Questions 7-9 refer to the following information. At 27°C, five identical rigid 2.0 L vessels are fi — States of Matter Chemistry Question

STEPS:

1. Identify the contents of Vessel 3: According to the background information, Vessel 3 is filled only with nitrogen gas ($\text{N}_2(g)$) and contains no solids. In the initial diagram at $27^\circ\text{C}$, the nitrogen is correctly represented as four diatomic molecules (pairs of bonded spheres).
2. Apply the Law of Conservation of Mass: Because the vessel is rigid and sealed, the system is closed. No gas can enter or leave, and nitrogen gas does not undergo any chemical reaction in this container. Therefore, the total number of nitrogen atoms and molecules must remain exactly constant; the diagram representing the heated state must still show exactly four diatomic molecules.
3. Connect temperature to molecular speed: According to the Kinetic Molecular Theory, the average kinetic energy of gas molecules is directly proportional to their absolute temperature in Kelvin ($\text{KE} \propto T$). Since kinetic energy is mathematically defined by the mass and velocity of the particles ($\text{KE} = \frac{1}{2}mv^2$), heating the gas from $27^\circ\text{C}$ to $127^\circ\text{C}$ increases the average kinetic energy of the molecules, which directly increases their average speed.
4. Determine the correct visual representation: The prompt states that the lengths of the arrows represent the speeds of the particles. Because the average speed of the $\text{N}_2$ molecules increases upon heating, the arrows in the heated diagram must be longer on average than they were in the initial diagram at $27^\circ\text{C}$. Diagram A correctly preserves the four diatomic molecules while showing them moving with longer velocity arrows.

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WHY_OTHERS_WRONG:

• Option B is incorrect: This diagram depicts the diatomic nitrogen molecules dissociated into individual, separate nitrogen atoms. Diatomic nitrogen ($\text{N}_2$) is held together by an exceptionally strong triple covalent bond and does not thermally dissociate into individual atoms at a mild temperature of $127^\circ\text{C}$.
• Option C is incorrect: This diagram represents the molecules with shorter arrows than the initial state, which represents a decrease in average speed and kinetic energy. This would occur if the vessel were cooled, not heated.
• Option D is incorrect: This diagram shows a greater number of gas molecules (eight diatomic molecules instead of four). Because the vessel is sealed, no additional nitrogen gas can enter, and the number of moles of gas cannot increase.