Ch.12 - Liquids, Solids & Intermolecular Forces

Tro - Chemistry: A Molecular Approach 6th Edition

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Tro 6th Edition

Ch.12 - Liquids, Solids & Intermolecular Forces

Problem 81

Chapter 12, Problem 81

Nitrogen has a normal boiling point of 77.3 K and a melting point (at 1 atm) of 63.1 K. Its critical temperature is 126.2 K and its critical pressure is 2.55×10⁴ torr. It has a triple point at 63.1 K and 94.0 torr. Sketch the phase diagram for nitrogen. Does nitrogen have a stable liquid state at 1 atm?

Verified step by step guidance

Identify the key points to plot on the phase diagram: the normal boiling point, melting point, critical point, and triple point. These points are given in the problem statement.

Plot the triple point at 63.1 K and 94.0 torr. This point represents the unique set of conditions where solid, liquid, and gas phases coexist.

Plot the melting point at 63.1 K and 1 atm (760 torr). This indicates the temperature at which solid nitrogen transitions to liquid nitrogen under atmospheric pressure.

Plot the normal boiling point at 77.3 K and 1 atm. This is the temperature at which liquid nitrogen boils into nitrogen gas under atmospheric pressure.

Plot the critical point at 126.2 K and 2.55 * 104 torr. This point represents the temperature and pressure above which nitrogen cannot exist as a liquid, regardless of the pressure applied.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Phase Diagram

A phase diagram is a graphical representation that shows the states of a substance (solid, liquid, gas) at various temperatures and pressures. It includes key features such as the lines of equilibrium between phases, the critical point, and the triple point, where all three phases coexist. Understanding how to read and interpret a phase diagram is essential for analyzing the behavior of substances under different conditions.

Triple Point

The triple point of a substance is the specific temperature and pressure at which all three phases (solid, liquid, and gas) can coexist in equilibrium. For nitrogen, this occurs at 63.1 K and 94.0 torr. This concept is crucial for understanding the conditions under which a substance can exist as a liquid, and it helps determine the stability of the liquid phase at various pressures.

Critical Temperature and Pressure

The critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of pressure, while the critical pressure is the minimum pressure required to maintain the liquid state at this temperature. For nitrogen, the critical temperature is 126.2 K and the critical pressure is 2.55 * 10^4 torr. These concepts are vital for determining whether nitrogen can exist as a stable liquid at 1 atm, which is below its critical temperature.

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