Ionization, Dissociation, and Acid/Base Strength

Ionization and Dissociation Reactions

• Ionization and dissociation reactions involve acids and bases.

Ionization and Dissociation Reactions

• Ionization and dissociation reactions involve acids and bases.

• These reactions relate to the concepts of weak and strong acids/bases, and their associated constants $K<em>a$ and $K</em>b$.

Strong vs. Weak Acids and Bases

• Strong Acids: Ionize 100% in solution, meaning all of the acid (HA) molecules break apart into ions. The reaction proceeds to completion ($\rightarrow$).

• Weak Acids: Only a portion of the acid (HA) molecules ionize. The reaction is reversible, proceeding in both directions ($\leftrightarrow$).

• Strong Bases: Dissociate 100% in solution; that is, all of the base (MOH) molecules separate into ions. The reaction proceeds to completion ($\rightarrow$).

• Weak Bases: Only a portion of the base (B) molecules dissociate. This reaction is also reversible ($\leftrightarrow$).

Ionization and Dissociation Reactions - Examples

• Ionization Reaction of Phosphoric Acid ($H<em>3PO</em>4$):

  $H<em>3PO</em>4(aq) + H<em>2O(l) \rightleftharpoons H</em>3O^+(aq) + PO_4^{-3}(aq)$

• Dissociation Reaction of Magnesium Hydroxide ($Mg(OH)_2$):

  $Mg(OH)_2(s) \rightleftharpoons Mg^{+2}(aq) + 2OH^-(aq)$

• Ionization Reaction of Oxalic Acid (Weak Acid):

  $H<em>2C</em>2O<em>4(aq) + 2H</em>2O(l) \rightleftharpoons 2H<em>3O^+(aq) + C</em>2O_4^{-2}(aq)$

• Dissociation Reaction of Iron (III) Hydroxide (Weak Base):

  $Fe(OH)_3(s) \rightleftharpoons Fe^{+3}(aq) + 3OH^-(aq)$

Acid Ionization and Base Dissociation in Water: Reactions

• Calcium Hydroxide (Strong Base): Dissociates completely.

  $Ca(OH)_2(s) \rightarrow Ca^{+2}(aq) + 2OH^-(aq)$

• Acetic Acid (Weak Acid): Ionizes partially.

  $CH<em>3COOH(aq) + H</em>2O(l) \rightleftharpoons H<em>3O^+(aq) + CH</em>3COO^-(aq)$

• Arsenic Acid (Weak Acid): Ionizes partially.

  $H<em>3AsO</em>4(aq) + 3H<em>2O(l) \rightleftharpoons 3H</em>3O^+(aq) + AsO_4^{-3}(aq)$

Acid and Base Ionization/Dissociation Constant ($K<em>a$ and $K</em>b$)

• $K<em>a$ and $K</em>b$ measure the extent to which an acid or base breaks apart into ions in solution.

• They are similar to the equilibrium constant, $K_{eq}$.

• Acid ionization and $K<em>a$: $HA(aq) + H</em>2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$

• Base dissociation and $K_b$:

  $BOH(aq) \rightleftharpoons B^+(aq) + OH^-(aq)$

$K_a$: Acid Ionization Constant

• For the reaction: $CH<em>3COOH(aq) + H</em>2O(l) \rightleftharpoons H<em>3O^+(aq) + CH</em>3COO^-(aq)$

• Remember that only gases and aqueous solutions are included in the equilibrium constant expressions. Liquids like water are excluded.

• $K_a = \frac{[Products]}{[Reactants]}$

Strong vs. Weak Acids and $K_a$

• Strong Acid: Ionizes 100%, resulting in a very large $K_a$ value.

• Weak Acid: Ionizes only to a small extent, resulting in a small $K_a$ value.

• Acid strength is relative, and the cutoff between strong and weak depends on the specific context.

$K_a$: Acid Ionization Constant - Properties

• Like $K{eq}$, $K_a$ is constant at a given temperature.

• Changing the temperature will change the $K_a$ value.

• The stronger the acid, the larger the $K<em>a$ value; for strong acids, the $K</em>a$ values are large and not really considered reversible.

• The weaker the acid, the smaller the $K_a$ value (much less than 1).

Examples of $K_a$ Values for Different Acids

Strong vs. Weak Acids and Equilibrium

• Strong Acid: Ionizes 100%. At equilibrium, the concentration of products is much higher than the concentration of reactants (products are favored).

• Weak Acid: Ionizes only partially. At equilibrium, the concentration of reactants is higher than the concentration of products (reactants are favored).

Oxalic Acid Example Question (Weak Acid)

• Write the ionization reaction for aqueous oxalic acid in water.

  $H<em>2C</em>2O<em>4(aq) + 2H</em>2O(l) \rightleftharpoons 2H<em>3O^+(aq) + C</em>2O_4^{-2}(aq)$

• Write the $K_a$ expression.

• If the concentration of hydronium ions is 0.0014M, the oxalate ion concentration is 7.0x10-4 M, and the $K_a$ for oxalic acid is 5.37x10-2, what is the initial concentration of oxalic acid that we placed in water?

Strong vs. Weak Bases

$K_b$ = Ionization Constant for a Weak Base

• $K_b$ describes the equilibrium between a base and its conjugate acid and hydroxide ion.

• Remember, $H_2O$ is a liquid and is not included in the equilibrium expression!

• $K_b$ =$\frac{[Products]}{[Reactants]}$

$K_b$ Example: Ammonia

• $NH<em>3(aq) + H</em>2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$

  • The water is not applicable in the equation.

• $K<em>b = \frac{[NH</em>4^+][OH^-]}{[NH_3]}$

$K_b$ = Base Ionization Constant - Properties

• The stronger the base, the larger the value for $K_b$.

• Since strong bases break apart completely, the values of $K<em>b$ for the strong bases are so huge that we really don’t consider them to be reversible; the value in the denominator approaches zero, $K</em>b$ approaches infinity.

• The weaker the base, the smaller the value for $K_b$ (much less than 1).

• The smaller the value of $K_b$, the weaker the base because the less $OH^-$ ions are present.

Example practice problem

What is the concentration of the hydroxide ion if the $K_b$ value of magnesium hydroxide is $7.9x10^{-6}$, $[Mg(OH)2]$= 0.59M and $[Mg^{+2}]$ = 0.036M

$Mg(OH)_2 \rightleftharpoons Mg^{+2}(</p><h4 id="d9327c8b-b03d-4e6c-ae0b-5a4827bc7fa9" data-toc-id="d9327c8b-b03d-4e6c-ae0b-5a4827bc7fa9" collapsed="false" seolevelmigrated="true">Strong vs. Weak Acids and Bases</h4><ul><li><p><strong>Strong Acids:</strong> Ionize 100$\rightarrow$).</p></li><li><p><strong>Weak Acids:</strong> Only a portion of the acid (HA) molecules ionize. The reaction is reversible, proceeding in both directions ($\leftrightarrow$).</p></li><li><p><strong>Strong Bases:</strong> Dissociate 100$\rightarrow$).</p></li><li><p><strong>Weak Bases:</strong> Only a portion of the base (B) molecules dissociate. This reaction is also reversible ($\leftrightarrow$).</p></li></ul><h4 id="6927e775-801d-4f4b-b168-7a05557d80b3" data-toc-id="6927e775-801d-4f4b-b168-7a05557d80b3" collapsed="false" seolevelmigrated="true">Ionization and Dissociation Reactions - Examples</h4><ul><li><p></p></li></ul><h5 id="edf952f5-076b-4c38-beb6-b19402f3616d" data-toc-id="edf952f5-076b-4c38-beb6-b19402f3616d" collapsed="false" seolevelmigrated="true">Ionization and Dissociation Reactions</h5><ul><li><p>Ionization and dissociation reactions involve acids and bases.</p></li><li><p>These reactions relate to the concepts of weak and strong acids/bases, and their associated constants$Ka$and$Kb$.</p></li></ul><h6 id="30e14790-9e42-4959-86e0-daa6b0025f97" data-toc-id="30e14790-9e42-4959-86e0-daa6b0025f97" collapsed="false" seolevelmigrated="true">Strong vs. Weak Acids and Bases</h6><ul><li><p><strong>Strong Acids:</strong> Ionize 100$\rightarrow$).</p></li><li><p><strong>Weak Acids:</strong> Only a portion of the acid (HA) molecules ionize. The reaction is reversible, proceeding in both directions ($\leftrightarrow$).</p></li><li><p><strong>Strong Bases:</strong> Dissociate 100$\rightarrow$).</p></li><li><p><strong>Weak Bases:</strong> Only a portion of the base (B) molecules dissociate. This reaction is also reversible ($\leftrightarrow$).</p></li></ul><h6 id="eabf57c3-6bf5-4384-895b-44d8f0207d7d" data-toc-id="eabf57c3-6bf5-4384-895b-44d8f0207d7d" collapsed="false" seolevelmigrated="true">Ionization and Dissociation Reactions - Examples</h6><ul><li><p><strong>Ionization Reaction of Phosphoric Acid ($H3PO4$):</strong><br></p><p>$H3PO4(aq) + H2O(l) \rightleftharpoons H3O^+(aq) + PO_4^{-3}(aq)$</p></li><li><p><strong>Dissociation Reaction of Magnesium Hydroxide ($Mg(OH)2$):</em></strong>*<br></p><p>$Mg(OH)*2(s) \rightleftharpoons Mg^{+2}(aq) + 2OH^-(aq)$</p></li><li><p><strong>Ionization Reaction of Oxalic Acid (Weak Acid):</strong><br></p><p>$H2C2O4(aq) + 2H2O(l) \rightleftharpoons 2H3O^+(aq) + C2O_4^{-2}(aq)$</p></li><li><p><strong>Dissociation Reaction of Iron (III) Hydroxide (Weak Base):</strong><br></p><p>$Fe(OH)_3(s) \rightleftharpoons Fe^{+3}(aq) + 3OH^-(aq)$</p></li></ul><h6 id="b401a14a-4b69-43ca-adf5-033d8437974a" data-toc-id="b401a14a-4b69-43ca-adf5-033d8437974a" collapsed="false" seolevelmigrated="true">Acid Ionization and Base Dissociation in Water: Reactions</h6><ul><li><p><strong>Calcium Hydroxide (Strong Base):</strong> Dissociates completely.<br></p><p>$Ca(OH)_2(s) \rightarrow Ca^{+2}(aq) + 2OH^-(aq)$</p></li><li><p><strong>Acetic Acid (Weak Acid):</strong> Ionizes partially.<br></p><p>$CH3COOH(aq) + H2O(l) \rightleftharpoons H3O^+(aq) + CH3COO^-(aq)$</p></li><li><p><strong>Arsenic Acid (Weak Acid):</strong> Ionizes partially.<br></p><p>$H3AsO4(aq) + 3H2O(l) \rightleftharpoons 3H3O^+(aq) + AsO_4^{-3}(aq)$</p></li></ul><h6 id="76f2395d-80bf-4c59-a706-b199e4f4161f" data-toc-id="76f2395d-80bf-4c59-a706-b199e4f4161f" collapsed="false" seolevelmigrated="true">Acid and Base Ionization/Dissociation Constant ($Ka$and$Kb$)</h6><ul><li><p>$Ka$and$Kb$measure the extent to which an acid or base breaks apart into ions in solution.</p></li><li><p>They are similar to the equilibrium constant,$K_{eq}$.</p></li><li><p>Acid ionization and$Ka$:$HA(aq) + H2O(l) \rightleftharpoons H_3O^+(aq) + A^-(aq)$</p></li><li><p>Base dissociation and$K_b$:<br></p><p>$BOH(aq) \rightleftharpoons B^+(aq) + OH^-(aq)$</p></li></ul><h6 id="f25e573f-89b6-4d7b-b63e-be1918511dae" data-toc-id="f25e573f-89b6-4d7b-b63e-be1918511dae" collapsed="false" seolevelmigrated="true">$K_a$: Acid Ionization Constant</h6><ul><li><p>For the reaction:$CH3COOH(aq) + H2O(l) \rightleftharpoons H3O^+(aq) + CH3COO^-(aq)$</p></li><li><p>Remember that only gases and aqueous solutions are included in the equilibrium constant expressions. Liquids like water are excluded.</p></li><li><p>$K_a = \frac{[Products]}{[Reactants]}$</p></li></ul><h6 id="fb7ef947-d36f-41d7-900d-190120de7dab" data-toc-id="fb7ef947-d36f-41d7-900d-190120de7dab" collapsed="false" seolevelmigrated="true">Strong vs. Weak Acids and$K_a$</h6><ul><li><p><strong>Strong Acid:</strong> Ionizes 100$K_a$value.</p></li><li><p><strong>Weak Acid:</strong> Ionizes only to a small extent, resulting in a small$K_a$value.</p></li><li><p>Acid strength is relative, and the cutoff between strong and weak depends on the specific context.</p></li></ul><h6 id="80f4f153-2cf6-49cb-9a50-9649e3fff22f" data-toc-id="80f4f153-2cf6-49cb-9a50-9649e3fff22f" collapsed="false" seolevelmigrated="true">$K_a$: Acid Ionization Constant - Properties</h6><ul><li><p>Like$K{eq}$,$Ka$is constant at a given temperature.</p></li><li><p>Changing the temperature will change the$K_a$value.</p></li><li><p>The stronger the acid, the larger the$Ka$value; for strong acids, the$Ka$values are large and not really considered reversible.</p></li><li><p>The weaker the acid, the smaller the$K_a$value (much less than 1).</p></li></ul><h6 id="45dcf4e3-cb8f-45cd-a245-970d0fb74bee" data-toc-id="45dcf4e3-cb8f-45cd-a245-970d0fb74bee" collapsed="false" seolevelmigrated="true">Examples of$K_a$Values for Different Acids</h6><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><th colspan="1" rowspan="1"><p></p></th><th colspan="1" rowspan="1"><p></p></th><th colspan="1" rowspan="1"><p></p></th></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr><tr><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td><td colspan="1" rowspan="1"><p></p></td></tr></tbody></table><h6 id="4911d4d6-c54f-4a0e-9671-d58056aac266" data-toc-id="4911d4d6-c54f-4a0e-9671-d58056aac266" collapsed="false" seolevelmigrated="true">Strong vs. Weak Acids and Equilibrium</h6><ul><li><p><strong>Strong Acid:</strong> Ionizes 100$H2C2O4(aq) + 2H2O(l) \rightleftharpoons 2H3O^+(aq) + C2O_4^{-2}(aq)$</p></li><li><p>Write the$K_a$expression.</p></li><li><p><em>If the concentration of hydronium ions is 0.0014M, the oxalate ion concentration is 7.0x10-4 M, and the$K_a$for oxalic acid is 5.37x10-2, what is the initial concentration of oxalic acid that we placed in water?</em></p></li></ul><h6 id="bac36b02-abda-4c0b-b544-7e60a7b5f973" data-toc-id="bac36b02-abda-4c0b-b544-7e60a7b5f973" collapsed="false" seolevelmigrated="true">Strong vs. Weak Bases</h6><table style="min-width: 75px"><colgroup><col style="min-width: 25px"><col style="min-width: 25px"><col style="min-width: 25px"></colgroup><tbody><tr><th colspan="1" rowspan="1" style="text-align:left;"><p></p></th><th colspan="1" rowspan="1" style="text-align:left;"><p>Complete Dissociation</p></th><th colspan="1" rowspan="1" style="text-align:left;"><p>Incomplete Dissociation</p></th></tr><tr><td colspan="1" rowspan="1" style="text-align:left;"><p><strong>Strong Bases</strong></p></td><td colspan="1" rowspan="1" style="text-align:left;"><p>Complete Dissociation</p></td><td colspan="1" rowspan="1" style="text-align:left;"><p>Incomplete Ionization – Reversible Reaction</p></td></tr><tr><td colspan="1" rowspan="1" style="text-align:left;"><p><strong>Weak Bases</strong></p></td><td colspan="1" rowspan="1" style="text-align:left;"><p>Complete Dissociation</p></td><td colspan="1" rowspan="1" style="text-align:left;"><p>Incomplete Dissociation – Reversible Reaction</p></td></tr></tbody></table><h6 id="8a463387-05db-4854-8ce7-e15540a1b8f1" data-toc-id="8a463387-05db-4854-8ce7-e15540a1b8f1" collapsed="false" seolevelmigrated="true">$K_b$= Ionization Constant for a Weak Base</h6><ul><li><p>$K_b$describes the equilibrium between a base and its conjugate acid and hydroxide ion.</p></li><li><p>Remember,$H_2O$is a liquid and is not included in the equilibrium expression!</p></li><li><p>$K_b$=$\frac{[Products]}{[Reactants]}$</p></li></ul><h6 id="51d90085-41e5-42ca-957c-f9b04549e9e6" data-toc-id="51d90085-41e5-42ca-957c-f9b04549e9e6" collapsed="false" seolevelmigrated="true">$K_b$Example: Ammonia</h6><ul><li><p>$NH3(aq) + H2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq)$</p><ul><li><p>The water is not applicable in the equation.</p></li></ul></li><li><p>$Kb = \frac{[NH4^+][OH^-]}{[NH_3]}$</p></li></ul><h6 id="8529ce18-1c35-4747-97af-7148e10a74a5" data-toc-id="8529ce18-1c35-4747-97af-7148e10a74a5" collapsed="false" seolevelmigrated="true">$K_b$= Base Ionization Constant - Properties</h6><ul><li><p>The stronger the base, the larger the value for$K_b$.</p></li><li><p>Since strong bases break apart completely, the values of$Kb$for the strong bases are so huge that we really don’t consider them to be reversible; the value in the denominator approaches zero,$Kb$approaches infinity.</p></li><li><p>The weaker the base, the smaller the value for$K_b$(much less than 1).</p></li><li><p>The smaller the value of$K_b$, the weaker the base because the less$OH^-$ions are present.</p></li></ul><h6 id="89260b54-27fe-43fe-94b4-c1c7e94cb333" data-toc-id="89260b54-27fe-43fe-94b4-c1c7e94cb333" collapsed="false" seolevelmigrated="true">Example practice problem</h6><p><em>What is the concentration of the hydroxide ion if the$Kb$value of magnesium hydroxide is$7.9x10^{-6}$,$[Mg(OH)2]$= 0.59M and$[Mg^{+2}]$= 0.036M</em></p><p>$Mg(OH)*2 \rightleftharpoons Mg^{+2}(