Problem 43 - Chemistry An Atoms First Approach

Question

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

Step-by-Step Solution

Answer

a. \(\mathrm{KrF}_{2}\): Carbon Tetrachloride (Nonpolar) b. \(\mathrm{SF}_{2}\): Water (Polar) c. \(\mathrm{SO}_{2}\): Water (Polar) d. \(\mathrm{CO}_{2}\): Carbon Tetrachloride (Nonpolar) e. \(\mathrm{MgF}_{2}\): Water (Polar) f. \(\mathrm{CH}_{2}\mathrm{O}\): Water (Polar) g. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\): Carbon Tetrachloride (Nonpolar)

Step 1: Determine the polarity of each molecule

To determine the polarity of each molecule, we will examine their molecular structures and electronegativities. Below, I have listed the polarity of each molecule, along with a brief explanation. a. \(\mathrm{KrF}_{2}\): Nonpolar (Linear molecular geometry, no net dipole moment) b. \(\mathrm{SF}_{2}\): Polar (Bent molecular geometry, the presence of lone pairs on sulfur creates a net dipole moment) c. \(\mathrm{SO}_{2}\): Polar (Bent molecular geometry, the presence of lone pairs on sulfur creates a net dipole moment) d. \(\mathrm{CO}_{2}\): Nonpolar (Linear molecular geometry, no net dipole moment) e. \(\mathrm{MgF}_{2}\): Polar (Ionic compound) f. \(\mathrm{CH}_{2}\mathrm{O}\): Polar (Separation of charges, oxygen has greater electronegativity than carbon and hydrogen) g. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\): Nonpolar (No electronegative atoms present, no net dipole moment)

Step 2: Choose the appropriate solvent based on the molecule's polarity

Since water is a polar solvent and carbon tetrachloride is a nonpolar solvent, we will choose the solvent accordingly. a. \(\mathrm{KrF}_{2}\): Carbon Tetrachloride (Nonpolar) b. \(\mathrm{SF}_{2}\): Water (Polar) c. \(\mathrm{SO}_{2}\): Water (Polar) d. \(\mathrm{CO}_{2}\): Carbon Tetrachloride (Nonpolar) e. \(\mathrm{MgF}_{2}\): Water (Polar) f. \(\mathrm{CH}_{2}\mathrm{O}\): Water (Polar) g. \(\mathrm{CH}_{2}=\mathrm{CH}_{2}\): Carbon Tetrachloride (Nonpolar)

Key Concepts

Molecular Polarity

Understanding the concept of molecular polarity is crucial when determining a molecule's solubility and behavior in a given solvent. Polarity refers to the distribution of electrical charge over the atoms within a molecule. A molecule may be classified as polar if there is a significant difference in electronegativity between its atoms, leading to an uneven distribution of charge. This results in one end of the molecule becoming partially negative, while the other end becomes partially positive, creating what is known as a dipole moment.

A classic example is water (H₂O), where the oxygen atom is more electronegative than the hydrogen atoms. This difference in electronegativity gives water its polar nature. In the context of your homework problem, compounds like sulfur dioxide (SO₂) and formaldehyde (CH₂O) exhibit polar characteristics due to differences in electronegativity and molecular geometry that do not cancel out, resulting in a net dipole moment.

On the other hand, molecules with symmetrical geometry and even distribution of electronegativity, such as carbon dioxide (CO₂) and ethene (CH₂=CH₂), are non-polar because they have no net dipole moment. Recognizing the polarity of molecules helps us predict their physical properties, such as solubility in various solvents.

Solvent Selection

Selecting the correct solvent is essential in many chemical processes, including the dissolution of compounds. The 'like dissolves like' principle is a simple yet fundamental rule that predicts solubility: polar solvents typically dissolve polar substances, while nonpolar solvents dissolve nonpolar substances. Water, being polar, is adept at dissolving ionic compounds and other polar molecules due to its ability to form hydrogen bonds, which stabilize the ions or polar molecules in solution.

In your textbook exercise, for example, water is the appropriate solvent for dissolving sulfur fluoride (SF₂) because of its polar nature. Water is also chosen for magnesium fluoride (MgF₂), an ionic compound, due to water's high polarity which allows it to solvate ions effectively. Conversely, non-polar molecules such as carbon tetrachloride (CCl₄) would be the solvent of choice for nonpolar molecules like krypton difluoride (KrF₂) because it can dissolve substances without a net dipole moment effectively.

Electronegativity

Electronegativity is a key concept when exploring molecular polarity and solubility. It quantifies an atom's tendency to attract the shared electrons in a chemical bond. Elements with high electronegativity, such as fluorine, oxygen, and nitrogen, tend to attract electrons more strongly, while elements with lower electronegativity, like the alkali metals, do not attract electrons as vigorously.

Understanding the differences in electronegativity among atoms in a molecule helps explain why molecules have certain shapes and exhibit polarity. For instance, the oxygen atom in sulfur dioxide (SO₂) is more electronegative than sulfur, leading to a polar molecule. This concept of electronegativity not only plays a pivotal role in determining the polarity but also influences the chemical behavior and reactivity of molecules. It's a fundamental tool for predicting molecular interactions with solvents and other substances, as seen in the exercise solutions provided.