Question

What are dispersion forces? How does the strength of dispersion forces relate to molar mass?

Short answer

Dispersion forces are temporary intermolecular forces caused by instantaneous dipoles. Their strength is positively correlated with molar mass, as heavier molecules with more electrons tend to have stronger dispersion forces.

Step-by-step solution

Define Dispersion Forces

Dispersion forces, also known as London dispersion forces, are the weakest type of intermolecular forces. They are temporary attractive forces that result from the constant motion of electrons within molecules, which can create instantaneous dipoles when the distribution of electrons around an atom is asymmetrical at any moment.

Relate Dispersion Forces to Molar Mass

The strength of dispersion forces increases with the increase in molar mass of the molecule. This is because larger atoms or molecules, which generally have higher molar masses, have more electrons. More electrons mean a greater likelihood of temporary dipoles occurring, which enhances the strength of the dispersion forces.

Key concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between different molecules or between non-bonded atoms. They are fundamentally important as they determine the physical properties of substances, like melting and boiling points, viscosity, and solubility. While there are several types of intermolecular forces, they generally fall into one of the following categories: dipole-dipole interactions, hydrogen bonds, and London dispersion forces.

Understanding these forces can help explain why some substances are gases at room temperature while others are liquids or solids. For example, water has strong hydrogen bonds that give it a high boiling point, whereas carbon dioxide, with weaker intermolecular forces, is gaseous under the same conditions.

London Dispersion Forces

London dispersion forces are a type of intermolecular force named after German-American physicist Fritz London. As the weakest intermolecular force, they occur in all molecular substances, whether polar or nonpolar. The forces originate from fluctuations in the electron distribution within molecules or atoms.

Creating Instantaneous Dipoles

These fluctuations can momentarily create more negative charge on one side of a molecule than the other, leading to a temporary dipole. When one molecule induces a dipole in another nearby molecule, the two molecules will experience an attraction known as a dispersion force. This effect is especially prominent in large, heavy atoms or molecules.

Role in Nonpolar Substances

London dispersion forces are crucial for nonpolar substances, like noble gases and nonpolar molecules, because they are the only type of intermolecular force present. In polar molecules, dispersion forces work together with other types of forces but still play a significant role in the overall intermolecular interaction.

Molar Mass

Molar mass is the mass of one mole of a substance, usually measured in grams per mole. It is a physical property that reflects the mass of atoms, ions, or molecules in a given sample and is directly related to the concept known as the Avogadro's number, which is \(6.022 \times 10^{23}\) entities per mole.

Calculating Molar Mass

To calculate the molar mass of a compound, the atomic mass of each element (from the periodic table) is multiplied by the number of atoms of that element in the compound and then added together. For example, the molar mass of water (H2O) is approximately \(18 g/mol\), calculated from the atomic masses of hydrogen and oxygen.

Importance in Chemistry

Molar mass serves as a bridge between the mass of a material and the number of moles present. It is essential for stoichiometry calculations in chemistry, which allow chemists to predict the amounts of substances consumed and produced in a given reaction.