Question

(ii) What are negative deviations from raoults
law and what is the cause of these
deviations? Explain with one example.


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Answer 1

Answer:

When the vapour pressure of a non-ideal solution is either higher or lower than that predicted by Raoullt's law, the solution exhibits deviations. These deviations are caused because of unequal intermolecular attractive forces between solute-solvent molecules and solute-solute or solvent-solvent molecules.

Answer 2

Answer:

Non-ideal solutions showing negative deviations  from Raoult’s law .

• In this type of deviations, the partial vapour  pressure of each component of the solution is less than  the vapour pressure as expected according to  Raoult’s law.

• These types of deviations are shown by the  solutions in which solvent-solvent and solute-solute  interactions are weaker than solvent-solute  interactions.
• In solution, the interactions among  molecules become stronger and therefore their  escaping tendency decreases which results in the  decrease in their partial vapour pressures.
• In these  solutions, the total vapour pressure of the solution  is also less than the vapour pressure as expected  according to Raoult’s law.

Examples of solutions showing negative  deviations

i) Acetone and Chloroform

ii) Chloroform and diethyl ether

iii) Chloroform and nitric acid

iv) Acetone and Aniline

v) Water and nitric acid

Explanation for negative deviation:-

• Consider a solution of acetone and chloroform.

• When acetone and chloroform are mixed, there  are new attractive forces due to intermolecular  hydrogen bonding.

• Thus, the attractive forces  become stronger and the escaping tendency of  each liquid from the solution decreases.
• Therefore,  the vapour pressure of the solution is less than as  expected according to Raoult’s law.

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