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Question
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
\[\begin{array}{cc}
\ce{H}\phantom{..............}\ce{Cl}\phantom{................}\ce{F}\phantom{........}\\
\backslash\phantom{...............}\backslash\phantom{................}\backslash\phantom{......}\\
\ce{H - C - COOH Cl - C - COOH F - C - COOH}\\
/\phantom{...............}/\phantom{...............}/\phantom{.......}\\
\phantom{.}\ce{\underset{Acetic acid}{H}}\phantom{.....}\ce{\underset{Trichloroacetic acid}{Cl}}\phantom{...}\ce{\underset{Trifluoroacetic acid}{F}}\phantom{.......}\end{array}\]
Among H, Cl, and F, H is least electronegative while F is most electronegative. Then, F can withdraw electrons towards itself more than Cl and H. Thus, trifluoroacetic acid can easily lose H+ ions i.e., trifluoroacetic acid ionizes to the largest extent. Now, the more ions produced, the greater is the depression of the freezing point. Hence, the depression in the freezing point increases in the order:
Acetic acid < trichloroacetic acid < trifluoroacetic acid
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