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Q1.31:
The depression in freezing point of water observed for the same amount of acetic acid, trichloroacetic acid and trifluoroacetic acid increases in the order given above. Explain briefly.

Solution :

Step 1: Theoretical Foundation of Freezing Point Depression

The depression in freezing point ($\Delta T_f$) is a colligative property defined by the relation:

$\Delta T_f = i \cdot K_f \cdot m$

[Per Raoult’s Law and the colligative properties of dilute solutions]. Here, $i$ represents the van't Hoff factor, $K_f$ is the cryoscopic constant of the solvent, and $m$ is the molality of the solution. Since the concentration ($m$) and the solvent ($K_f$) are constant for the three acids, the magnitude of $\Delta T_f$ is directly proportional to the van't Hoff factor ($i$).

Step 2: Analysis of the van't Hoff Factor ($i$)

For weak electrolytes (like organic acids), the van't Hoff factor is expressed as:

$i = 1 + \alpha(n - 1)$

Where $\alpha$ is the degree of dissociation and $n$ is the number of ions produced per molecule ($n=2$ for these monoprotic acids). Thus, $i = 1 + \alpha$. As the acid strength (tendency to dissociate into ions) increases, $\alpha$ increases, which consequently increases $i$ and $\Delta T_f$.

Step 3: Inductive Effect and Acidity Comparison

The acidity of carboxylic acids is determined by the stability of the conjugate base (carboxylate ion) formed after the release of a proton ($H^+$). Electron-withdrawing groups (EWGs) stabilize the negative charge on the carboxylate group via the inductive effect ($-I$ effect).

Acid Substituent Inductive Effect
Acetic Acid ($CH_3COOH$) $-CH_3$ Weak $+I$ (Electron donating)
Trichloroacetic Acid ($CCl_3COOH$) $-Cl$ (3 atoms) Strong $-I$
Trifluoroacetic Acid ($CF_3COOH$) $-F$ (3 atoms) Very Strong $-I$

Step 4: Logical Derivation of Dissociation Order

  • Acetic Acid: The methyl group is electron-donating ($+I$ effect), which destabilizes the carboxylate ion, leading to minimal dissociation ($\alpha$ is smallest).
  • Trichloroacetic Acid: Three chlorine atoms exert a strong $-I$ effect, stabilizing the conjugate base significantly, leading to higher dissociation than acetic acid.
  • Trifluoroacetic Acid: Fluorine is more electronegative than chlorine. The three fluorine atoms exert the strongest $-I$ effect among the trio, providing the greatest stabilization to the conjugate base. Thus, $\alpha$ is largest.

Step 5: Visual Representation of Freezing Point Depression Mechanism

Magnitude of $\Delta T_f$ vs Acid Strength Acetic Trichloroacetic Trifluoroacetic

The dissociation constants ($K_a$) follow the order: $CH_3COOH < CCl_3COOH < CF_3COOH$.

Final Solution: The depression in freezing point increases in the order of acetic acid < trichloroacetic acid < trifluoroacetic acid because the electron-withdrawing inductive effect of the substituent groups increases in that order. This leads to an increase in the degree of dissociation ($\alpha$), a higher van't Hoff factor ($i$), and consequently a larger value for the colligative property $\Delta T_f$.


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