12.4.1 Expression for Ka

• The Ka expression is derived from the equilibrium constant expression.
• The Ka expression for a generic acid HA is Ka = \frac{[H^{ + }][A^{ - }]}{[HA]}</latex>.
• What does a lower Ka value indicate about an acid?

  Weaker acid
• What is the Ka value of Hydrochloric Acid ($HCl$)?


  $1.3 \times 10^{6}$
• Acetic Acid ($CH_{3}COOH$) is a weak acid.
• The symbol $[H^{ + }]$ in the Ka expression represents the concentration of hydrogen ions.
• What is the acid dissociation constant, Ka, a quantitative measure of?

  Acid strength
• The expression for $Ka$ is given by $Ka =$$\frac{[H^{ + }][A^{ - }]}{[HA]}$, where $[HA]$ represents the concentration of the undissociated acid
• A higher $Ka$ value indicates a stronger acid.
• Match the acid with its $Ka$ value and strength:


  Hydrochloric Acid ($HCl$) ↔️ $1.3 × 10^{6}$ ||| Strong

  Acetic Acid ($CH_{3}COOH$) ↔️ $1.8 × 10^{ - 5}$ ||| Weak

  Hydrocyanic Acid ($HCN$) ↔️ $6.2 × 10^{ - 10}$ ||| Very Weak
• The dissociation equation for a weak acid $HA$ in solution is $HA(aq) \rightleftharpoons H^ +$$(aq) +$$A^ - (aq)$, where A^ -</latex> represents the conjugate base.
• The double arrow in the dissociation equation of a weak acid indicates that the reaction is reversible.
• Write the dissociation equation for acetic acid $(CH_{3}COOH)$.


  $CH_{3}COOH(aq) \rightleftharpoons H^ +$$(aq) +$$CH_{3}COO^ - (aq)$
• The expression for $Ka$ is derived from the equilibrium constant expression: $Ka =$$\frac{[H^{ + }][A^{ - }]}{[HA]}$, where $[HA]$ is the concentration of the undissociated acid
• What is the dissociation equation for a generic acid $HA$ in water?


  $HA(aq) \rightleftharpoons H^{ + }(aq) +$$A^{ - }(aq)$
• The acid dissociation constant $Ka$ is derived from the equilibrium constant expression.
• The Ka expression for a generic acid HA is $Ka =$$\frac{[H^{ + }][A^{ - }]}{[HA]}$.
• Give an example of a strong acid.

  Hydrochloric Acid ($HCl$)
• Match the component in the Ka expression with its meaning:

  $[H^{ + }]$ ↔️ Concentration of hydrogen ions

  $[A^{ - }]$ ↔️ Concentration of conjugate base

  $[HA]$ ↔️ Concentration of undissociated acid
• Steps to derive the Ka expression from the dissociation equation

  1️⃣ Write the dissociation equation for a generic acid $HA$

  2️⃣ Express the equilibrium constant for the dissociation

  3️⃣ Define the terms in the equilibrium constant expression

  4️⃣ Write the final Ka expression
• The acid dissociation constant $Ka$ is a quantitative measure of the strength of an acid in solution.
• What does a higher Ka value indicate about an acid?

  Stronger acid
• Hydrochloric acid has a higher Ka value than acetic acid.
• $HA$ in the dissociation equation represents the undissociated weak acid.
• Write the dissociation equation for acetic acid ($CH_{3}COOH$).


  $CH_{3}COOH(aq) \rightleftharpoons H^ +$$(aq) +$$CH_{3}COO^ - (aq)$
• Strong acids undergo irreversible reactions, while weak acids undergo reversible reactions.
• List the key components in the Ka expression for a generic acid $HA$.


  $[H^{ + }], [A^{ - }], [HA]$
• Match the components of the Ka expression with their meanings:

  $[H^{ + }]$ ↔️ Concentration of hydrogen ions

  $[A^{ - }]$ ↔️ Concentration of conjugate base

  $[HA]$ ↔️ Concentration of undissociated acid
• What does a higher Ka value indicate about an acid's strength?

  Stronger acid
• A lower Ka value indicates a weaker acid.
• Order the following acids from strongest to weakest based on their Ka values:

  1️⃣ Hydrochloric Acid (1.3 \times10^{6}</latex>)

  2️⃣ Acetic Acid ($1.8 \times 10^{ - 5}$)

  3️⃣ Hydrocyanic Acid ($6.2 \times 10^{ - 10}$)
• Match the acid with its approximate Ka value and acid strength:

  Hydrochloric Acid ↔️ $1.3 \times 10^{6}$, Strong

  Acetic Acid ↔️ $1.8 \times 10^{ - 5}$, Weak

  Hydrocyanic Acid ↔️ $6.2 \times 10^{ - 10}$, Very Weak
• Why is Hydrochloric Acid considered a strong acid based on its Ka value?

  High Ka value
• The Ka value is significant in understanding the relative strengths of acids.
• The Ka expression is derived from the equilibrium constant for the dissociation of an acid.
• What does the Ka value help predict in acid-base equilibria?

  Extent of dissociation