How do the boiling points of the alkanes change with relative molecular mass?

The boiling points of alkanes increase with an increase in relative molecular mass. Here's a step-by-step explanation:

1. Alkanes are hydrocarbons, which means they are made up of hydrogen and carbon atoms. The simplest alkane is methane (CH4), and as you add more carbon atoms to the chain, the relative molecular mass increases.

2. The boiling point of a substance is the temperature at which it changes from a liquid to a gas. For alkanes, this is determined by the strength of the forces between the molecules, known as intermolecular forces.

3. In alkanes, the primary intermolecular force is the London dispersion force, which is a type of van der Waals force. These forces are temporary and occur when electrons in the molecule create a temporary dipole moment, which induces a dipole in a neighboring molecule.

4. The strength of the London dispersion forces increases with the size and shape of the molecule. Larger molecules have more electrons and thus can have larger temporary dipoles, leading to stronger London dispersion forces.

5. Therefore, as the relative molecular mass of the alkanes increases (i.e., as you add more carbon atoms to the chain), the strength of the London dispersion forces also increases. This means it takes more energy (and thus a higher temperature) to overcome these forces and change the substance from a liquid to a gas.

6. Hence, the boiling points of the alkanes increase with an increase in relative molecular mass.