Which of these gases cannot appropriately be collected by water displacement? (A) CO2, (B) Ar, (C) O2, (D) H2, (E) SO2.
The gases referred to in this question are carbon dioxide, argon, oxygen, hydrogen, and sulfur dioxide. Carbon dioxide, oxygen, hydrogen, and sulfur dioxide are gases that are commonly encountered or generated in the college laboratory. Like oxygen and hydrogen, argon is an element, but it is a noble gas. It has many industrial applications including a big use in welding. The apparatus we see in the diagram is a common setup used in school or college laboratories to generate and collect various gases. The generator is usually a round-bottom flask that will contain the reacting chemicals. The funnel, which is attached to the generator via a bung, is used to introduce liquids to the mixture.
The funnel tube must be below the liquid level so that the gas can only exit via the delivery tube. The delivery tube directs the gas flow to the trough of water where the bubbles are collected by water displacement in a suitable collection vessel like a gas jar. This common method of gas collection called water displacement works most appropriately for gases that did not dissolve in water. That is to say, that the gases collected must have limited or very low solubility in water. Water is a polar molecule. This is due to the electronegativity difference between the oxygen and hydrogen atoms found within the covalent bonds in the molecule. This polarity is indicated in part of the liquid water structure here using the 𝛿-positive and 𝛿-negative symbols.
Liquid water is also extensively hydrogen bonded. These intermolecular hydrogen bonds exist between water molecules. They’re indicated by the dashed lines in part of the structure of liquid water here. Whether or not a gas molecule will dissolve in liquid water depends upon the molecule’s ability to disrupt the extensive network of hydrogen bonding in the liquid water. And then whether it will fit into this network once it’s there. In general, we find that polar molecules dissolve best in polar solvents like water. So to best answer the question, we need to decide upon the solubility of each gas molecule based upon whether the molecule is polar or nonpolar.
CO2 or carbon dioxide is a linear molecule. It does contain polarized bonds due to the electronegativity difference between the carbon and the oxygen atoms found within these bonds. Due to the linear symmetry of the molecule, the dipoles cancel each other out, and carbon dioxide is considered to be a nonpolar molecule. Carbon dioxide has limited solubility in water and normal atmospheric pressure. It is dissolved in water under higher pressures in fizzy drinks. Therefore, it would be fine to collect carbon dioxide by water displacement.
Since the question is asking us which gases cannot be collected by water displacement, carbon dioxide is not the correct answer here. Argon is a noble gas. It contains monatomic spherical atoms. Argon is therefore a nonpolar gas, and it would not be soluble in water. It can be collected by water displacement, and therefore it’s not the correct answer. Oxygen is a diatomic element and the oxygen-oxygen double bonds that the molecules contain are not polarized. It is therefore a nonpolar molecule. Oxygen has some solubility in water certainly enough for fish to survive in lakes and streams. But it is trivial, and therefore it’s okay to collect oxygen by water displacement. Oxygen is not the correct answer either.
Hydrogen is another diatomic element. Its covalent bonds are not polarized, and it is not a polar molecule. Hydrogen is absolutely fine to collect by water displacement due to its very limited solubility in water. Hydrogen is not the correct answer. SO2 or sulfur dioxide is a polar molecule. The molecule has a lack of symmetry due to its nonlinear shape. And there is a dipole across the molecule. SO2 is emitted in vast quantities from volcanoes. This is a natural source of sulfur dioxide. It is also released when you burn coal which contains sulfur which is a man-made source of sulfur dioxide.
Either way, the sulfur dioxide is highly soluble in atmospheric water. It dissolves to form sulfurous acid which can fall as acid rain. Due to the high solubility of sulfur dioxide in water, it cannot appropriately be collected by water displacement. Sulfur dioxide is therefore the correct answer.