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Understanding the Strengths of Acids and Bases: Strong vs Weak

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emory
9 diapositives · 287 Views · Jul 21, 2014

This guide explores the strengths of acids and bases, focusing on how strong acids fully ionize in water versus weak acids that only partially ionize. Key examples demonstrate the behavior of strong acids like HCl and H2SO4, and weak acids such as acetic acid (CH3COOH). We introduce the concept of the acid dissociation constant (Ka) and the significance of equilibrium in acid-base reactions. Additionally, we cover strong bases, their dissociation in aqueous solutions, and how to determine the Ka of an acid through equilibrium expressions.

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Understanding the Strengths of Acids and Bases: Strong vs Weak
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  1. Chemistry 122 Strengths of Acids and Bases

  2. Strong Acids and Bases • The strength of an acid depends on how much it ionizes in water • Strong acids completely ionize, releasing all their H+ • Ex. HCl and H2SO4 • HCl(g) + H2O(l)→H3O+(aq) + Cl-(aq) (100% ionized) • Weak acids only ionize slightly in solution, only releasing a small amount of H+ • Ex. Acetic acid, CH3COOH • CH3COOH(aq) + H2O(l)↔ H3O+(aq) + CH3COO-(aq)

  3. Acid Dissociation Constant • With a weak acid like vinegar, less than 1% of the H+(aq) or H3O+(aq) are being ionized at any given moment • The level of ionization determines the strength of the acid • The more it ionizes, the greater its strength • The equilibrium constant can be written as follows: Keq = [H3O+]x[CH3COO-]/[CH3COOH]x[H2O]

  4. Dilute Solutions • The concentration of water is constant • It can be combined with Keq to give an acid dissociation constant • This is the ratio of the dissociated (ionized) form of the acid to the concentration of the undissociated form of the acid Keq = Ka = [H3O+]x[CH3COO-]/[CH3COOH] • Strong acids have high Ka values and weak acids are small

  5. Table 19.7, p. 607 • Name the monoprotic, diprotic and triprotic acid. • What do you notice about the polyprotic acids with respect to their Ka? • What is considered to be the strongest of the weak acids? Which ionization constant is considered to be the weakest?

  6. What makes a strong acid – strong? • In solution, the negative ion of an acid is a base • In strong acids, the negative ion is a very weak base • Its ability to combine with a hydrogen ion is slight • Weak acids contain an anion that is considered to be a strong base • This negative ion will combine with a hydrogen ion readily to form an acid again • Concentration is different from acid and base strength. The equilibrium of a strong acid or base favours products. Weak acids and bases' equilibrium favours reactants.

  7. Base Dissociation Constant • Strong bases completely dissociate into metal ions and hydroxide ions in aqueous solution • Even though they are not always soluble, what does dissolve, dissociates completely in water • Ex. Ca(OH)2 • Weak bases – NH3(aq) + H2O(l)↔ NH4+(aq) + OH-(aq) • The reactants are favoured at equilibrium.

  8. Base Dissociation Constant Keq = [NH4+]x[OH-]/[NH3]x[H2O] • Again, water is a constant in dilute solutions… Keq = Kb = [NH4+]x[OH-]/[NH3] The general form of the equation is as follows: Kb = [conjugate acid]x[OH-]/[base]

  9. Determining the Ka of an acid Ka = [H+] x [A-]/[HA] Sample Problem 19.5, p. 610 Questions 22, 23