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Acids and Bases
CHEM 102!
T. Hughbanks!
Basic Definitions & Concepts
Most basic concepts are given clearly in your text - these notes will only list these as topics discussed, so there will be less detail.! Brønsted acids, bases (and distinction with Arrhenius and Lewis definitions)! Strong acids and bases! !(eg., HCl, H 2 SO 4 ; NaOH, KNH 2 )! In Chapter 3, there are two definitions of acids and bases: the Arrhenius and the Brønsted– Lowry definition.! Arrhenius acid: Any substance that when dissolved in water increases the concentration of hydrogen ions, H + .! Arrhenius base: Any substance that increases the concentration of hydroxide ions, OH, when dissolved in water.! A Brønsted–Lowry acid is a proton (H + ) donor.! A Brønsted–Lowry base is a proton acceptor.!
Acids & Bases: A Review Strong & Weak Acids/Bases
HNO 3 , HCl, H 2 SO 4 and HClO 4 are classified as strong acids.!
Strong & Weak Acids/Bases
HCl(aq) + H 2 O(ℓ) → H 3 O+(aq) + Cl–(aq)!
Other common strong bases include KOH and Ca(OH) 2 .! CaO (lime) + H 2 O →! Ca(OH) 2 (slaked lime)!
Strong & Weak Acids/Bases
Acidic and Basic Oxides!
The oxides that one uses to form acids and bases in aqueous solution often have reactivity that reflects their acidic or basic character.! Examples: Li 2 O, CaO, and BaO react with water to form basic solutions and can react with acids directly to form salts. Likewise, SO 3 , CO 2 , and N 2 O 5 form acidic aqueous solutions and can react directly with bases to give salts.!
Basic Definitions & Concepts
Weak acids and bases! !(eg., HCN, HF, CH 3 COOH; NH 3 , pyridine, CH 3 NH 2 )! Conjugate acids and Bases, eg.,!
HCN(aq) + H 2 O ⇌ H 3 O+(aq) + CN–(aq)!
acid conjugate base conjugate base acid
Water Autoionization (Autoprotolysis)!
“[H 2 O]” is constant, so a H 2 O = 1.! In pure water, what are [H 3 O+] & [OH–]?! What is ∆G˚ for this reaction?! Water self-dissociates, even in the absence! of added acids or bases:! !H 2 O + H 2 O ⇌ H 3 O+(aq) + OH–(aq)!
Keq = [H 3 O+^ ][OH–^ ] = 1.0 × 10 −^14 = Kw
pH Scale
In pure H 2 O,! ! ![H 3 O+] = 10–7, pH = -log 10–7^ = 7! pOH is defined similarly: pOH = -log[OH-]! H 3 O+^ concentration is conveniently measured! using the logarithmic pH scale:! !!! pH = -log[H 3 O+]!
pH and pOH
In aqueous solutions, [H 3 O+][OH-] = 10–14! This is true even with added acids or bases.! -log {[H 3 O+][OH-]} = -log {10–14}! -log [H 3 O+] - log [OH-] =14! pH + pOH = 14! pH = -log [H 3 O+]! pOH = -log [OH-]!
The acid-base “Seesaw” in Water
- the pH Scale!
The autoionization equilibrium always operates in aqueous solution, even when other reactions are occurring.! It is convenient to work with “-log 10 ” (pH) scale:!
2 H 2 O H 3 O+^ ( aq ) + OH–^ ( aq )
Keq = [H 3 O+^ ][OH–^ ] = 1.0 × 10 −^14 = Kw
−log Kw = - log[H 3 O+^ ] − log[OH–^ ] = 14 p Kw = pH + pOH = 14
Example 1!
For acetic acid, p Ka = 4.74. What is the pH of a 0.1 M solution?!
CH 3 COOH(aq) + H 2 O ⇌!
!! !H 3 O+(aq) + CH 3 COO–(aq)!
Ka =
[H 3 O+^ ][CH 3 COO–^ ]
[CH 3 COOH]
= 1.8 × 10 −^5
Example 2!
For ammonia, p Kb = 4.74. What is the pH of a 0.1 M solution? What % of NH 3 is converted to NH 4 +?! NH 3 (aq) + H 2 O ⇌ NH 4 +(aq) + OH–(aq)!
Kb =
[NH 4 +^ ][OH–^ ]
[NH 3 ]
= 1.8 × 10
− 5
Example 3!
For hydrofluoric acid, p Ka = 3.46. What is the pH of a 10-3^ M solution?! HF(aq) + H 2 O ⇌ H 3 O+(aq) + F–(aq)!
pH - case of high dilution!
The set-up (what we know):! [Li+] = 5 × 10 -8^ ; ![H 3 O+][OH–] = 1 × 10 -14! Charge balance: [OH–] = [Li+] + [H 3 O+]! Solve, [OH–] = 1.2808 × 10 -7^ pOH = 6.89! So, [H 3 O+] = [OH–] – [Li+] = 0.7808 × 10 -7^! pH = 7.11 (check: pH + pOH = 14)! 5 × 10 -8^ M LiOH - two sources of OH–! LiOH → Li+(aq) + OH–(aq)!
2 H 2 O ⇌ H 3 O+(aq) + OH–(aq)!
Conjugate Acids and Bases
HCN is a rather weak acid, Ka = 4.9 × 10 - (pKa = 9.31). What is the general relationship between the strengths of conjugate acid- base pairs?! What is Kb for the conjugate base, CN-?! HCN(aq) + H 2 O ⇌ H 3 O+(aq) + CN–(aq)! acid conjugate base conjugate base acid HClO 2 (aq) + H 2 O ⇌ H 3 O+(aq) + ClO 2 – (aq)! HCO 2 H(aq) + H 2 O ⇌ H 3 O+(aq) + HCO 2 – (aq)! HClO(aq) + H 2 O ⇌ H 3 O+^ (aq) + ClO–(aq)! NH 4 +(aq) + H 2 O ⇌ H 3 O+^ (aq) + NH 3 (aq)! CH 3 NH 3 +(aq) + H 2 O ⇌ H 3 O+^ (aq) + CH 3 NH 2 (aq)!
Conjugate acid-base pairs!
Increase strength! Increase strength! Ionization Constants for Acids/Bases The strongest acids are at the upper left. They have the largest Ka values.! Ka values become smaller on descending the chart as the acid strength declines.! The strongest bases are at the lower right. They have the largest Kb values.! Kb values become larger on descending the chart as base strength increases.! Equilibrium Constants for Acids & Bases (Guide to Table 17.3)
