Question

Determine if each pair of compounds forms a homogeneous solution when combined. For those that form homogeneous solutions, indicate the type of forces that are involved. a. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \mathrm{aHd} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) \(\mathrm{b} . \mathrm{CBr}_{4} \mathrm{and} \mathrm{H}_{2} \mathrm{O}\) c. LiNO \(_{3}\) and \(\mathrm{H}_{2} \mathrm{O}\) d. \(\mathrm{CH}_{3} \mathrm{OH}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\)

Short answer

a. Homogeneous, London dispersion forces; b. Heterogeneous, different intermolecular forces; c. Homogeneous, ion-dipole forces; d. Heterogeneous, different intermolecular forces.

Step-by-step solution

- Evaluate solubility of nonpolar compounds

Determine if both compounds are nonpolar. Nonpolar compounds will generally dissolve in nonpolar solvents to form a homogeneous solution. For a. the two compounds are pentane (C5H12) and hexane (C6H14), both of which are nonpolar hydrocarbons. As they are both nonpolar, they will likely dissolve in each other due to London dispersion forces.

- Evaluate solubility of polar and nonpolar compounds

Assess if one compound is polar and the other is nonpolar; such mixtures usually form heterogeneous solutions. For b., CBr4 is nonpolar while H2O is polar. They will not form a homogeneous mixture because they have different polarities and thus different kinds of intermolecular forces.

- Evaluate solubility of ionic compounds in water

Determine if an ionic compound will dissolve in water, forming a homogeneous solution. Ionic compounds generally dissolve in polar solvents like water due to ion-dipole forces. For c., LiNO3 is an ionic compound and H2O is a polar solvent. LiNO3 will dissolve in H2O, forming a homogeneous solution facilitated by ion-dipole forces.

- Evaluate solubility of polar and nonpolar organic compounds

If one organic compound is polar while the other is nonpolar, they typically do not mix well. For d., CH3OH (methanol) is polar and CH3(CH2)3CH3 (pentane) is nonpolar. They will not form a homogeneous solution because of the difference in polarity and the types of intermolecular forces. Methanol will form hydrogen bonds, while pentane only experiences London dispersion forces.

Key concepts

Solubility of Compounds

Understanding solubility of compounds is fundamental in chemistry, as it helps predict which substances can be mixed together to form a homogeneous solution. A homogeneous solution is one in which the composition is uniform throughout, meaning that the dissolved substance, or solute, is evenly distributed in the solvent. The solubility of a compound depends on the chemical nature of the solute and solvent, particularly on whether they are polar or nonpolar.
For example, mixing two nonpolar compounds, such as hexane and pentane, results in a homogeneous solution because they share similar intermolecular interactions, called London dispersion forces. On the contrary, mixing polar and nonpolar compounds usually creates a heterogeneous mixture, where the substances do not mix evenly, as seen with water (H₂O) and carbon tetrabromide (CBr₄). A deeper understanding of solubility involves recognizing these patterns and applying the 'like dissolves like' principle.

Intermolecular Forces

The concept of intermolecular forces is crucial in predicting solubility and ultimately whether a mixture will be homogeneous or heterogeneous. Intermolecular forces are the attractions between molecules that influence the physical properties of substances. These forces include London dispersion forces, dipole-dipole interactions, and hydrogen bonds.
London dispersion forces are a type of van der Waals force, which are the weakest intermolecular attractions occurring between nonpolar molecules, as in the case of pentane and hexane. Dipole-dipole interactions occur between polar molecules, and hydrogen bonds, which are a subset of dipole-dipole interactions, form between molecules containing hydrogen attached to a highly electronegative atom, such as oxygen, nitrogen, or fluorine, which is why methanol can form hydrogen bonds. Understanding these forces helps in identifying why some substances mix well while others do not.

Polarity and Solubility

The relationship between polarity and solubility is a cornerstone in understanding why certain substances dissolve in others. Polarity refers to the distribution of electrical charge across a molecule. Polar molecules have a significant difference in electronegativity between atoms, resulting in a dipole moment, whereas nonpolar molecules have a more uniform distribution of charge.
Solubility is influenced by polarity because polar solvents are better at dissolving polar solutes due to their ability to form dipole-dipole interactions and hydrogen bonds, as seen with water's ability to dissolve ionic compounds like LiNO₃. In contrast, nonpolar solvents dissolve nonpolar solutes through induced dipole interactions. This concept explains why methanol, being polar, would not mix to form a homogeneous solution with the nonpolar pentane.

Ionic and Molecular Compounds

The solubility difference between ionic and molecular compounds is yet another aspect critical to understanding homogeneous solutions. Ionic compounds consist of positive and negative ions held together by ionic bonds, which give them diverse properties such as high melting and boiling points and good conductivity when dissolved. Their solubility in water is due to water's polarity and ability to stabilize the ions, which is reflected in the dissolution of LiNO₃ in water.
Molecular compounds, on the other hand, are composed of atoms linked by covalent bonds and may be polar or nonpolar. The solubility of molecular compounds depends on their polarity in relation to the solvent's polarity. Recognizing whether a substance is ionic or molecular is crucial for predicting its solubility behavior and interaction with other compounds.