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Acid-Base Theories

Acid-Base Theories. The “Boyz”. Arrhenius Theory of Acids. Acid: molecular substances that breaks-ups in aqueous solution into H+ and anions H + (“hydrogen ions” or “protons”) H + gives acids its protons Example: HNO 3(aq)  H + (aq) + NO 3 – (aq). Common Acids.

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Acid-Base Theories

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  1. Acid-Base Theories The “Boyz”

  2. Arrhenius Theory of Acids • Acid: molecular substances that breaks-ups in aqueous solution into H+ and anions • H+ (“hydrogen ions” or “protons”) • H+ gives acids its protons • Example: HNO3(aq) H+(aq) + NO3–(aq) Acid and Base Theories

  3. Common Acids Acid and Base Theories

  4. Arrhenius Theory of Bases • Base: releases OH– ions in aqueous solution • OH– (hydroxide) • OH– gives bases their properties • Example: NaOH (aq) Na+ (aq) + OH– (aq) Acid and Base Theories

  5. Common Bases Acid and Base Theories

  6. Limitations of Arrhenius Theory • H+ does not exist in solution • More likely to find H+ attached to H2O (hydrated) • H3O+ • Some bases, like ammonia, do not fit this definition, the solution is basic, but the compound does not dissociate, forming hydroxide ion • NH3(g) + H2O (l) NH4+(aq) + OH-(aq) • Limited to the solvent water, but acid-base reactions can occur in other solvents Acid and Base Theories

  7. Bronsted and Lowry • Thomas Lowry • London (England) Johannes Bronsted Copenhagen (Denmark) Acid and Base Theories

  8. Brønsted–LowryTheory of Acids and Bases • Acid: proton (H+) donor HCl + NH3 NH4+ + Cl– • HCl donates a H+ to NH3 • H+ does not exist by itself Acid and Base Theories

  9. Bases: accept a proton H2O + NH3 NH4+ + OH– • NH3 accepts a H+ from H2O • A Brønsted -Lowry acid must have a H in its • formula (like an Arrhenius acid) • Any negative ion can be a Brønsted -Lowry • base Acid and Base Theories

  10. For an acid-base reaction: • There must be a transfer of a proton • A substance can behave as an acid, if another substance behaves as a base • i.e. there is a proton donor (acid) and a proton acceptor (base) HCl (aq) + H2O (l) H3O+(aq) + Cl-(aq) acid base conjugate conjugate acid base Acid and Base Theories

  11. Conjugate acid-base pair • The two molecules or ions related by transfer of a proton from one to the other Example. HCl (aq) and H2O (l) • HCl donates the proton and H2O receives the proton Acid and Base Theories

  12. Conjugate base of an acid • The particle remaining when the proton is removed from the acid • HCl (aq) – acid • Remove proton  Cl-(aq) (conjugate base) Conjugate acid of a base • The particle produced when a base receives a proton • H2O (l) – base • Add proton  H3O+(aq) (conjugate acid) Acid and Base Theories

  13. Conjugate Acid-Base Pair Practce H2SO4 + H2O  HSO4- + H3O+ H2SO4 & HSO4- H3O+ & H2O HCl + NH3  Cl- + NH4+ HCl & Cl- NH4+ & NH3 NH3 + H2O  NH4+ + OH- H2O + OH- NH4+ & NH3 Acid and Base Theories

  14. Table 1: Comparing the Arrhenius and Brønsted-Lowry Theory Acid and Base Theories

  15. The Acid Ionization Constant, Ka • The reaction of a weak acid, HA, with water forms a dynamic equilibrium involving H3O+ and a conjugate base, A- HA(aq) + H2O(l) ⇋ H3O+(aq) + A-(aq) Acid and Base Theories

  16. The equilibrium constant, Ka, is called the acid ionization constant • Ka is used for reactions in which an acid, HA(aq), reacts with water to form a conjugate base, A-(aq) Acid and Base Theories

  17. The equilibrium law for HA(aq) + H2O(l) ⇋ H3O+(aq) + A-(aq) Ka = Acid and Base Theories

  18. Ka = • We know H+(aq) does not exist as individual ions in aqueous solution; rather, every hydrogen ion is bound to water molecule as a hydronium ion, H3O+(aq) • For convenience, we can simply write HA(aq) separating into H+(aq) and A-(aq) Acid and Base Theories

  19. Ka= • Second, when the concentration of a substance remains constant during a reaction, that substance is omitted from the equilibrium law equation • In a dilution solution, we assume the concentration of water remains constant, therefore we can omit H2O(l) Acid and Base Theories

  20. Ka = Simplified… where [H+(aq)] is equivalent to [H3O+(aq)] Acid and Base Theories

  21. A Competition for Protons HA(aq) + H2O(l) ⇋ H3O+(aq) + A-(aq) acid base conjugate conjugate acid base • If H2O has a much greater affinity for H+ than does A- (if it is a stronger base), the equilibrium position will be far to the right • Most dissolved acid will be in the ionized form at equilibrium Acid and Base Theories

  22. A Competition for Protons HA(aq) + H2O(l) ⇋ H3O+(aq) + A-(aq) acid base conjugate conjugate acid base • If A- has a much greater affinity for H+ than does H2O, the equilibrium position will be far to the left • Most dissolved acid will be present at equilibrium as HA(aq) molecules Acid and Base Theories

  23. Learning Checkpoint p. 532 Practice UC # 1, 2, 3

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