Revision in shorts Raoult’s Law: “The partial vapour | CBSE OFFICIAL ✅
Revision in shorts
Raoult’s Law:
“The partial vapour pressure of any component in the solution is directly proportional to its mole fraction”.
For a binary solution of two components A and B,
PA = XA
PB = XB
Where
P0A = vapour pressure of component A in pure state.
PA = vapour pressure of component A in the solution.
P0B = vapour pressure of component B in pure state.
PB = vapour pressure of component B in the solution
Limitations of Raoult’s Law:-
Raoult’s law is applicable only to very dilute solutions.
It is applicable to solutions containing non-volatile solute only.
It is not applicable to solutes which dissociate or associate in a particular solution
Raoult’s Law in Combination with Dalton’s Law of Partial Pressure:
[[PT = XA P0A + XB P0B = P0B + (P0A -P0B) XA ]]]]
Where
PT = Total Vapour Pressure of the Solution.
Ideal and Non-Ideal Solutions:
Ideal Solution:
These solutions obey Raoult’s law at all compositions of solute in solvent at all temperature
Two liquids A and B form and ideal solution when A –A and B–B molecular attractions will be same and hence A–B molecular attraction will be almost same as A–A and B–B molecular attraction.
For Ideal Solution:
1. dHmixing = 0, i.e. no heat should be absorbed or evolved during mixing
2. dVmixing = 0, i.e. no expansion or contraction on mixing
Examples , Ethyl chloride and ethyl bromide, n–hexane and n–heptane , CCl4 and SiCl4
Non-Ideal Solution:
These solutions deviate from ideal behaviour and do not obey Raoult’s law over entire range of composition.
For non ideal solutions,
1. dHmixing ≠ 0
2. dHmixing ≠ 0
Here we may have two cases
A) Positive Deviation:
1. PA > XA & PB > XB
2. dHmix > 0
3. dVmix > 0
Example: Cyclohexane and Ethanol
B) Negative Deviation:
1. PA > XA & PB > XB
2. dHmix < 0
3. dVmix < 0
