Question

True or false: (a) For molecules with similar molecular weights, the dispersion forces become stronger as the molecules become more polarizable. (b) For the noble gases the dispersion forces decrease while the boiling points increase as you go down the column in the periodic table. (c) In terms of the total attractive forces for a given substance dipole- dipole interactions, when present, are always larger than dispersion forces. (d) All other factors being the same, dispersion forces between linear molecules are greater than dispersion forces between molecules whose shapes are nearly spherical.

Short answer

(a) True: Dispersion forces become stronger as molecules become more polarizable for those with similar molecular weights. (b) False: For noble gases, dispersion forces and boiling points both increase as you go down the column in the periodic table. (c) False: Dipole-dipole interactions are not always larger than dispersion forces; it depends on the specific substance and its molecular structure. (d) True: Dispersion forces between linear molecules are greater than those between nearly spherical molecules, assuming all other factors are the same.

Step-by-step solution

Statement (a)

For molecules with similar molecular weights, the dispersion forces become stronger as the molecules become more polarizable. This statement is true. Dispersion forces, also known as London forces, depend on the polarizability of a molecule. Polarizability is the ability of a molecule's electron cloud to be distorted under the influence of nearby charges. Larger, more complex molecules typically have higher polarizability, which results in stronger dispersion forces between the molecules.

Statement (b)

For the noble gases, the dispersion forces decrease while the boiling points increase as you go down the column in the periodic table. This statement is false. As we go down the column in the periodic table, the atomic size increases, resulting in a greater number of electrons and higher polarizability. This leads to stronger dispersion forces between the noble gas atoms. As a result, the boiling points of noble gases also increase down the group, as more energy is needed to overcome the stronger dispersion forces holding the atoms together.

Statement (c)

In terms of the total attractive forces for a given substance, dipole-dipole interactions, when present, are always larger than dispersion forces. This statement is false. While dipole-dipole interactions can be stronger than dispersion forces, it depends on the specific substance and its molecular structure. In polar molecules, dipole-dipole interactions can be significant and sometimes stronger than dispersion forces. However, in nonpolar molecules, dispersion forces are the primary intermolecular forces in play.

Statement (d)

All other factors being the same, dispersion forces between linear molecules are greater than dispersion forces between molecules whose shapes are nearly spherical. This statement is true. Linear molecules with similar molecular weights have larger surface contact areas compared to nearly spherical molecules, which leads to stronger dispersion forces due to increased interaction between the electron clouds of the molecules.

Key concepts

Dispersion Forces

Dispersion forces, often referred to as London dispersion forces, are the weakest type of intermolecular forces. They arise due to temporary fluctuations in electron density within a molecule or atom, creating a momentary dipole which induces a dipole in neighboring molecules.

These fleeting attractions are present in all molecules, whether polar or nonpolar, but are particularly significant in nonpolar compounds where other types of intermolecular forces are absent. The strength of dispersion forces depends on the polarizability of the molecule, which is the ease with which its electron cloud can be distorted. Larger molecules with more electrons exhibit greater polarizability and therefore stronger dispersion forces, which can lead to higher boiling points and greater solubility in nonpolar solvents.

Polarizability

Polarizability is an important concept in understanding not only dispersion forces but also how molecules interact with one another in general. It refers to the ability of an atom or a molecule to form instantaneous dipoles due to the shifting of the electron cloud with respect to the positively charged nucleus. Factors affecting polarizability include the number of electrons and the volume of the electron cloud.

The greater the number of electrons and the larger the volume, the more easily the electron cloud can be distorted, leading to stronger temporary dipoles. This is why larger atoms or molecules, and those with more diffuse electron clouds (such as those in higher periods on the periodic table), tend to have higher polarizability, which directly influences properties like melting and boiling points.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, which have permanent dipoles. These forces result from the electrostatic attraction between the positive end of one dipole and the negative end of another. These are stronger than dispersion forces but weaker than ionic and covalent bonds.

The strength of dipole-dipole interactions increases with the polarity of the molecule, meaning the greater the difference in electronegativity between the atoms in the molecule, the stronger the interaction. However, it's a common misconception that dipole-dipole interactions are always the main contributor to the total intermolecular forces within a substance. The reality is more nuanced, and in some cases, dispersion forces may predominate.

Boiling Points

Boiling points are a key physical property of substances and provide valuable insight into the intermolecular forces at play. A substance boils when its vapor pressure equals the external pressure, typically atmospheric pressure. The energy needed to change a liquid into a gas is largely determined by the strength of its intermolecular forces.

Substances with strong intermolecular forces require more energy to separate the molecules, resulting in higher boiling points. This concept is evident when comparing molecules of similar molecular weight but different types of intermolecular forces. For example, polar molecules with strong dipole-dipole interactions have higher boiling points than nonpolar molecules with only dispersion forces.

Periodic Table Trends

The structure of the periodic table helps us predict various trends in element properties, including polarizability and, consequently, the strength of intermolecular forces such as dispersion forces. As you move down a group in the periodic table, elements have more electron shells, increasing the size of the electron cloud and the polarizability.

This increase in polarizability down a group leads to stronger dispersion forces, which ties into the observable increases in boiling points for elements like the noble gases. However, as you move across a period, the electron cloud does not get significantly larger, but the effective nuclear charge increases, pulling the electron cloud closer and making the elements less polarizable. Understanding these trends is crucial in predicting and explaining the physical properties of different substances.