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Section 18-2

Section 18.2 Strengths of Acids and Bases. Relate the strength of an acid or base to its degree of ionization. Compare the strength of a weak acid with the strength of its conjugate base.

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Section 18-2

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  1. Section 18.2 Strengths of Acids and Bases • Relate the strength of an acid or base to its degree of ionization. • Compare the strength of a weak acid with the strength of its conjugate base. • Explain the relationship between the strengths of acids and bases and the values of their ionization constants. In solution, strong acids and bases ionize completely, but weak acids and bases ionize only partially. Section 18-2

  2. Acid Dissociation HA  H+ + A- AcidProtonConjugate base Alternately, H+ may be written in its hydrated form, H3O+ (hydronium ion)

  3. Strengths of Acids (cont.) • The equilibrium constant, Keq, provides a quantitative measure of the degree of ionization of an acid. • The acid ionization constant is the value of the equilibrium constant expression for the ionization of an acid, Ka. • Kaindicates whether products or reactants are favored at equilibrium. Section 18-2

  4. Dissociation of Strong Acids Large Ka or small Ka? Reactant favored or product favored?

  5. Acids are Proton (H+ ion) Donors Strong acids are assumed to be 100% ionized in solution (good H+donors). HCl H2SO4 HNO3 • Because they produce the maximum number of hydrogen ions, strong acids are good conductors of electricity (electrolytes).

  6. Strengths of Acids (cont.) • With a strong acid, the conjugate base is a weak base. • Equilibrium lies almost completely to the right in the equation because the conjugate base has a weaker attraction for the H+ ion than does the base in the forward reaction. Section 18-2

  7. Acids that ionize only partially in dilute aqueous solutions are called weak acids. Weak acids are usually less than 5% ionized in solution (poor H+donors). H3PO4 HC2H3O2 Organic acids

  8. Dissociation of Weak Acids Large Ka or small Ka? Reactant favored or product favored?

  9. Strengths of Acids (cont.) • In a weak acid, the ionization equilibrium lies to the far left in the ionization equation because the conjugate base has a greater attraction for H+ ions than does the base in the forward reaction. Section 18-2

  10. Strengths of Acids (cont.) • For weak acids, the products tend to be smaller compared to the un-ionized molecules (reactant). • Weaker acids have a smaller Ka. Section 18-2

  11. Strengths of Bases • A base that dissociates completely into metal ions and hydroxide ions is known as a strong base. • A weak base ionizes only partially in dilute aqueous solution. Section 18-2

  12. Strengths of Bases (cont.) • The base ionization constant, Kb, is the value of the equilibrium constant expression for the ionization of a base. Section 18-2

  13. A B C D Section 18.2 Assessment A solution with a small Kb is a ____. A.weak acid B.weak base C.strong acid D.strong base Section 18-2

  14. A B C D Section 18.2 Assessment Where is the equilibrium point in the ionization equation for a strong acid? A.far right B.far left C.slightly right D.slightly left Section 18-2

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