Problem 24 - Chemical Principles: The Quest for Insight

Question

Which would be the better solvent, \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{CCl}_{4}\), for each of the following: (a) \(\mathrm{NH}_{5} ;\) (b) \(\mathrm{HCl} ;\) (c) \(\mathrm{I}_{2}\) ?

Step-by-Step Solution

Answer

The better solvents are: (a) H_{2}O for NH_{3}; (b) H_{2}O for HCl; (c) CCl_{4} for I_{2}.

Step 1 - Understanding Solvent and Solute Interactions

To determine the better solvent for a solute, we must consider the polarity of both the solvent and the solute. Polar solutes dissolve better in polar solvents, and non-polar solutes dissolve better in non-polar solvents. This is based on the 'like dissolves like' principle.

Step 2 - Analyzing the Polarity of Solvents

Water (H_{2}O) is a polar molecule due to its bent shape and the difference in electronegativity between hydrogen and oxygen. Carbon tetrachloride (CCl_{4}), however, is nonpolar because its molecular geometry is symmetrical, causing the polarities of the C-Cl bonds to cancel out.

Step 3 - Analyzing Each Solute

Determine the polarity of each solute: (a) NH_{3} (ammonia) is polar due to its trigonal pyramidal shape and a lone pair of electrons, making water a better solvent. (b) HCl (hydrogen chloride) is also polar because of the dipole moment between hydrogen and chlorine, so water is a better solvent. (c) I_{2} (iodine) is non-polar because it is a diatomic molecule of the same element, making CCl_{4} the better solvent.

Key Concepts

Polarity of Molecules

Understanding the polarity of molecules is crucial when it comes to predicting and explaining how substances will interact with one another. In essence, polarity refers to the distribution of electrical charge over the molecules. When there are differences in electronegativity between bonded atoms, polar molecules such as water (\(\mathrm{H}_{2} \mathrm{O}\)) result, showing partial positive and negative charges at different parts of the molecule. This is largely related to the geometry of the molecule and the atoms involved.

For instance, the oxygen atom in water has a higher electronegativity than the hydrogen atoms, pulling the shared electrons towards itself and creating a negative pole around the oxygen and positive poles around the hydrogens. This uneven charge distribution allows water to form hydrogen bonds with other polar substances, which is essential for many of its solvent properties.

Like Dissolves Like Principle

The 'like dissolves like' principle is a simple way to predict whether a solvent can dissolve a given solute. This aphorism means that polar solvents, such as water, are good at dissolving polar solutes, due to the attraction between the opposite charges of the polar molecules. Conversely, non-polar solvents like carbon tetrachloride (\(\mathrm{CCl}_{4}\)) are better at dissolving non-polar solutes, because there is no charge repulsion to overcome.

• If you're trying to dissolve table salt (\(\mathrm{NaCl}\)), which is polar, water is your best bet.
• However, for something like oil, which is non-polar, you'd want to use a non-polar solvent, such as \(\mathrm{CCl}_{4}\).

The principle is used widely in chemistry to design experiments and also in industries such as pharmaceuticals, where drugs must be dissolved in solvents that match their polarity.

Molecular Geometry

Molecular geometry plays a significant role in determining whether a molecule is polar or non-polar. This is because the shape of a molecule determines how the charge is distributed, and therefore, its interactions with other substances. The VSEPR (Valence Shell Electron Pair Repulsion) theory aids in predicting the 3-dimensional shape of molecules.

For example, water has a bent or V-shaped geometry, leading to an uneven distribution of charge, thus making it polar. Carbon tetrachloride, on the other hand, with its symmetrical tetrahedral structure, is non-polar because the bond polarities cancel each other out. \(\mathrm{NH}_{3}\) (ammonia), with its trigonal pyramidal shape, has a lone electron pair that contributes to its polarity. Recognizing these shapes helps us understand why certain molecules behave as they do in solution.