Chapter 17: Problem 83

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

Expert verified

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.

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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

Step by step solution

Understand the characteristics of alkanes

Investigate intermolecular forces in alkanes

Relate molecular size and London dispersion forces

Understand the relationship between intermolecular forces and states of matter

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

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