Phase Change Equilibrium — Overview & Examples

We are studying phase change diagrams. We've seen how substances change phases at different temperatures and pressure. So far, we haven't discussed the boundary lines in detail. Today, we'll examine what is happening at each phase boundary.

Have you heard of chemical equilibrium? If not, you need to know that chemical reactions can sometimes happen in two directions. The forward reaction is the one we know. The reactants become the products. But, sometimes, the products revert to the reactants. We call it a reverse reaction. A reaction is at equilibrium when the forward and reverse reactions have the same rate.

To reach the equilibrium state, we need a closed system. Imagine you're boiling water in a covered pot. As the water boils, the steam hits the lid and condenses. You've achieved equilibrium when the rate of boiling and condensation are equal. So, we should always have the same volume of liquid water. If we didn't have a lid, we would never reach equilibrium. The water would constantly vaporize, and the steam would never condense.

At the phase boundary lines, we are at equilibrium. We'll explore each transition using water's phase diagram as an example.

Solid-Liquid Boundary

At the solid-liquid boundary, melting and freezing are happening at the same rate. The reaction would be, $H_{2}O_{(s)} \rightleftharpoons H_{2}O_{(l)}$. At atmospheric pressure, the solid-liquid boundary occurs at the melting temperature. For water, that's $0^{o}C$. At equilibrium, both the water and ice are at zero degrees celsius. Recall water's heating and cooling curve. At zero celsius, we have a plateau. That is the temperature we are breaking or reforming the hydrogen bonds. Some of the liquid molecules have a low enough energy that they hydrogen bond to the ice. But, some of the ice molecules have enough energy to become liquid. Both processes are happening at the same rate. So, the mass of liquid and ice would never change.

Let's think of a real-world situation that uses solid-liquid equilibrium. I think of insulated water bottles, like Hydro Flasks. They have two stainless steel walls. Before they seal the walls together, they suck the air out. Having no air between the walls minimizes heat conduction. Heat can only escape by radiating. If you fill the Hydro Flask with ice water, it reaches equilibrium. So, your drink stays cold all day. In reality, a Hydro Flask is not a perfectly closed system. But, even my knock-off brand has ice for about six hours.

Liquid-Gas Boundary

We've already thought about the liquid-gas boundary when we boiled water in a covered pot. It occurs when vaporization and condensation are at equilibrium. The chemical equation looks like, $H_{2}O_{(l)} \rightleftharpoons H_{2}O_{(g)}$. At atmospheric pressure, the liquid-gas equilibrium temperature is the boiling point. Water's boiling point is $100^{o}C$. Again, the heating and cooling curve has a plateau. So, some liquid molecules have enough translational energy to escape the hydrogen bonds. But, some gas molecules collide with the liquid's surface. They are attracted to the liquid and reenter the liquid phase.

Solid-Gas Boundary

The equilibrium equation for this phase boundary is $H_{2}O_{(s)} \rightleftharpoons H_{2}O_{(g)}$. It is the hardest to understand. We don't have everyday experiences with sublimation and deposition. But you may have seen dry ice, which is solid carbon dioxide. At atmospheric pressure, it sublimates at $-78^{o}C$. That's cold! Remember, carbon dioxide is a nonpolar covalent molecule. It's held together by dispersion forces. So, at ordinary temperatures, the molecules have enough kinetic energy to become gaseous.

The sublimation and deposition of water only happen at low pressures. Remember, a gas phase transition occurs when vapor pressure equals external pressure. To transition from solid to gas requires low temperatures and pressures. The molecules need enough energy to escape the surface.

The Triple Point

The triple point is where all three boundary lines meet. So, all three phases are in equilibrium. There is no single equation for the triple point. Instead, all three of the equilibrium reactions are occurring. At water's triple point, we would see ice floating in boiling water. The solid phase must contact the liquid and gas phases. The ice is both sublimating to steam and melting into liquid water. Meanwhile, the vapor condenses to water and deposits onto the ice. Finally, the liquid water is both freezing and vaporizing.

A Few Concluding Points

Remember, the phase boundaries are lines. So they occur at various temperatures and pressures. The molecules' kinetic energies balance the external pressure. The result is that the phase changes are in equilibrium. As we saw, the heating and cooling curves have plateaus. We are rearranging the electrostatic interactions. So, some molecules transition between the two phases.

(Video) Phase Diagrams | General Chemistry

Chad's Prep

In this section of the video, Chad explains the basics of phase change diagrams. During his explanation, he describes the phase equilibrium that occurs at the boundaries. This video will help you connect phase change diagrams, phase transitions, and equilibrium.

(Video) Phase Transitions | Physical Chemistry I | 054

By Professor Derricotte

This video is an advanced explanation of phase equilibrium and phase transitions. But, it gives a clear explanation of the chemistry. If you're looking for a deeper understanding, take a look at this video.