Problem 11 - General Chemistry: Principles and Modern Applications

Question

Which of the following substances would obey Trouton's rule most closely: HF, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CH}_{3}\) (toluene), or \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol)? Explain your reasoning.

Step-by-Step Solution

Answer

Toluene (\(C_6H_5CH_3\)) would obey Trouton's rule most closely, as it is a non-polar molecule, while HF and methanol (\(CH_3OH\)) are polar.

Step 1: Identify the Polarity of HF

HF is a polar molecule. The difference in electronegativity between hydrogen and fluorine result in a polar bond, leading to a dipole in the molecule.

Step 2: Identify the Polarity of C6H5CH3 (toluene)

Toluene (C6H5CH3) is a non-polar molecule. While the C-H bonds can be considered slightly polar, the symmetrical cyclic structure of the molecule results in the cancellation of dipole moments, leading to a non-polar molecule.

Step 3: Identify the Polarity of CH3OH (methanol)

Methanol (CH3OH) is a polar molecule. It contains a -OH group, which is a polar functional group, resulting in a polar molecule.

Step 4: Apply Trouton's Rule

Given that Trouton's Rule applies best to non-polar molecules, toluene (C6H5CH3), being a non-polar molecule, would obey Trouton's Rule most closely compared to HF and methanol (CH3OH), which are polar.

Key Concepts

Polarity

Polarity in chemistry refers to the distribution of electric charge around atoms, molecules, or chemical groups. Molecules can be polar or non-polar depending on how their atoms share electrons.
When atoms in a molecule share electrons equally, the molecule is non-polar. This typically happens when the atoms have similar electronegativity. In contrast, polar molecules have unequal sharing of electrons because of differences in electronegativity between bonded atoms.
Polarity is a key concept because it affects molecular interactions and properties such as solubility, boiling and melting points, and chemical reactions. Understanding polarity helps predict how substances will behave in different situations.

Non-Polar Molecules

Non-polar molecules are those where the electrons are shared equally between the atoms. This results in no net dipole moment. The absence of a dipole makes these molecules often symmetrical.
Examples of non-polar molecules include gases like nitrogen (N\(_2\)) and oxygen (O\(_2\)), as well as hydrocarbons like toluene ( \(C_6H_5CH_3\)).
Non-polar molecules have certain characteristics:

• They tend to be hydrophobic, or water-repelling.
• They often have lower boiling points compared to polar molecules.
• Generally, they mix well with other non-polar substances.

In the case of Trouton's Rule, which relates the entropy of vaporization of liquids, non-polar molecules like toluene are typically better suited as they have less complex intermolecular forces compared to polar molecules.

Polar Molecules

Polar molecules occur when there is an unequal distribution of electrons between the atoms, resulting in partial positive and negative charges. Think of it like a magnet where there is a positive end and a negative end. This property is vital for understanding molecular interactions.
Common examples of polar molecules are water ( \(H_2O\)) and methanol ( \(CH_3OH\)). In the case of HF (hydrogen fluoride), the difference in electronegativity between hydrogen and fluorine leads to a strong polar bond.
Some characteristics of polar molecules are:

• They are generally hydrophilic, meaning they dissolve easily in water.
• They tend to have higher boiling points because of the stronger intermolecular forces between them, like hydrogen bonding.
• In reactions, they might interact more dynamically due to their partial charges.

Applying these concepts, polar molecules like HF and methanol display stronger molecular forces that diverge from Trouton's Rule, which applies more directly to non-polar substances.