Problem 13 - Chemistry and Chemical Reactivity

Question

Which pairs of liquids will be miscible? (a) \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (b) \(\mathrm{C}_{6} \mathrm{H}_{6}\) (benzene) and \(\mathrm{CCl}_{4}\) (c) \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\)

Step-by-Step Solution

Answer

(b) and (c) are miscible.

Step 1: Understand Miscibility

Miscibility refers to the ability of two liquids to mix and form a homogeneous solution. The general rule is 'like dissolves like': polar solvents mix with polar solvents, and non-polar solvents mix with non-polar solvents.

Step 2: Analyze Pair (a)

Water ( H_{2}O ) is a polar solvent because of its molecular structure, where the oxygen atom pulls electrons away from the hydrogen atoms. CH_{3}CH_{2}CH_{2}CH_{3} (butane) is a non-polar compound as its electron distribution is fairly even along its chain, and it lacks a significant dipole moment. Hence, Water and butane will not be miscible.

Step 3: Analyze Pair (b)

Benzene ( C_{6}H_{6} ) is non-polar due to its symmetric ring structure. CCl_{4} (carbon tetrachloride) is also non-polar because of its symmetrical tetrahedral shape. Since both benzene and carbon tetrachloride are non-polar, they are miscible with each other.

Step 4: Analyze Pair (c)

Water ( H_{2}O ) is polar. Acetic acid ( CH_{3}CO_{2}H ) has a carboxyl group which makes it polar. Polar solvents (like water and acetic acid) are generally miscible. Therefore, Water and acetic acid will be miscible with each other.

Key Concepts

Polar and Non-Polar Solvents

When considering the miscibility of liquids, a fundamental concept to understand is the nature of polar and non-polar solvents. Polar solvents, such as water, have molecules with a distinct separation of electric charges. This means that one end of the molecule carries a slight positive charge, while the other end carries a slight negative charge. This feature enables them to engage in hydrogen bonding and other dipole-dipole interactions with other polar molecules.

• Water ( H_2O ) is a classic example of a polar solvent, where the oxygen atom attracts electrons more than the hydrogen.
• Non-polar solvents, like butane or benzene, have an even distribution of electrons, resulting in no net dipole moment.
• This is why non-polar solvents, such as carbon tetrachloride, mix well with other non-polar solvents but not with polar solvents.

The general rule 'like dissolves like' arises from these interactions: polar mixes with polar, and non-polar mixes with non-polar.

Chemical Structure

The chemical structure of a compound plays a vital role in determining whether liquids will be miscible. Structures influence the polarity of molecules, thereby affecting their interaction.

• Water, with a bent shape, has a clear polar nature due to uneven distribution of electron density.
• Benzene ( C_6H_6 ), with its symmetrical hexagonal ring, is non-polar because electrons are evenly shared across its structure.
• Acetic acid ( CH_3CO_2H ) contains a carboxyl group, which gives it a strong polar characteristic, allowing it to mix with other polar substances like water.

Understanding these structural elements is crucial when predicting if two liquids will form a homogeneous solution.

Homogeneous Solutions

A homogeneous solution is a mixture where the composition is consistent throughout. Miscibility between two liquids usually results in a homogeneous mixture. The intermolecular forces between similar polarities contribute to the creation of such solutions.

• When water and acetic acid combine, the polar nature of each leads to hydrogen bonds forming effectively within the mixture.
• Benzene and CCl_4 result in a homogeneous solution as both are non-polar, allowing London dispersion forces to act uniformly throughout.
• However, a polar and non-polar pair like water and butane do not form a homogeneous solution, as their intermolecular forces are not compatible.

Thus, it is the uniform interaction at the molecular level that dictates whether or not a solution is homogeneous.