases Acid-Base Properties of Salt Solutions 3. Salts derived from a weak base and strong acid are acidic. Example: NH 4 Cl Mg(NO 3 ) 2 weak base ion/strong acid ion weak base ion/strong acid ion [ NH 3 + HCl NH 4 Cl + H 2 O ] [ Mg(OH) 2 + HNO 3 Mg(NO 3 ) 2 + 2H 2 O ] 4. Salts derived from a weak base and weak acid, both anions and cations are hydrolyzed. So it’s the extent of which each ion is hydrolyzed- compare Ka and Kb for both Example: NH 4 CN weak base ion/weak acid ion NH 4 + has K a of 5.6 x 10 -10 and CN - has K b of 2.0 x 10 -5 The solution has a higher Kb so its basic.
Acids and Bases 16.73 Predict whether aqueous solutions of the following compounds are acidic, basic or neutral; a.NH 4 Br_____________ b.FeCl 3 _____________ c. Na 2 CO 3 _____________ d. KClO 4 _____________ e. NaHC 2 O 4 _________ Weak base – strong acid Acid Weak base – strong acid Acid Strong base – weak acid Base Strong base – strong acid Neutral Strong base – weak acid Base
Acids and Bases What makes a strong acid strong? WHY oh WHY are these STRONG ACIDS??? Strong acids: HCl HBr HI HNO 3 HClO 4 H 2 SO 4
Acids and Bases Factors that Affect Acid Strength Consider the acid with the formula, H-X. For this substance to be an acid we need the following: POLARITY- 1.Keep in mind that acids are made of nonmetals. 2.The difference in electronegativity between the 2 covalently bonded determines if it will hydrolyze and pull the H+ off the water creating a “stronger” acid 3.NON-Polar bonds can not be hydrolyzed so they are neutral.
Acids and Bases Factors that Affect Acid Strength Consider the acid with the formula, H-X. For this substance to be an acid we need the following: BONDING STRENGTH- 1. HF is a weak acid because the H-F bond is sooo tight it has a hard time hydrolyzing. 2. HCl is a strong acid because the H-Cl bond is not as tight and will hydrolyze in water AND create more H+
Acids and Bases Factors that Affect Acid Strength Consider the acid with the formula, H-X. For this substance to be an acid we need the following: CONJUGATE STABILITY 2. The H-X bond must be weak enough to be broken 3. The conjugate base, X −, must be stable. (The more stable the anion, the more acidic it will be.)
Acids and Bases Strength of Binary Acids Acid strength increases across a period and down a group. Conversely, base strength decreases across a period and down a group. HF is a weak acid because the bond is highly polar so the bond energy is high. Therefore, H-F doesn’t dissociate as much as HCl. The electronegativity difference between C and H is so small that the C-H bond is non-polar so CH 4 is neither an acid nor a base.
Acids and Bases Strength of Oxyacids As the number of O atoms attached to Y increases, the O–H bond polarity increases. Consequently, the electrons in the O-H bond are pulled away from the H, and this makes it easier to ionize! The strength of the acid will therefore increase.
Acids and Bases Strength of Carboxylic Acids Why can CH 3 COO–H ionize, but CH 3 CH 2 O–H cannot? 1. The additional oxygen atom on the carboxyl group increases the polarity of the O–H bond and stabilizes the conjugate base. 2. The conjugate base of CH 3 COO − exhibits resonance further increasing the stability of the conjugate base. The carboxylic acid strength also increases as the number of electronegative groups in the acid increases. Example: Acetic acid is much weaker than trichloroacetic acid. CH 3 COOH vs. CCl 3 COOH weaker stronger − −
Acids and Bases Lewis Acids and Bases There is one last way to define acids and bases… (3) Lewis Definition: - acids: electron pair acceptor - bases: electron pair donor (Notice that it is the exact opposite of Brønsted-Lowry’s definition…acids are proton donors; bases are proton acceptors.) Note: Lewis acids and Lewis bases do not need to contain protons, therefore, the Lewis definition is the most general definition of acids and bases we have. Lewis acids generally have an incomplete octet…(Example: BF 3 ) Lewis acids must have a vacant orbital (into which the electron pairs can be donated).
Acids and Bases Here’s an example of a Lewis acid and Lewis base… H – N: + B – F H – N – B – F Compounds with multiple bonds can act as Lewis acids. - For example, consider the reaction: H 2 O (l) + CO 2(g) H 2 CO 3(aq) Water acts as the electron pair donor and carbon dioxide as the electron pair acceptor in this reaction. Lewis Acids and Bases H H H HF F F F e- pair donor (base) e- pair acceptor (acid) +
Acids and Bases Hydrolysis of Metal Ions Metal ions are (+) charged and attract water molecules (via the lone pairs on oxygen). The higher the charge, the smaller the metal ion and the stronger the M-OH 2 interaction. Both factors increase K a, so… Summary: (1) Smaller ions are more acidic. (2) Higher charged ions are more acidic. Practice Problems: (1) Ca 2+ vs. Zn 2+ Which is more acidic in water? As you go across the periodic table, size generally decreases, so Zn +2 is smaller than Ca +2 which makes Zn +2 more acidic. (2) Ca +2 vs. Al +3 Which is more acidic in water? Al +3 is more acidic. Not only is its charge larger, but it is also a smaller cation.
Acids and Bases Hydrolysis of Metal Ions Weaker acid Smaller K a Higher pH Stronger acid Larger K a Lower pH
Acids and Bases 16.83 Based on all the notes in section 16.10, which is the stronger base among the pair? a. BrO - or ClO - : Why___________________________________ b BrO - or BrO 2 - : Why___________________________________ c. HPO 4 -2 or H 2 PO 4 -1 - : Why__________________________________ Think of it as who is the weakest acid. ClO - because acid strength increase as we move down a group BrO - - because it has less oxygens around the central atom HPO 4 -2 - because it can accept more H+
Acids and Bases 16.89. Identify the Lewis acid and Lewis base among the reactions in each of the following reactions. A. Fe(ClO 4 ) 3 (s) + 6H 2 O (l) Fe(H 2 O) 6 +3 (aq) + 3ClO 4 - (aq) B. CN - (aq) + H 2 O (l) HCN (aq) + OH - (aq) C. (CH 3 ) 3 N (g) + BF 3 (g) (CH 3 ) 3 NBF 3 (s) E-pair donor is base and e-pair acceptor is acid. L-acidL-base L-acid L-baseL-acid