Problem 44 - Chemistry An Atoms First Approach

Question

Which solvent, water or hexane \(\left(\mathrm{C}_{6} \mathrm{H}_{14}\right),\) would you choose to dissolve each of the following? a. \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) b. \(\mathrm{CS}_{2}\) c. \(\mathrm{CH}_{3} \mathrm{OH}\) d. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{CH}_{2} \mathrm{OH}\) e.\(\mathrm HCl\) {f .} \( \mathrm{C}_{6} \mathrm{H}_{6}\)

Step-by-Step Solution

Answer

a. Water b. Hexane c. Water d. Hexane e. Water f. Hexane

Step 1: Identifying compound polarity

Copper nitrate, Cu(NO3)2, is an ionic compound because it consists of a metal (Cu) and a polyatomic ion (NO3−). Ionic compounds are generally considered to be polar.

Step 2: Choosing the appropriate solvent

Since Cu(NO3)2 is polar, it will dissolve better in the polar solvent water than in the nonpolar solvent hexane. #b. CS2#

Step 1: Identifying compound polarity

Carbon disulfide, CS2, is a nonpolar compound as the electronegativity difference between carbon and sulfur is small.

Step 2: Choosing the appropriate solvent

Since CS2 is nonpolar, it will dissolve better in the nonpolar solvent hexane than in the polar solvent water. #c. CH3OH#

Step 1: Identifying compound polarity

Methanol, CH3OH, is a polar compound because the oxygen atom forms a polar bond with the hydrogen atom due to their significant electronegativity difference.

Step 2: Choosing the appropriate solvent

Since CH3OH is polar, it will dissolve better in the polar solvent water than in the nonpolar solvent hexane. #d. CH3(CH2)16CH2OH#

Step 1: Identifying compound polarity

This molecule, which is a long-chain alcohol, is largely nonpolar because it consists of a long hydrocarbon chain. The hydroxyl group at the end of the molecule is polar, but its contribution to the overall polarity of the molecule is fairly small due to the size of the hydrocarbon chain.

Step 2: Choosing the appropriate solvent

Since the hydrocarbon chain dominates the overall polarity of the molecule, it is considered more nonpolar than polar. Therefore, it will dissolve better in the nonpolar solvent hexane than in the polar solvent water. #e. HCl#

Step 1: Identifying compound polarity

Hydrogen chloride, HCl, is a polar compound due to the significant electronegativity difference between hydrogen and chlorine atoms.

Step 2: Choosing the appropriate solvent

Since HCl is polar, it will dissolve better in the polar solvent water than in the nonpolar solvent hexane. #f. C6H6#

Step 1: Identifying compound polarity

Benzene, C6H6, is a nonpolar compound as it consists of a symmetrical ring of carbon and hydrogen atoms that create an overall equal distribution of electron density.

Step 2: Choosing the appropriate solvent

Since C6H6 is nonpolar, it will dissolve better in the nonpolar solvent hexane than in the polar solvent water.

Key Concepts

Polar and Nonpolar Solvents

Solvents are substances that dissolve other materials to form a solution. They can be broadly categorized into two types: polar and nonpolar. The key difference between polar and nonpolar solvents lies in their molecular structure and the distribution of electrical charge.

Polar solvents, like water, have a partial positive charge on one end and a partial negative charge on the other. This is because the molecules in polar solvents have an unequal sharing of electrons among their atoms. As a result, polar solvents can dissolve ionic and other polar compounds effectively.

• Water is the most common polar solvent and is known as the "universal solvent."
• It can easily dissolve salts, acids, and many organic compounds like alcohols.

Nonpolar solvents, on the other hand, have an equal distribution of electrical charge. Their molecules share electrons more equally, resulting in little to no partial charges within the molecule. Hexane is an example of a nonpolar solvent.

• Hexane is good for dissolving nonpolar substances like oils and fats.

The rule of thumb "like dissolves like" helps determine the solubility of a compound in polar and nonpolar solvents.

Compound Polarity

Polarity in compounds is determined by the electronegativity of the atoms and the shape of the molecule. When there is a significant difference in electronegativity between the atoms in a molecule, the electrons are not shared equally. This leads to a polar bond.

• In polar compounds, there is a clear separation of charges, creating dipoles.
• Water, with its oxygen-hydrogen bonds, is a perfect example of a polar compound.

Nonpolar compounds have either very small or no difference in electronegativity between their atoms, and their shapes allow for an equal distribution of electrons.

• Molecules like carbon disulfide (CS2) and benzene (C6H6) are nonpolar because their electron clouds are evenly distributed over the entire molecule.

Understanding the polarity of a compound is crucial when predicting its solubility in various solvents.

Dissolution Process

The dissolution process is the interaction between solute molecules and solvent molecules. This interaction leads to the solute dissolving and distributing uniformly within the solvent.

• For polar solutes, dissolution involves interactions like hydrogen bonding and dipole-dipole attractions with polar solvents.
• In the case of ionic compounds, the solvent's polar molecules surround and separate the solute ions.

In nonpolar solutes, dissolution relies on dispersion forces (also known as Van der Waals forces) between the solute and solvent molecules.

• Nonpolar solutes, such as long-chain alcohols, prefer nonpolar solvents where these dispersion forces play a significant role.

Dissolution is essentially a balance between solvent-solvent, solute-solute, and solute-solvent interactions, guiding the overall solubility of a substance. Understanding the molecular interactions during the dissolution process helps in selecting the suitable solvent for dissolving a particular solute efficiently.