Problem 92 - Chemistry

Question

When a gas was heated under atmospheric conditions, its color deepened. Heating above \(150^{\circ} \mathrm{C}\) caused the color to fade, and at \(550^{\circ} \mathrm{C}\) the color was barely detectable. However, at \(550^{\circ} \mathrm{C},\) the color was partially restored by increasing the pressure of the system. Which of the following best fits the above description? Justify your choice. (a) A mixture of hydrogen and bromine, (b) pure bromine, (c) a mixture of nitrogen dioxide and dinitrogen tetroxide. (Hint: Bromine has a reddish color and nitrogen dioxide is a brown gas. The other gases are colorless.)

Step-by-Step Solution

Answer

The best fit for the description in the question is option (c) - a mixture of nitrogen dioxide and dinitrogen tetroxide. The change in color of the gas under different conditions of temperature and pressure matches with these two substances showing an equilibrium, with the brown nitrogen dioxide partially converting to colorless dinitrogen tetroxide and vice versa.

Step 1: Analyze each of the options

Start with taking a look at each of the options under the conditions described in the problem.\n\n(a) A mixture of hydrogen and bromine - Hydrogen is colorless and Bromine is reddish in color.\n\n(b) Pure bromine - Bromine is reddish in color, but no other gases are present for any possible reactions.\n\n(c) A mixture of nitrogen dioxide and dinitrogen tetroxide - Both these compounds are colored; Nitrogen dioxide is a brown gas, and dinitrogen tetroxide is colorless.

Step 2: Compare the options with the conditions in the problem

Now that we understand the properties of each option, we can compare them with the conditions described in the question.\n\nThe exercise mentioned that the color of the gas deepens under heating but then fades away above \(150^{\circ} \mathrm{C}\), and at \(550^{\circ} \mathrm{C}\), the color was barely detectable. However, at the same temperature, by increasing the pressure, the color was partially restored. None of the conditions relate to the properties of options (a) and (b), as their colors would not normally change in this way with changes in temperature and pressure.

Step 3: Choose the option which fits best

After comparing, the best fitting option would be (c) - a mixture of nitrogen dioxide and dinitrogen tetroxide.\n\nThe deepening color of the gas under heating, fading away above \(150^{\circ} C\), and barely detectable at \(550^{\circ} C\), matches with the fact that as the temperature increases, the brown nitrogen dioxide partially converts to colorless dinitrogen tetroxide. This also resonates with the fact that at \(550^{\circ} C\), the color was partially restored by increasing the pressure, as the colorless dinitrogen tetroxide would partially convert back to brown nitrogen dioxide under increased pressure. This shows an equilibrium between the two substances depending on temperature and pressure.

Key Concepts

Nitrogen Dioxide

Nitrogen dioxide (NO₂) is a brown gas that is significant in its reaction behavior and visual properties. It is a common atmospheric pollutant and is characterized by its distinct brownish color.
This gas plays a vital role in various chemical reactions, especially in the environment and its interaction with other gases. In the context of chemical equilibrium, nitrogen dioxide is known for participating in a reversible reaction.

• In lower concentrations that can occur naturally, NO₂ provides a noticeable brown hue to air, which is often seen in polluted urban areas.
• The visual cue of its color change with conditions like temperature and pressure is crucial to understanding its equilibrium behavior.

When you heat nitrogen dioxide, its ability to shift into another compound becomes evident as part of its equilibrium nature, which points out the role of thermal energy in its transformation.

Dinitrogen Tetroxide

Dinitrogen tetroxide (N₂O₄) is a chemical compound in equilibrium with nitrogen dioxide. It is colorless at lower temperatures and color becomes apparent as it dissociates into nitrogen dioxide.
The relationship between these two molecules showcases the basic principles of chemical equilibrium. The process involves transitions between two forms:

• At lower temperatures, N₂O₄ is stable and mostly does not display color.
• As you increase the temperature, N₂O₄ dissociates into brown NO₂, which results in the mixture appearing colored.

This pair exhibits an insightful example of how color and temperature are intertwined in chemical equilibria. Such behavior clarifies the concept of dynamic equilibrium, where forward and reverse reactions occur until a balance is acheived.

Pressure Effect on Equilibrium

The effect of pressure on equilibrium is an essential concept in chemical reactions involving gases. According to Le Chatelier’s principle, a change in pressure can shift the position of equilibrium to counteract the pressure change.
In the case of nitrogen dioxide and dinitrogen tetroxide, pressure plays a crucial role:

• When the pressure is increased, the system adjusts itself to reduce the number of gas molecules, favoring the formation of N₂O₄ from NO₂.
• This results in a change from brown to a less intense color as more dinitrogen tetroxide forms.

Such shift shows how external conditions like pressure can dynamically influence gas phase equilibria. It underscores the reversible nature of chemical reactions and provides a tangible example of how equilibrium can be manipulated by changing conditions.