Problem 95 - Chemistry and Chemical Reactivity

Question

Which of the following substances is/are likely to dissolve in water, and which is/are likely to dissolve in benzene \(\left(\mathrm{C}_{6} \mathrm{H}_{6}\right) ?\) (a) \(\mathrm{NaNO}_{3}\) (b) diethyl ether, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3}\) (c) naphthalene, \(\mathrm{C}_{10} \mathrm{H}_{8}\) (see page 454 for structure) (d) \(\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

(a) and (d) dissolve in water; (b) and (c) dissolve in benzene.

Step 1: Identify Solvent Polarity

Water is a polar solvent due to its bent shape and the difference in electronegativity between hydrogen and oxygen. Benzene (\( C_6H_6 \)) is a non-polar solvent because it is symmetric and lacks regions of positive and negative charge.

Step 2: Determine Solubility of \( \mathrm{NaNO}_{3} \)

\( \mathrm{NaNO}_{3} \) is a polar ionic compound composed of \( \mathrm{Na}^{+} \) and \( \mathrm{NO}_{3}^{-} \) ions. Polar substances generally dissolve in polar solvents like water due to similar intermolecular forces.

Step 3: Determine Solubility of Diethyl Ether

Diethyl ether (\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OCH}_{2} \mathrm{CH}_{3} \)) is slightly polar but mostly non-polar because of the ether group. It is more likely to dissolve in non-polar solvents like benzene, according to the principle 'like dissolves like'.

Step 4: Determine Solubility of Naphthalene

Naphthalene (\( \mathrm{C}_{10} \mathrm{H}_{8} \)) is a non-polar molecule as it is composed entirely of non-polar carbon-hydrogen bonds. It will dissolve better in non-polar solvents like benzene.

Step 5: Determine Solubility of \( \mathrm{NH}_{4} \mathrm{Cl} \)

\( \mathrm{NH}_{4} \mathrm{Cl} \) is an ionic compound and is polar due to the presence of charged particles. Thus, it tends to dissolve in polar solvents like water.

Key Concepts

Polar and Non-Polar Solvents

Solubility depends significantly on the nature of the solvent, which can be either polar or non-polar. Water, for example, is a polar solvent. This means it has positive and negative ends due to the shape of its molecule and the difference in electronegativity between the oxygen and hydrogen atoms. This makes water excellent at dissolving other polar substances or ionic compounds, because the opposite charges attract each other and help break down the solute's structure.
In contrast, benzene ( C_6H_6 ) is a non-polar solvent. Non-polar molecules like benzene have a more symmetrical charge distribution, leading to an absence of significant charge regions. Such solvents are better suited for dissolving other non-polar substances, as there are no charges to interact with polar molecules. When dealing with solubility, considering the nature of the solvent is key to predicting whether a substance will dissolve.

Ionic Compounds

Ionic compounds are substances made up of positive and negative ions. A classic example is sodium nitrate ( NaNO_3 ), which breaks up into Na^+ and NO_3^- ions in solution. Because ionic compounds consist of charged particles, they are typically soluble in polar solvents like water. The water molecules can surround and stabilize these ions through interactions between their partial charges and the ions' full charges.
Ionic compounds such as ammonium chloride ( NH_4Cl ) follow this solubility pattern as well. They dissolve effectively in water by splitting apart into their constituent ions, which are stabilized by the water's polar nature. Understanding the ionic nature of a compound is crucial to predict its solubility behavior in different solvents.

Intermolecular Forces in Solutions

The intermolecular forces between solutes and solvents play a vital role in determining solubility. Polar solvents, like water, have strong intermolecular forces such as hydrogen bonding and dipole-dipole interactions. These forces are effective in dissolving ionic or polar compounds by surrounding and stabilizing them.
Non-polar solvents, such as benzene, depend on dispersion forces or Van der Waals forces. These interactions are relatively weaker and are most effective among non-polar molecules. For instance, diethyl ether and naphthalene, which possess non-polar characteristics, will interact more favorably with benzene. When considering solubility, it's essential to analyze the dominant intermolecular forces in play.

Like Dissolves Like Principle

The 'like dissolves like' principle is a simple yet powerful guideline for predicting solubility. It states that polar solvents tend to dissolve polar compounds, while non-polar solvents dissolve non-polar compounds. This concept arises because similar types of intermolecular forces in solute and solvent favorably interact.
For example, sodium nitrate dissolves in water due to the attraction between the ionic and polar charges. Diethyl ether and naphthalene dissolve better in benzene because all are non-polar, allowing dispersion forces to predominate. Understanding this principle makes it easier to predict and explain solubility outcomes across a range of substances and solvents.