Question

If you compare the solubilities of the noble gases in water, you find that solubility increases from smallest atomic weight to largest, \(\mathrm{Ar}<\mathrm{Kr}<\mathrm{Xe}\). Which of the following statements is the best explanation? [Section 13.3] (a) The heavier the gas, the more it sinks to the bottom of the water and leaves room for more gas molecules at the top of the water. (b) The heavier the gas, the more dispersion forces it has, and therefore the more attractive interactions it has with water molecules. (c) The heavier the gas, the more likely it is to hydrogenbond with water. (d) The heavier the gas, the more likely it is to make a saturated solution in water.

Short answer

The best explanation for the solubility trend of noble gases in water, increasing from smallest atomic weight to largest, is (b): The heavier the gas, the more dispersion forces it has, and therefore the more attractive interactions it has with water molecules.

Step-by-step solution

Start with the properties of noble gases

Noble gases are inert and unreactive, and they do not form any chemical bonds with other atoms, including water molecules. However, they can still be dissolved to some extent thanks to intermolecular forces.

Compare the given explanations

Here, let's go through each explanation given: (a) Solubility in water is not generally determined by the position of the gas molecules within the water; it relies on the intermolecular forces between the water and gas molecules. So this explanation can be eliminated. (b) Dispersion (or London) forces are the sort of intermolecular forces that would affect noble gases, which are nonpolar molecules. More massive molecules generally have larger dispersion forces, which can result in an increased attraction to water molecules. This statement seems plausible, so keep it aside for now. (c) Hydrogen bonds are formed between polar molecules containing hydrogen and other electronegative atoms such as oxygen, nitrogen, or fluorine. Noble gases are not involved in hydrogen bonding, so this explanation can be dismissed. (d) The solubility trend in the statement could indicate saturated solutions, but this statement does not explain the reason for the trend in solubility as we are looking for.

Choose the correct explanation

From the above discussion, the only plausible explanation is (b): The heavier the gas, the more dispersion forces it has, and therefore the more attractive interactions it has with water molecules. Thus, the solubility of noble gases in water increases from the smallest atomic weight (Argon) to the largest (Xenon).

Key concepts

Intermolecular Forces

Intermolecular forces are the attractions and repulsions between molecules or ions in a material. These forces hold molecules together and determine many properties of substances including boiling point, melting point, and solubility.

Noble gases, even though they are inert and do not form chemical bonds, experience intermolecular forces when in proximity to other substances like water.
- These forces are responsible for the states of matter and help explain why substances like noble gases can be dissolved in liquids like water.
- Understanding intermolecular forces helps us comprehend why noble gases increase in solubility with increasing atomic mass.

The primary type of intermolecular force acting on noble gases is the London dispersion force. This is due to their nonpolar nature, as noble gases do not have permanent dipoles akin to polar molecules.

Dispersion Forces

Dispersion forces, also known as London forces, are a type of weak intermolecular force that arises from temporary dipoles. These temporary dipoles occur due to the movement of electrons within a neutral atom or molecule.

For noble gases, which are nonpolar, dispersion forces are significant because they are the only type of intermolecular forces these atoms can experience.
- They occur in all atoms and molecules, regardless of polarity and are generally stronger in larger atoms or molecules because they have more electrons that can move, creating temporary dipoles.
- So, heavier noble gases like xenon exhibit stronger dispersion forces compared to lighter ones like helium.

When noble gases dissolve in water, the dispersion forces between the noble gas atoms and water molecules allow for some interaction, increasing their solubility.
- The greater the dispersion forces due to the larger atomic size, the more they can attract water molecules, enhancing solubility.

Noble Gas Characteristics

Noble gases include helium, neon, argon, krypton, xenon, and radon, located in Group 18 of the periodic table. They are known for being odorless, colorless, and having very low reactivity.

A few key characteristics of noble gases include:

• They have complete electron shells, which makes them highly stable and unreactive.
• Noble gases do not easily form compounds with other elements, which is why they are often deemed 'inert'.
• Despite their lack of chemical reactivity, they exhibit varying physical properties like solubility in water, influenced by intermolecular dispersion forces.

Heavier noble gases are more soluble in water than lighter ones.
- This is due to stronger dispersion forces in heavier atoms, allowing them to interact more effectively with water molecules.

Understanding the characteristics of noble gases, particularly their chemical inertness and how they dissolve because of physical forces, paints a clearer picture of their unique behavior in solutions.