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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.
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Solution 1
\[\begin{array}{cc}
\ce{H}\phantom{..............}\ce{Cl}\phantom{................}\ce{F}\phantom{........}\\
\backslash\phantom{...............}\backslash\phantom{................}\backslash\phantom{......}\\
\ce{H - C - COOH Cl - C - COOH F - C - COOH}\\
/\phantom{...............}/\phantom{................}/\phantom{.......}\\
\phantom{.}\ce{\underset{acid}{\underset{Acetic}{H}}}\phantom{.....}\ce{\underset{acid}{\underset{Trichloroacetic}{Cl}}}\phantom{..}\ce{\underset{acid}{\underset{Trifluoroacetic}{F}}}\phantom{......}\end{array}\]
Among H, Cl, and F, H is the least electronegative, while F is the 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
Solution 2
The depression in freezing point for acetic acid, trichloroacetic acid and trifluoroacetic acid is in the order.
\[\begin{array}{cc}
\phantom{.........}\ce{Cl}\phantom{...................}\ce{F}\\
\phantom{.........}|\phantom{.....................}|\\
\ce{CH3COOH < Cl - C - COOH < F - C - COOH}\\
\phantom{.........}|\phantom{.....................}|\\
\phantom{.........}\ce{Cl}\phantom{...................}\ce{F}\\
\end{array}\]
In freezing points, trifluoroacetic acid records the greatest depression. This is because fluorine has the greatest effect, leading to the largest ionisation of the acid due to its high electronegativity. It exhibits the greatest freezing point depression because of the maximum amount of ions present (depression in F.P. is a colliding property that depends upon the number of particles present in the solution). Chlorine has a smaller effect and is less electronegative. Consequently, there is less trichloroacetic acid ionisation. Acetic acid is the least ionised and shows the slightest decrease in freezing point because it lacks an electronegative group bonded to its carbon atom.
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