Question

When ethanol mixes in cyclohexane, the intermolecular forces reduce between ethanol molecules. In this, liquid pair shows:
A. Positive deviation from Raoult’s law
B. Negative deviation from Raoult’s law
C. No deviation from Raoult’s law
D. Decrease in volume

Answer 1

Hint: When a non-volatile solute is added to a solvent, its vapor pressure is lowered. The quantitative relationship between the vapor pressure of the solution and the amount of the solute dissolved is given in the form of Raoult’s law.

Raoult’s law for a solution containing non- volatile solutes: - The vapor pressure of a solvent over a solution containing non- volatile solute is directly proportional to the mole fraction of the solvent at a particular temperature. ${p^s}$(Vapour Pressure of solution) $ = {p^o}$ (Vapour Pressure of pure solvent) $ \times {X_A}$ (mole fraction of solvent)

Raoult’s law for volatile solutes: - Let the solution contain two miscible liquids A and B having mole fraction ${x_A}$ and ${x_B}$. Also, let $p_A^o$ and $p_B^o$ be the vapor pressures of liquids A and B in the pure state at a certain temperature. Then according to Raoult’s law. Vapour pressure of component A in solution $ = $ Vapour pressure of liquid A in pure state $ \times $ mole fraction of A in solution

Thus, ${p_A} = p_A^o{X_A}$

Similarly ${p_B} = p_B^o{X_B}$

Thus, Raoult’s law can be defined as below:
In a solution containing volatile solute, the vapor pressure of a component at a given temperature is equal to the product of mole fraction in solution and the vapor pressure in a pure state.

Complete step by step answer:
In Ethanol (Polar) – Cyclohexane (Non- Polar), the intermolecular forces of attraction are strong in ethanol due to hydrogen bonding. On adding cyclohexane, its molecule gets in between the molecules of ethanol, thus breaking the hydrogen bonds and reducing the ethanol- ethanol attractions considerably. This process requires energy and thus some energy is absorbed and $\Delta {H_{mix}}$ is positive. Moreover, the reduction in hydrogen bonding causes an increase in volume. Thus, $\Delta {V_{mix}}$ is also positive.
The vapor pressure of such a solution is maximum at an intermediate composition and thus at this stage, its boiling point will be minimum.
The non- ideal solution of such composition distills over a constant temperature without any change in composition. Such liquid mixtures are called azeotropic mixtures and such solutions are called minimum boiling point azeotropes.

So when ethanol mixes in cyclohexane the liquid pair shows a positive deviation from Raoult’s law.

So, the correct answer is Option A .

Note: 1.Ideal solution: - An ideal solution is the one in which no volume and no enthalpy changes take place when the components are mixed in any proportion. Moreover, Raoult’s law is obeyed.
Examples of ideal solution are Benzene + Toluene, Chlorobenzene and Bromobenzene.

2.Non- ideal solution: - The solution which does not obey Raoult’s law is called ideal solutions. Unlike ideal solutions, there is a difference in the polarity and the structure of the molecules of the solute and the solvent. Depending upon the nature of the solute and the solvent, non-ideal solutions may show positive or negative deviations from ideal behavior.
Examples of non- ideal solutions showing positive deviation are Benzene and Acetone, Ethanol, and Cyclohexane.
Examples of non- ideal solutions showing negative deviation are Benzene and Chloroform, Chloroform and Acetone, Diethyl ether, and Chloroform.