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Acids, Bases and Equilibria

Acids, Bases and Equilibria. Overview. Definitions Strong acids pH Water equilibrium Weak acids Buffers Other equilibria LeChatlier’s Principle. Defining Acids and Bases. Arrhenius model Acid – Proton donor – e.g. HCl Base – Hydroxide donor – e.g. NaOH But how about Sodium Carbonate?.

tobias-gregory
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Acids, Bases and Equilibria

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  1. Acids, Bases and Equilibria

  2. Overview • Definitions • Strong acids • pH • Water equilibrium • Weak acids • Buffers • Other equilibria • LeChatlier’s Principle

  3. Defining Acids and Bases • Arrhenius model • Acid – Proton donor – e.g. HCl • Base – Hydroxide donor – e.g. NaOH • But how about Sodium Carbonate?

  4. Defining Acids and Bases - 2 • Brønsted-Lowery model • Acid – Proton donor – same as Arrhenius • Base – Proton ACCEPTOR • Aha – so Na2CO3 IS basic! Na2CO3 + 2HCl  2NaCl + H2CO3

  5. Strong Acids and Bases • Ionic solids like NaOH; completely form ions in water: NaOH + H2O  Na+ + OH- + H2O • Covalent molecules like HCl completely IONIZE in water: HCl + H2O  H3O+ + Cl- • H3O+ is “hydronium” ion – no bare protons

  6. Defining pH • Remember pH? • Less than 7 = acid • More than 7 = base • But what does it mean? • pH is a measure of the concentration of hydronium ion in water pH = - log [H3O+]

  7. Translation: - log • Suppose we have 0.1M HCl solution • Since it is fully ionized, we have 0.1M H3O+ • 0.1 = 10-1 • -log (10-1) = 1! • Therefore pH of this acid solution is 1

  8. Getting the pH of a base • Even in base, pH measures hydronium ion • H3O+ and OH- are related by the equilibrium of water

  9. See p. 611

  10. So, what’s equilibrium? • Second grade analogy – see-saw • In an equilibrium situation, reactions or changes go both ways • Hold ice and water at 0o • Water melts and ice freezes at the same time • “Dynamic” equilibrium

  11. Equilibrium 2 • Form a saturated solution of NaCl • NaCl dissolves; • Same time, NaCl forms new crystals

  12. Water is amphoteric • H2O + H2O  H3O+ + OH- • Reaction moves to right at same rate as to the left • Water is being both an acid and a base • On the other side, “conjugates” are formed • H3O+ is the conjugate acid of H2O • OH- is the conjugate base of H2O

  13. Water’s “Equilibrium Constant” • K = [H3O+][OH-] • K = 10-14 • Square root of 10-14 = 10-7 • [H3O+] = [OH-] = 10-7 • Therefore pH of pure water = 7!

  14. So now to pH of bases: • Find the pH of 0.01M NaOH • Fully ionized; therefore 0.01M OH- • [OH-] = 10-2 • K = [H3O+][OH-] • 10-14 = [H3O+] * 10-2 • 10-12 = [H3O+]; pH = 12 • OR pK = pH + POH • 14 = pH + 2 • 12 = pH

  15. And Weak Acids (or Bases) • A weak acid is one which is NOT fully ionized • Acetic Acid == HAc (or CH3COOH) • HAc + H2O  H3O+ + Ac- • Acetate ion is the conjugate base of Acetic acid • At equilibrium, HAc is largely NOT ionized • Because the reaction goes both ways, Acetate can accept a proton: from H3O+ OR from H2O Ac- + H2O  HAc + OH- • Yes, a salt made from a weak acid and a strong base is basic!

  16. Typical weak acids: • Acetic acid CH3COOH • Carbonic acid H2CO3 • Second or third H+ of phosphoric: H2PO4-1, HPO4-2

  17. So let’s make a “Buffer” • A buffer is a solution of a weak acid and the strong base salt of its conjugate base: • Acetic acid and sodium acetate 0.1M 0.1M CH3COOH + H2O  H3O+ + CH3COO-

  18. And let’s add some acid • First to water: • Add 0.01M HCl to water • pH becomes 2, right? ([H3O+] = 10-2) • But add the same acid to the buffer: 0.1M 0.1M CH3COOH + H2O  H3O+ + CH3COO- 0.11M 0.09M • [H3O+] is almost unaffected! pH stays “same”

  19. Buffer: definition • A buffer is a solution of a weak acid and its conjugate base OR • A buffer is a solution of a weak base and its conjugate acid • Which resists changes in pH when small amounts of strong acid or base are added • Blood is (or contains) a buffer!

  20. LeChatlier’s Principle • Notice that a buffer takes advantage of a reversible reaction which shifts away from the species we add: H3O+ or OH- • LeChatlier said ANY system in equilibrium will shift in such a way as to minimize the effect of a stress applied

  21. Illustration of Principle

  22. Wasn’t that fun????? • Definitions • Strong acids • pH • Water equilibrium • Weak acids • Buffers • Other equilibria • LeChatlier’s Principle

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