Understanding Acid-Base Chemistry

Overview of Acid-Base Chemistry

Deprotonation Process

• First step involves deprotonation of carbonic acid.

• Reaction:

  • Carbonic acid ($H<em>2CO</em>3$) reacts with water ($H_2O$):

  • $H<em>2CO</em>3 + H<em>2O ightleftharpoons H</em>3O^+ + HCO_3^-$.

Predicting pH

• To predict pH, utilize ICE tables and the $K_a$ values.

• Calculation Steps:

  • Set up ICE table for the first step of ionization.

  • Starting concentrations for bicarbonate ($HCO<em>3^-$) and hydronium ($H</em>3O^+$) are defined as $x$.

  • For carbonate ($CO_3^{2-}$), initial concentration is 0.

Second ICE Table

• After determining $x$ from the first ICE table, set up a second ICE table using the second $K_a$ value.

• Introduce new variable $y$:

  • Change in concentrations of $H<em>3O^+$ and $CO</em>3^{2-}$ will both increase by $y$.

• Equilibrium concentrations will be:

  • $[H_3O^+] = x + y$

  • $[CO_3^{2-}] = y$

  • $[HCO_3^-] = x - y$

Simplification of Concentration Calculations

• Total $[H_3O^+]$ concentration is approximately equal to $x$ since $y$ is negligible ($y ext{ is several orders of magnitude less than } x$).

• Therefore, $[H_3O^+] ext{ can be approximated as } x$.

Assumptions in pH Estimation

• Assumption is valid when $K<em>{a1}$ and $K</em>{a2}$ differ by at least three orders of magnitude.

• Example: if $K<em>{a1} = 10^{-7}$ and $K</em>{a2} = 10^{-10}$, the assumption holds.

• If $K<em>{a1} = 10^{-7}$ and $K</em>{a2} = 10^{-8}$, the assumption is not valid.

• $ ext{Strong acids like sulfuric acid have $K<em>{a1}$ very large, leading to significant }[H3O^+] ext{ contributions from the first ionization.}$

Example Problem: Estimating pH of 12M Solution

• Reaction:

  • Pure liquid reaction involves 12M$concentration.</p></li></ul></li><li><p>Set up equilibrium expression:</p><ul><li><p>$Ka = rac{[H3O^+][H2PO4^-]}{[H3PO4]}$</p></li></ul></li><li><p>Substituting into the expression:</p><ul><li><p>$K_a = rac{x^2}{12 - x}$</p></li></ul></li><li><p>Proceed to solve for$x$(using approximation to simplify).</p></li></ul><h4 id="4ea84271-73f5-49db-af15-17243ba85839" data-toc-id="4ea84271-73f5-49db-af15-17243ba85839" collapsed="false" seolevelmigrated="true">Additional Checks and Considerations</h4><ul><li><p>After calculations, ensure$x$exceeds$10^{-5}$.</p></li><li><p>If not, account for the contribution from water’s ionization ($1 imes 10^{-7}$).</p></li></ul><h4 id="68b71318-51af-4eb0-b116-85bd306dbf1c" data-toc-id="68b71318-51af-4eb0-b116-85bd306dbf1c" collapsed="false" seolevelmigrated="true">Lewis Acid-Base Theory</h4><ul><li><p>Lewis acid-Base definitions differ from traditional definitions.</p></li><li><p>A Lewis base donates a lone pair, whereas a Lewis acid accepts a lone pair.</p></li></ul><h5 id="ae82c7d3-57f6-4b8e-b629-d1da64e35f87" data-toc-id="ae82c7d3-57f6-4b8e-b629-d1da64e35f87" collapsed="false" seolevelmigrated="true">Examples of Lewis Acids</h5><ul><li><p>$H^+$ ions and metal cations (e.g., $Na^+, Fe^{3+}$) can act as Lewis acids.

  • Coordination compounds form when Lewis bases donate electrons to Lewis acids.

  • Example:

    • $Fe^{3+}$ can coordinate with water, pulling electron density and increasing acidity of protons in water.

  Dative Bonds

  • Dative bonds occur when both electrons in a bond come from one atom.

  • Important for understanding coordination chemistry.

  Acid-Base Definition Summary

  • Arrhenius Acid: Produces $H^+$ in water.

  • Arrhenius Base: Produces $OH^-$ in water.

  • Bronsted Acid: $H^+$ donor.

  • Bronsted Base: $H^+$ acceptor.

  Hydrolysis Example: Ammonia

  • $NH<em>3 + H</em>2O <br>ightleftharpoons NH_4^+ + OH^-$

  • Lone pair in ammonia attacking $H^+$ creates a hydroxide ion.

  • The electrophilic sites and electron movements dictate reaction pathways.