Question

In research that required the careful measurement of gas densities, John Rayleigh, a physicist, found that the density of \(\mathrm{O}_{2}(\mathrm{g})\) had the same value whether the gas was obtained from air or derived from one of its compounds. The situation with \(\mathrm{N}_{2}(\mathrm{g})\) was different, however. The density of \(\mathrm{N}_{2}(\mathrm{g})\) had the same value when the \(\mathrm{N}_{2}(\mathrm{g})\) was derived from any of various compounds, but a different value if the \(\mathrm{N}_{2}(\mathrm{g})\) was extracted from air. In \(1894,\) Rayleigh enlisted the aid of William Ramsay, a chemist, to solve this apparent mystery; in the course of their work they discovered the noble gases. (a) Why do you suppose that the \(\mathrm{N}_{2}(\mathrm{g})\) extracted from liquid air did not have the same density as \(\mathrm{N}_{2}(\mathrm{g})\) obtained from its compounds? (b) Which gas do you suppose had the greater density: \(\mathrm{N}_{2}(\mathrm{g})\) extracted from air or \(\mathrm{N}_{2}(\mathrm{g})\) prepared from nitrogen compounds? Explain. (c) The way in which Ramsay proved that nitrogen gas extracted from air was itself a mixture of gases involved allowing this nitrogen to react with magnesium metal to form magnesium nitride. Explain the significance of this experiment. (d) Calculate the percent difference in the densities at \(0.00^{\circ} \mathrm{C}\) and 1.00 atm of Rayleigh's \(\mathrm{N}_{2}(\mathrm{g})\) extracted from air and \(\mathrm{N}_{2}(\mathrm{g})\) derived from nitrogen compounds. [The volume percentages of the major components of air are \(78.084 \% \mathrm{N}_{2}, 20.946 \% \mathrm{O}_{2}, 0.934 \% \mathrm{Ar},\) and \(0.0379 \% \mathrm{CO}_{2} .\)

Short answer

The nitrogen gas extracted from air has a higher density because it invariably contains traces of noble gases which have higher atomic masses. Nitrogen prepared from nitrogen compounds is largely free of such contaminants, leading to a lower density. Ramsay used this difference to aim to prove the presence of other gases in the nitrogen extracted from air by reacting this nitrogen with magnesium to form magnesium nitride. The percent difference in densities can be calculated using the volume percentages given for the different gas components of air.

Step-by-step solution

Analyzing N2(g) from Air and Compounds

The density of a gas is dependent on the type of gas particles involved. The density difference of nitrogen extracted from air and nitrogen derived from compounds can be attributed to an additional component in the nitrogen derived from air, which turned out to be the noble gases. These noble gases increased the overall density of the nitrogen extracted from air.

Determining which N2(g) is Denser

Given that nitrogen gas extracted from air also contains noble gases, it would have greater density than nitrogen gas prepared from nitrogen compounds. The reason is that the noble gases, being monoatomic and having larger atomic masses than nitrogen, contribute to an increased average mass of gas particles, resulting in an increased density.

Significance of Reaction with Magnesium Metal

When the nitrogen gas extracted from air reacts with magnesium to form magnesium nitride, it's evidence that the gas extracted from air is slightly different from pure nitrogen. The remaining unreacted gas is likely the additional component (noble gas) that caused the initial density variation. This experiment was critical in leading to the discovery of noble gases.

Calculating Percent Difference in Densities

To calculate the percent difference in densities, we first calculate the average density of Nitrogen in air using the given volume percentages. Since the density is proportional to the molar mass for a given pressure and temperature, the average molar mass (and hence the density) of air is the sum of the molar masses of its components, each multiplied by its volume percentage. The density of pure nitrogen can be obtained from the periodic table. Then, the percent difference in densities = \((Density_{air} - Density_{N_{2}}) / Density_{N_{2}} * 100\) . Note: Nitrogen from nitrogen compounds is expected to have the same density as pure nitrogen unless specified otherwise.

Key concepts

Noble Gases

Noble gases are a unique group of elements found in Group 18 of the periodic table. These gases are characterized by their lack of reactivity due to having complete valence electron shells, meaning they do not easily form bonds with other elements. This special feature makes them useful in various industrial and scientific applications. When John Rayleigh and William Ramsay discovered noble gases during their research on nitrogen's density, they unveiled a whole new category of elements. Noble gases such as helium, neon, and argon, each contribute to the density of nitrogen gas extracted from the air. As monoatomic gases, their larger atomic masses compared to diatomic nitrogen molecules result in a greater overall density.
This inert behavior, however, is what leads them to be present in small but consistent amounts in the atmosphere. Their substantial atomic mass plays a crucial role in altering the properties of gas mixtures they are part of, especially noticeable in precision experiments and measuring atmospheric gases.

Nitrogen Compounds

Nitrogen compounds are diverse and significant in both nature and industry. Compounds of nitrogen involve its bonding with various other elements, resulting in different molecular sizes and structures. In the context of Rayleigh and Ramsay's experiment, nitrogen compounds provided a baseline for pure nitrogen's properties because of the lack of noble gases. Nitrogen derived from these compounds does not exhibit the same density irregularities as those in atmospheric nitrogen, making it an essential reference point for comparison measurements.
Common examples of nitrogen compounds include ammonia ( H_3N ) and nitrous oxide ( N_2O ), which illustrate the ability of nitrogen to form multiple bonds, sharing its electrons. These compounds demonstrate nitrogen's versatility, forming complex structures and participating in key biological processes, such as amino acid formation and the nitrogen cycle.

Gas Mixtures

Gas mixtures are prevalent in many natural and artificial environments. They are formed when two or more gases are combined without reacting chemically. Each component of a gas mixture retains its own properties, contributing to the overall characteristics of the mixture. In Rayleigh and Ramsay's study, the nitrogen extracted from air was determined to be a gas mixture, with noble gases such as argon being part of it. This revelation changed our understanding of the atmospheric composition and gas behavior.
Gas mixtures can vary greatly in density due to the presence of heavier or lighter gases, affecting calculations and analyses in scientific experiments. The ease with which gases intermingle allows for complex interactions, often requiring precise methods to separate or identify them. Such understanding is crucial in fields like environmental science, where the composition of the air impacts climate, pollution, and human health.