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. \(\mathrm{C}_{6} \mathrm{H}_{6}\)

Short answer

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

Step-by-step solution

Determine the polarity of each substance

. To decide the best solvent for each substance, first, we need to determine whether each substance is polar or non-polar. a. Cu(NO₃)₂: This is an ionic compound, as it has a metal (Cu) and a polyatomic ion (NO₃⁻). Ionic compounds are polar. b. CS₂: This molecule has a linear shape and has no polar bonds, making it a non-polar molecule. c. CH₃OH: Methanol contains a polar O-H bond and has a polar molecular geometry, making it a polar molecule. d. CH₃(CH₂)₁₆CH₂OH: This compound is a long-chain alcohol, primarily non-polar due to the long hydrocarbon chain, but with a polar O-H bond at the end. e. HCl: HCl is a simple covalent molecular compound with a polar bond, making it a polar molecule. f. C₆H₆: Benzene contains non-polar C-C and C-H bonds, and has a symmetric molecular geometry, making it a non-polar molecule.

Choose the best solvent for each substance based on polarity

. Now that we have determined the polarity of each substance, we can choose the best solvent for each one based on the "like dissolves like" concept. a. Cu(NO₃)₂: As an ionic compound (polar), it would dissolve better in water, a polar solvent. b. CS₂: Being a non-polar molecule, it would dissolve better in hexane, a non-polar solvent. c. CH₃OH: This polar molecule would dissolve better in water, a polar solvent. d. CH₃(CH₂)₁₆CH₂OH: This compound has both polar and non-polar regions. Due to the majority of the molecule being non-polar, it would dissolve better in hexane, a non-polar solvent. e. HCl: HCl is a polar molecule, so it would dissolve better in water, a polar solvent. f. C₆H₆: As a non-polar molecule, benzene would dissolve better in hexane, a non-polar solvent.

Key concepts

Polarity

Polarity is a fundamental concept in chemistry that relates to how a molecule or compound is oriented in space. Polar molecules have an uneven distribution of electron density, which results in a partial positive charge on one side and a partial negative charge on the other. This occurs because of differences in electronegativity between the atoms within the molecule.
For example, a water molecule (H extsubscript{2}O) is polar because of the significant difference in electronegativity between oxygen and hydrogen, creating a bent shape. At the same time, the oxygen atom carries a slightly negative charge, while hydrogen atoms have a slightly positive charge.
Understanding polarity helps in predicting solubility, boiling and melting points, and even reactivity of molecules. It is a key factor in determining how substances interact with each other, especially when it comes to dissolving them in various solvents.

Ionic Compounds

Ionic compounds consist of a conglomeration of ions held together by ionic bonds, which are the electrical attractions between positively charged cations and negatively charged anions. These strong attractions form a lattice structure, which is usually crystalline, contributing to the compounds' high melting and boiling points.
A classic example of an ionic compound is sodium chloride (NaCl), where sodium (Na extsuperscript{+}) donates an electron to chlorine (Cl extsuperscript{-}), resulting in the stable ionic bond. Because ionic compounds are polar, they tend to dissolve well in polar solvents like water.
This behavior aligns with the solubility rule "like dissolves like," meaning polar substances are more likely to dissolve in polar solvents due to similar intermolecular forces, while non-polar substances will not mix well with ionic compounds.

Like Dissolves Like

"Like dissolves like" is a guiding principle in chemistry that helps predict solubility. It suggests that solvents will dissolve solutes that have similar polarities because of compatible intermolecular forces at play. This rule is especially crucial when working with mixtures of different substances.
For instance, water is a polar solvent and typically dissolves polar substances and ionic compounds well. On the other hand, non-polar solvents like hexane are better suited to dissolve non-polar molecules.
This principle helps in choosing the correct solvent to dissolve a particular compound, making processes such as extraction, purification, and reaction predictability more efficient in both industrial and laboratory settings.

Polar and Non-Polar Solvents

Solvents play a key role in chemistry by providing a medium in which reaction takes place, or in which substances can be dissolved.

• **Polar solvents**, like water and ethanol, are characterized by molecules that have a significant difference in electronegativity between their atoms, leading to a dipole moment. They tend to dissolve ionic and other polar substances due to strong interactions such as hydrogen bonding or dipole-dipole interactions.


• **Non-polar solvents**, such as hexane and benzene, consist of molecules that have little to no difference in electronegativity across their structure. Consequently, they are better at dissolving non-polar substances due to van der Waals forces.

This differentiation in solvents is crucial for conducting chemical synthesis and analyses that require precision in solubility characteristics.