Question

Which substance in each pair would be expected to be less volatile? Explain your reasoning. a. \(\mathrm{C}_{4} \mathrm{H}_{10}(l)\) or \(\mathrm{H}_{2} \mathrm{O}(l)\) b. \(\mathrm{NH}_{3}(l)\) or \(\mathrm{NH}_{2} \mathrm{OH}(l)\) c. \(\mathrm{CH}_{3} \mathrm{OH}(l)\) or \(\mathrm{CH}_{4}(l)\)

Short answer

In each pair, the less volatile substance is: a. \(\mathrm{H}_{2} \mathrm{O}(l)\) because it has hydrogen bonding which results in stronger intermolecular forces. b. \(\mathrm{NH}_{2} \mathrm{OH}(l)\) because it has a larger molecular structure, leading to stronger dispersion forces. c. \(\mathrm{CH}_{3} \mathrm{OH}(l)\) because it has hydrogen bonding which results in stronger intermolecular forces.

Step-by-step solution

Identify the Intermolecular Forces

Start by identifying the types of intermolecular forces present in each substance, focusing on dispersion forces, dipole-dipole interactions, and hydrogen bonding. a. \(\mathrm{C}_{4} \mathrm{H}_{10}(l)\) has only dispersion forces. \(\mathrm{H}_{2} \mathrm{O}(l)\) has dispersion forces, dipole-dipole interactions, and hydrogen bonding. b. \(\mathrm{NH}_{3}(l)\) has dispersion forces, dipole-dipole interactions, and hydrogen bonding. \(\mathrm{NH}_{2} \mathrm{OH}(l)\) has dispersion forces, dipole-dipole interactions, and hydrogen bonding. c. \(\mathrm{CH}_{3} \mathrm{OH}(l)\) has dispersion forces, dipole-dipole interactions, and hydrogen bonding. \(\mathrm{CH}_{4}(l)\) has only dispersion forces.

Compare Intermolecular Forces

Now, we'll compare the strengths of the intermolecular forces in each substance within each pair to understand their volatility differences. a. \(\mathrm{C}_{4} \mathrm{H}_{10}(l)\) vs. \(\mathrm{H}_{2} \mathrm{O}(l)\): Since \(\mathrm{H}_{2} \mathrm{O}(l)\) has hydrogen bonding, it will have stronger intermolecular forces than \(\mathrm{C}_{4} \mathrm{H}_{10}(l)\), which has only dispersion forces. b. \(\mathrm{NH}_{3}(l)\) vs. \(\mathrm{NH}_{2} \mathrm{OH}(l)\): Both have hydrogen bonding, but \(\mathrm{NH}_{2} \mathrm{OH}(l)\) has a larger molecular structure than \(\mathrm{NH}_{3}(l)\), which results in stronger dispersion forces. c. \(\mathrm{CH}_{3} \mathrm{OH}(l)\) vs. \(\mathrm{CH}_{4}(l)\): \(\mathrm{CH}_{3} \mathrm{OH}(l)\) has hydrogen bonding, but \(\mathrm{CH}_{4}(l)\) does not, resulting in stronger intermolecular forces for \(\mathrm{CH}_{3} \mathrm{OH}(l)\).

Determine the Less Volatile Substance

Finally, we can determine the less volatile substance in each pair based on the intermolecular forces comparison. a. \(\mathrm{H}_{2} \mathrm{O}(l)\) is less volatile than \(\mathrm{C}_{4} \mathrm{H}_{10}(l)\) because it has stronger intermolecular forces due to hydrogen bonding. b. \(\mathrm{NH}_{2} \mathrm{OH}(l)\) is less volatile than \(\mathrm{NH}_{3}(l)\) because it has stronger dispersion forces due to its larger molecular structure. c. \(\mathrm{CH}_{3} \mathrm{OH}(l)\) is less volatile than \(\mathrm{CH}_{4}(l)\) because it has stronger intermolecular forces due to hydrogen bonding.

Key concepts

Volatility in Chemistry

Volatility refers to how easily a substance can vaporize, or turn into a gas, at a given temperature. In chemistry, volatility is highly dependent on the strength of the intermolecular forces present within a substance.

Substances with weak intermolecular forces become gases more readily which makes them more volatile. On the other hand, strong intermolecular forces hold molecules tightly together and resist vaporization, resulting in lower volatility. Understanding the concept of volatility is essential when predicting how substances will behave under various conditions, such as in distillation processes or when assessing the evaporation rate of liquids.

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole interaction and occurs when a hydrogen atom is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. The electronegative atom attracts electrons, creating a region of partial negative charge that can interact with a hydrogen atom attached to another molecule.

This interaction is particularly strong compared to other dipole-dipole interactions and is responsible for many unique properties of substances, such as the high boiling point of water. Substances with hydrogen bonds are less volatile because the molecules are held together strongly, requiring more energy to separate them and transition into the gas phase.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another molecule. These forces are significant when the molecules are close together but decrease rapidly as the distance between the molecules increases.

Substances with strong dipole-dipole interactions are less volatile than those with only dispersion forces because the polar molecules are attracted to each other, inhibiting their escape to the gas phase.

Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest intermolecular forces and are present in all molecules, whether polar or nonpolar. These forces result from temporary fluctuations in electron distribution within molecules, leading to a momentary dipole that induces a dipole in neighboring molecules.

The strength of dispersion forces increases with the size (or molar mass) of the molecule, as larger molecules have more electrons that can contribute to the fluctuations. While these forces are comparatively weak, in molecules with significant molar mass or surface area, they can become strong enough to affect volatility, making such substances less volatile than one would expect based on their molecular structure alone.