Question

Of \(F_{2}, C F_{4}\) and \(S F_{2}\), which substance is most soluble in water? Explain.

Short answer

Among F₂, CF₄, and SF₂, the most soluble substance in water is SF₂. This is because SF₂ is a polar molecule and can form dipole-dipole interactions with the polar water molecules, resulting in higher solubility compared to the nonpolar molecules F₂ and CF₄.

Step-by-step solution

Determine the polarity of the three substances

To determine the polarity of a molecule, we need to consider the electronegativity difference between the atoms and the molecular geometry. Let's analyze each substance: -F₂: It consists of two fluorine atoms with equal electronegativity, so the molecule is nonpolar. -CF₄: It has a tetrahedral molecular geometry with four identical carbon-fluorine bonds. Since the electronegativity difference between carbon and fluorine is canceled out by the symmetry, this molecule is also nonpolar. -SF₂: In this case, we have a sulfur atom bonded to two fluorine atoms. The electronegativity difference between sulfur and fluorine creates a polar bond. The molecular geometry of this compound is bent, and the dipoles do not cancel each other out, so this molecule is polar.

Compare the interaction of these substances with water

Water is a polar solvent, which means that its interactions with solutes are influenced by their polarity. A famous concept in solubility is "like dissolves like," which means that polar solutes tend to dissolve better in polar solvents and nonpolar solutes in nonpolar solvents. Therefore, we will now compare the interaction of the three substances with water based on their polarity: -F₂ and CF₄: As nonpolar molecules, they will not have strong interactions with the polar water molecules. Therefore, their solubility in water will be low. -SF₂: Being a polar molecule, it can form dipole-dipole interactions with water, and thus it is expected to have higher solubility in water compared to the other two substances.

Identify the most soluble substance in water

Based on the polarity analysis and the interaction with water, we can conclude that SF₂, as a polar molecule, will have the strongest interaction with polar water molecules and therefore will be the most soluble substance among F₂, CF₄, and SF₂.

Key concepts

Polarity of Molecules

When trying to understand why certain substances dissolve well in water and others do not, we need to start with the concept of molecular polarity. Polarity refers to the distribution of electric charge around a molecule. Molecules with unequal distribution of charges are considered polar, meaning they have a positive end and a negative end.

Polar molecules arise in two main scenarios: when there are electronegativity differences between bonded atoms and when the molecular geometry leads to an asymmetric distribution of these polar bonds. Electronegativity is a property of atoms that reflects how strongly they can attract electrons in a bond; fluorine, for example, is highly electronegative.

In the given exercise, SF₂ is polar because the bond between sulfur and fluorine creates a difference in electron density, and the 'bent' shape of the molecule means these differences don't cancel out. Contrastingly, F₂ is nonpolar because it consists of two identical atoms sharing electrons equally, and CF₄ is nonpolar due to its symmetrical 'tetrahedral' shape balancing out any charge differences.

Molecular Geometry

The shape or molecular geometry of a molecule has a significant impact on its polarity and consequently its solubility in water. The three-dimensional arrangement of atoms in a molecule determines how the charge is distributed and whether or not the polarities of individual bonds are neutralized by symmetry.

For instance, in a tetrahedral molecule like CF₄, the four C-F bonds are arranged symmetrically around the carbon atom, effectively canceling out each other's dipole moment and resulting in a nonpolar molecule. However, SF₂ with its 'V-shaped' or 'bent' geometry doesn't have the symmetry to cancel out the polarities of the S-F bonds, leading to an overall polar molecule.

Understanding molecular geometry isn't just about knowing the shape; it's about visualizing how different parts of the molecule interact and influence each other to produce a collective property, such as polarity, which ultimately affects solubility.

Dipole-Dipole Interactions

Among the various types of intermolecular forces, dipole-dipole interactions are particularly relevant when discussing the solubility of polar substances in water. These interactions occur between the positive end of one polar molecule and the negative end of another.

In the context of our exercise, SF₂'s polarity allows it to interact with water molecules through dipole-dipole attractions. Water itself is a highly polar molecule, with a partial negative charge near the oxygen and partial positive charges near the hydrogens. When a substance like SF₂ is introduced into water, the negatively charged fluorine atoms can attract the positively charged hydrogen atoms of water molecules, and vice versa, leading to a mixture, or solution, that is indicative of SF₂'s greater solubility compared to nonpolar F₂ and CF₄.

It's dipole-dipole interactions that often help polar molecules like sugars and salts dissolve in water, and a lack of these interactions that leave oils and fats floating on the surface, unable to mix.