Problem 132 - Chemistry

Question

Which of the following statements about intermolecular forces is(are) true? a. London dispersion forces are the only type of intermolecular force that nonpolar molecules exhibit. b. Molecules that have only London dispersion forces will always be gases at room temperature \(\left(25^{\circ} \mathrm{C}\right)\) c. The hydrogen-bonding forces in \(\mathrm{NH}_{3}\) are stronger than those in \(\mathrm{H}_{2} \mathrm{O}\) . d. The molecules in \(\mathrm{SO}_{2}(g)\) exhibit dipole-dipole intermolecular interactions. e. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) has stronger London dispersion forces than does \(\mathrm{CH}_{4}\) .

Step-by-Step Solution

Answer

Statements a, d, and e are true about intermolecular forces.

Step 1: Statement a

London dispersion forces are temporary attractive forces resulting from fluctuations in electron density in nonpolar molecules. These forces are present in all molecules, but they dominate in nonpolar ones. Hence, statement a is true.

Step 2: Statement b

Molecules with only London dispersion forces can be gases, liquids, or solids at room temperature depending on their molecular mass and strength of London dispersion forces. Larger nonpolar molecules may have stronger dispersion forces due to more electrons present, and they can be liquids or solids. Therefore, statement b is false.

Step 3: Statement c

Hydrogen bonding occurs when hydrogen is bonded to a highly electronegative element such as nitrogen, oxygen, or fluorine. In NH3, hydrogen atoms are bonded to nitrogen, while in H2O, hydrogen atoms are bonded to oxygen. Oxygen is more electronegative than nitrogen, which means that hydrogen bonds in H2O are stronger than those in NH3. Therefore, statement c is false.

Step 4: Statement d

SO2 is a polar molecule because the oxygen atoms have higher electronegativity than the sulfur atom, resulting in a net dipole moment. In polar molecules, there are electrostatic interactions called dipole-dipole forces between their positive and negative ends that help to stabilize them. Hence, statement d is true.

Step 5: Statement e

CH3CH2CH3 (propane) and CH4 (methane) are both nonpolar molecules, so they exhibit London dispersion forces. Propane has a larger molecular mass than methane, meaning it has a higher number of electrons and a larger electron cloud. This leads to stronger London dispersion forces in propane compared to methane. Thus, statement e is true. In conclusion, statements a, d, and e are true while statements b and c are false.

Key Concepts

London Dispersion Forces

London dispersion forces are a type of intermolecular force that arises due to temporary shifts in the electron density of molecules. These forces occur in all types of molecules, but they are the only source of attraction between nonpolar molecules. Imagine a nonpolar molecule, where electrons are evenly distributed. Sometimes, these electrons may move slightly, causing a fleeting imbalance in the charge distribution. This temporary dipole can induce a similar shift in neighboring molecules, leading to a weak attraction.

Key points to remember about London dispersion forces include:

• They exist in all molecules but are predominant in nonpolar molecules.
• The strength of these forces increases with the number of electrons and the size of the electron cloud.
• Larger molecules tend to have stronger dispersion forces due to their larger electron clouds.

Understanding these forces helps explain behaviors and properties of different substances, such as why some nonpolar molecules are liquids or solids at room temperature.

Dipole-Dipole Interactions

Dipole-dipole interactions are another type of intermolecular force that occurs between polar molecules. Unlike London dispersion forces, dipole-dipole interactions are caused by permanent dipoles due to the unequal sharing of electrons between atoms within a molecule. This results from differences in electronegativity, where one atom pulls electrons more strongly than the other.

In polar molecules, these permanent dipoles create regions of partial positive and negative charges. Dipole-dipole interactions occur when the positive end of one molecule attracts the negative end of another, aligning themselves in a stabilizing configuration.

• These interactions are much stronger than London dispersion forces but weaker than hydrogen bonds.
• The strength of dipole-dipole interactions depends on the magnitude of the dipole moment.
• These interactions significantly affect boiling points and solubility of polar compounds.

Recognizing when these forces are present can help explain why some molecules, like \( ext{SO}_{2}\), exhibit particular physical properties.

Hydrogen Bonding

Hydrogen bonding is a special type of dipole-dipole interaction that occurs in molecules where hydrogen is covalently bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. This type of bonding is particularly strong because the hydrogen atom, being very small, can get very close to the electronegative atom in the neighboring molecule.

For example, in water (H₂O), hydrogen bonds form between the hydrogen of one water molecule and the oxygen of another, leading to the unique properties of water.

• These bonds are stronger than typical dipole-dipole interactions due to the high electronegativity and small size of the involved atoms.
• Hydrogen bonding is responsible for the unique boiling points, melting points, and solubility in water.
• Commonly found in water, ammonia, and proteins, this interaction is crucial for the structure and function of biomolecules.

Understanding hydrogen bonding aids in explaining many biological processes and the physical properties of substances.

Nonpolar Molecules

Nonpolar molecules are molecules that do not have a significant difference in electronegativity between their atoms, leading to an even distribution of electrical charge. This means there are no resulting regions of partial positive or negative charge on the molecule.

Nonpolar molecules are primarily held together by London dispersion forces, since they lack permanent dipoles. Such molecules often consist of similar atoms bonded together, like diatomic gases (e.g., \( ext{N}_2\)) or hydrocarbons like methane (CH₄).

• They do not interact strongly with polar substances, which is why nonpolar and polar liquids often do not mix, like oil and water.
• Nonpolar molecules tend to be gases or liquids with relatively low boiling points unless they have high molecular masses.
• To identify nonpolar molecules, look for symmetric structures or atoms with similar electronegativities.

Recognizing nonpolar molecules is key to understanding solubility and state at room temperature.

Molecular Polarity

Molecular polarity refers to the distribution of electrical charge over the atoms in a molecule. Polarity arises when there is a significant difference in electronegativity between bonded atoms, leading to the formation of dipoles. If these dipoles are not symmetrically arranged, the molecule becomes polar.

Polarity affects many molecular properties, such as solubility, boiling point, and intermolecular interaction types. It is a crucial factor in determining the behavior of molecules in different environments.

• To determine polarity, examine both the bond dipoles and the molecule's symmetry.
• Polar molecules have higher boiling points due to stronger intermolecular forces compared to nonpolar molecules.
• Consider the electronegativity of atoms and the geometry of the molecule for polarity evaluation.

Grasping the concept of molecular polarity helps in predicting physical properties and chemical behavior of molecules in solutions and mixtures.