Question

The alkanes containing one, two, three, and four carbons are gases at room temperature. However, as alkanes get larger, they become liquids and then solids at room temperature. Why?

Short answer

As the number of carbon atoms in an alkane increases, the molecular size and mass also grow, resulting in stronger London dispersion forces between the molecules. At room temperature, the larger alkanes require more energy to break these intermolecular forces. Alkanes with one to four carbon atoms have weaker forces and exist as gases, while larger alkanes with stronger forces transition from gases to liquids and eventually solids at room temperature.

Step-by-step solution

Understand the characteristics of alkanes

Alkanes are hydrocarbons, meaning they're composed of hydrogen and carbon atoms. They are generally nonpolar molecules due to the similar electronegativity between carbon and hydrogen. As the number of carbon atoms in an alkane increases, the molecular size and mass also increase.

Investigate intermolecular forces in alkanes

Alkanes exhibit van der Waals forces, also known as London dispersion forces, which are the weakest type of intermolecular forces. These forces are created by temporary dipoles as a result of electron distribution fluctuation. Since alkanes are nonpolar, London dispersion forces are the only significant intermolecular forces present.

Relate molecular size and London dispersion forces

As the number of carbon atoms in an alkane increase, so does the molecular size and mass. Larger molecules have a greater number of electrons and a larger surface area. Both of these factors contribute to stronger London dispersion forces between molecules.

Understand the relationship between intermolecular forces and states of matter

The state of a substance depends on the balance between its intermolecular forces and the energy of its particles. At room temperature, substances with weak intermolecular forces typically exist as gases, while those with stronger forces tend to be liquids or solids.

Explain the change in states for alkanes based on carbon atom count

Since the London dispersion forces increase with the size of the alkane, the larger alkanes require more energy to break their intermolecular forces than the smaller alkanes. At room temperature, alkanes with just one to four carbon atoms don't have strong enough intermolecular forces to be present as liquids or solids, so they exist as gases. However, as the number of carbon atoms grows, so too do the intermolecular forces, leading to a transition from gases to liquids and eventually to solids at room temperature.

Key concepts

Alkanes

Alkanes are a fascinating group of hydrocarbons made solely of carbon and hydrogen atoms. Their composition leads to some unique properties. One of the most notable is their nonpolarity. This nonpolarity comes from the similar electronegativities of carbon and hydrogen, which means they share electrons fairly evenly.
This even distribution makes alkanes fairly simple in terms of intermolecular interactions. However, simplicity doesn't mean they're not interesting! As the number of carbon atoms in an alkane increases, the molecular structure becomes more complex with increased size and mass.
This increase in carbon atoms directly affects the physical properties of alkanes, particularly their state of matter at room temperature. Essentially, tiny alkanes may start as gases, but grow large enough, and they shift into liquids or solids. So, changes in the length and size of alkanes are crucial in how we see these compounds behave.

London Dispersion Forces

One key type of intermolecular force in alkanes is the London dispersion force. These forces are particularly worth our attention when talking about nonpolar molecules like alkanes. Originating from temporary shifts in electron density, they create instantaneous dipoles that attract each other momentarily.
Although London dispersion forces are the weakest intermolecular forces, they're still significant enough to influence the physical properties of materials. Each alkane molecule's larger size results in more electrons and a bigger surface area. Both factors increase the strength of these forces.
In alkanes, where no other intermolecular forces act strongly, London dispersion forces define much of the interaction between molecules. The role they play grows more noticeable as alkanes increase in size. So even weak forces wield a significant impact when enough molecules get together.

States of Matter

The states of matter—solid, liquid, and gas—depend heavily on molecular interactions and energy. In alkanes, this relationship is fascinating because of changes in intermolecular forces as their size increases.
At room temperature, gases and liquids are naturally in balance between intermolecular forces and kinetic energy. For alkanes with fewer carbon atoms, weak London dispersion forces lead to a gaseous state since the energy easily overcomes the forces holding them together.
As alkanes get larger, they have stronger London dispersion forces, so they can transition into liquids or even solids. It takes more energy to break these stronger forces, which is why a large alkane might be solid at room temperature.
So, our state of matter for alkanes isn't fixed but changes with size-related adjustments in intermolecular forces.