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Acids and Bases

Acids and Bases. Theories. Acids donate protons HCl ( aq ) + H 2 O (l)  Cl - ( aq ) + H 3 O + ( aq ) Bases accept protons NH 3( aq ) + H 2 O (l) ↔ NH 4 + ( aq ) + OH - ( aq )

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Acids and Bases

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  1. Acids and Bases

  2. Theories • Acids donate protons • HCl(aq) + H2O(l) Cl-(aq) + H3O+(aq) • Bases accept protons • NH3(aq) + H2O(l)↔ NH4+(aq) + OH-(aq) • When acids dissolve in water they produce H3O+ (or H+). When bases dissolve in water, they produce OH-.

  3. Conjugate Pairs • A conjugate pair is an acid and a base which differ by one proton. • HCl and Cl- • H2SO4and HSO4- • H2O and OH- • H3O+ and H2O • NH3 and NH4+ • HCl + NH3↔ Cl- + NH4+ • Acid base conj. base of HCl conj. acid of NH3

  4. Acid Dissociation • Strong acids completely dissociate in water. • HA A-(aq) + H+(aq) • Acidconjugate base • The equilibrium expression is: • Note: H+ and H3O+ are interchangable.

  5. Strong Acids • Strong acids completely dissociate. • Will strong acids have a large or small Ka? • The Big 6: • HCl, HBr, HI, HClO4, HNO3, H2SO4 (first dissociation only) • Have weak conjugate bases

  6. Weak Acids • Very slight dissociation • Very small Ka. • All acids that aren’t part of the Big 6 are weak. • Includes lots of ions. • Have strong conjugate bases

  7. Acidic Hydrogens • Hydrogen atoms bonded to highly electronegative elements can be easily donated. • These are acidic hydrogens • Acids with more than one acidic hydrogen are called polyprotic • The second dissociation is always much weaker than the first.

  8. Relative Acid Strength • Two factors can affect acid strength: • 1. The stability of the conjugate base. • I- is larger than Cl-, which makes it more stable. HI is a stronger acid than HCl. • 2. The electron density opposite the hydrogen. • HClO3 has more oxygens pulling the electron density from the hydrogen than HClO… making it stronger. • The F in fluoroacetic acid draws more electron density away from the hydrogen, making it more acidic than acetic acid.

  9. Self-Ionization of Water • Pure water still conducts a minuscule amount of electricity. • 2 H2O ↔ H3O+ + OH- • Kw = [H3O+][OH-] = 1.0 x10-14

  10. “p” scales • p means –log • Calculating pH, pOH • pH = -log [H3O+] • pOH = -log [OH-] • Converting between pH and pOH • pH + pOH = 14 • Strong acids and bases allow you to calculate pH and pOH directly from concentrations. • Weak acids/bases require you to use ICE tables!

  11. Practice • What is the pH of a 0.100M solution of methylamine (CH3NH2)? Kb = 4.38 x10-4

  12. Salt Hydrolysis • Reacting an acid with a hydroxide base forms a salt and water. • HCl + NaOH NaCl + H2O • Strong Acid + Strong Base = neutral salt (pH = 7) • Strong Acid + Weak Base = acidic salt • Weak Acid + Strong Base = basic salt • Weak/Weak combination… check Ka and Kb

  13. Lewis Acids/Bases • Lewis Acids are electron pair acceptors • Look for things with positive charges or incomplete octets • Lewis Bases are electron pair donors • Look for lone pairs on the central atom.

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