Solutions
8.0 Abnormal Colligative Properties
8.0 Abnormal Colligative Properties
For ionic (electrolytic) solutes number of particles are different then the number of particles actually added, due to association or disassociation of solute.
For example: $1$ mole of $KCl$ breaks in ${K^ + }$ and $C{l^ - }$ which contributes as two particles and as we know that colligative properties depends on number of particle, so the observed/actual/experimental value of colligative properties will be different from theoretically predicted values. This is known as Abnormal colligative property.
The extent of association or disassociation is different for different solutes.
For example: $NaCl$(strong electrolyte) breaks completely in ions($N{a^ + }$ and $C{l^ - }$) while $C{H_3}COOH$ (Weak electrolyte) in not going to completely ionise in ions in water.
The extent of disassociation or association is expressed as a correction factor called vant hoff factor ($i$).
$$i = \frac{{moles\;of\;particles\;after\;disassociation\;or\;association}}{{moles\;of\;solute\;dissolved}}$$
As If solute associates or disassociates in solution then experimental/observed/actual value of colligative property is different from theoretical value and this is related to vant-hoff factor.
$$i = \frac{{experimental\;or\;observed\;or\;actual\;value\;of\;colligative\;property}}{{theoretical\;value\;of\;colligative\;properties}}$$
$$i = \frac{{experimental\;or\;observed\;or\;actual\;number\;of\;particles\;or\;concentration}}{{theoretical\;number\;of\;particle\;or\;concentration}}$$
$$i = \frac{{theoretical\;molar\;mass\;of\;solute}}{{experimental\;molar\;mass\;of\;solute}}$$
If,
- $i < 1$ $ \Rightarrow $ disassociation of solute
- $i > 1$ $ \Rightarrow $ association of solute
- $i = 1$ $ \Rightarrow $ Normal colligative property
Case 1:
If disassociation occurs ($i > 1$). $\alpha $ = degree of disassociation
then, vant-hoff factor is given by,
$$i = 1 + (n - 1)\alpha $$
where $n$ is number of particles in which one molecule of electrolyte disassociates.
For example,
- $n$ for $NaCl$ is $2$ and $n$ for $MgC{l_2}$ is $3$ .
- $NaCl$ $100\% $ ionised (means $\alpha $ is $1$) then $i = 1$
- $MgC{l_2}$ $100\% $ ionised then $i = 3$ (same as value of '$n$' if $100\% $ ionised)
- ${K_4} [\kern-0.15em[ Fe{(CN)_6} ]\kern-0.15em] $ $75\% $ ionised ($\alpha $ = $0.75$) then $i = 4$ (by using formula $n=5$).
Case 2:
If disassociation occurs ($i < 188$). $\beta $ = degree of association
then, vant-hoff factor is given by,
$$i = 1 + \left( {\frac{1}{{n - 1}}} \right)\beta $$
If, molecules dimerise, $n= 2$, trimerise $n =3$, tetramerise $n= 3$
- $C{H_3}COOH$ $100\% $ dimerise in benzene then $i = 0.5$,
- ${C_6}{H_5}COOH$ is dimerise in benzene then $i = 0.5$.
