Question

Which solvent, water or carbon tetrachloride, would you choose to dissolve each of the following? a. \(\mathrm{KrF}_{2}\) e. \(\mathrm{MgF}_{2}\) b. \(\mathrm{SF}_{2}\) f. \(\mathrm{CH}_{2} \mathrm{O}\) c. \(\mathrm{SO}_{2}\) g. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) d. \(\mathrm{CO}_{2}\)

Short answer

The following substances dissolve better in the given solvents: a. \(\mathrm{KrF}_{2}\) in carbon tetrachloride. e. \(\mathrm{MgF}_{2}\) in water. b. \(\mathrm{SF}_{2}\) in water. f. \(\mathrm{CH}_{2} \mathrm{O}\) in water. c. \(\mathrm{SO}_{2}\) in water. g. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\) in carbon tetrachloride. d. \(\mathrm{CO}_{2}\) in carbon tetrachloride.

Step-by-step solution

Solubility of \(\mathrm{KrF}_{2}\)

\(\mathrm{KrF}_{2}\) is a nonpolar molecule because the electron distribution between the two fluorine atoms and the krypton atom is even. Therefore, it will dissolve better in a nonpolar solvent like carbon tetrachloride.

Solubility of \(\mathrm{MgF}_{2}\)

\(\mathrm{MgF}_{2}\) is an ionic compound due to the presence of magnesium and fluoride ions. Ionic compounds are generally soluble in polar solvents. Thus, it will dissolve better in water.

Solubility of \(\mathrm{SF}_{2}\)

\(\mathrm{SF}_{2}\) is a polar molecule as sulfur has a partial positive charge and the two fluorine atoms have partial negative charges. Since the molecule is polar, it will dissolve better in a polar solvent like water.

Solubility of \(\mathrm{CH}_{2} \mathrm{O}\)

\(\mathrm{CH}_{2} \mathrm{O}\), also known as formaldehyde, is a polar molecule due to the presence of a polar carbonyl group and polar carbon-hydrogen bonds. As it is polar, it will dissolve better in a polar solvent like water.

Solubility of \(\mathrm{SO}_{2}\)

\(\mathrm{SO}_{2}\) is a polar molecule due to the presence of two polar sulfur-oxygen bonds. As such, it will dissolve better in a polar solvent like water.

Solubility of \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\)

\(\mathrm{CH}_{2}=\mathrm{CH}_{2}\), also known as ethene or ethylene, is a nonpolar molecule because there is no net separation of charge between the atoms. Therefore, it will dissolve better in a nonpolar solvent like carbon tetrachloride.

Solubility of \(\mathrm{CO}_{2}\)

\(\mathrm{CO}_{2}\) is a nonpolar molecule even though the individual carbon-oxygen bonds are polar. This is due to the linear geometry of the molecule, which results in the net distribution of charge being uniform across the molecule. As such, it will dissolve better in a nonpolar solvent like carbon tetrachloride.

Key concepts

Solvent Selection

The selection of an appropriate solvent is a critical step in many chemical processes, including dissolving substances. Solvents can be broadly categorized as polar or nonpolar. Polar solvents, like water, have molecules with an uneven distribution of electrical charge, making them effective at dissolving ionic compounds and other polar substances. In contrast, nonpolar solvents such as carbon tetrachloride have molecules with a more even distribution of electrical charge, allowing them to dissolve nonpolar substances effectively. The key to solvent selection lies in the 'like dissolves like' principle, meaning that a polar solvent will better dissolve polar substances and a nonpolar solvent will better dissolve nonpolar substances.

When faced with a difficult dissolution challenge, understanding the nature of both the solute and the solvent is imperative. For substances that do not fit neatly into one category, solubility might vary, requiring a deeper analysis of molecular interactions and geometry to pick the most suitable solvent.

Polar and Nonpolar Molecules

Molecules are classified as polar or nonpolar based on their geometry and the electronegativity of the atoms involved. Polar molecules have regions of positive and negative charge due to an unequal distribution of electrons across their bonds. Common polar bonds involve hydrogen bonding with oxygen, nitrogen, or fluorine. On the other hand, nonpolar molecules have a more symmetric charge distribution and their bond dipoles cancel out, resulting in no overall polarity.

For instance, CO₂ is nonpolar because of its linear shape and equal bond dipoles that cancel each other out. This concept is crucial when predicting solubility and interactions between molecules. The electronegativity difference between atoms and the symmetry of the molecule are significant factors that determine whether a molecule is polar or nonpolar.

Ionic Compounds

Ionic compounds consist of positively charged ions (cations) and negatively charged ions (anions) held together by strong ionic bonds. These compounds typically have high melting and boiling points due to the strong attraction between the ions. Because of their charge, ionic compounds are well-suited to dissolve in polar solvents, which can stabilize the separate ions through electrostatic interactions. For example, MgF₂ is an ionic substance that will dissolve in water, as water's polar molecules are effective at surrounding and stabilizing the magnesium and fluoride ions, allowing the compound to dissociate and dissolve.

Molecular Geometry

Molecular geometry has a profound impact on the polarity and, in turn, the solubility of molecules. The shape of a molecule is determined by the positions of its constituent atoms in three-dimensional space. This geometry dictates how charge is distributed across the molecule, influencing whether molecular dipoles cancel out or reinforce each other. For instance, the linear geometry of CO₂ allows the two polar C-O bonds to cancel out, leading to a nonpolar molecule overall. Understanding the VSEPR (Valence Shell Electron Pair Repulsion) theory can provide insight into molecular geometry, helping us predict the physical and chemical properties of molecules, including their solubility in different solvents.