Phase EquilibriumReading, Interpreting, and Drawing Phase Diagrams

Review: Energy and Phase Changes

Two-Phase Systems• A system is a set of components that are being studied.

• In this system, are phases, or parts of matter that are uniform.

• A lava lamp has two phases (components) • The layers in a lava lamp stay apart because their chemical

compositions are different

• What makes the two phases of ice water different from each other?

Equilibrium• Equilibrium involves constant interchange of particles.

• If you open rubbing alcohol, you can smell it. This is because the alcohol diffuses out in the ________________ form.

• After you recap the alcohol, an equilibrium is quickly reached again.

• A dynamic EQUILIBRIUM is reached when particles are entering the gaseous phase at an equal rate as the particles go back to the liquid phase.

• At 0°C, water is freezing at the same rate as the ice is melting.

Vapor Pressure and Temp• In a closed container with a sample of liquid in the bottom,

there are gaseous particles striking all sides of the container.

• These particles are also striking the liquid component.

• The pressure exerted by the molecules of a gas, or vapor, phase in equilibrium with a liquid is called the vapor pressure.

• As the temp increases, the molecules move ____________, giving them more kinetic energy. This allows more particles to break out of the liquid phase and enter the gas above.

• As temp increases, vapor pressure ___________________.

Phase Diagrams• The substance’s state depends on the temperature and pressure.

• A phase diagram is a graph of the relationship between the physical state of a substance and the temperature and pressure of the substance.

• A phase diagrams has three lines.• One line is a vapor pressure curve for the liquid-gas equilibrium.• The second line is for the liquid-solid equilibrium.• And the third is for the solid-gas equilibrium.

• The temp and pressure conditions at which the solid, liquid, and gaseous phases of a system coexist at equilibrium is known as the triple point.

Phase Diagrams• The x-axis shows temperature• The y-axis shows pressure.

• For any given point (x,y) [T, P] you can tell if the substance will be a solid, liquid, or gas.

• An area above the gas phase lies the critical point

• The critical point is the temperature and pressure at which the gas and liquid states of a substance become identical and form one phase.

Water, H2O

Phase Diagrams• The lines in between the phases represent the phase

equilibrium

• i.e. the line between the solid and liquid represents the Solid-Liquid Equilibrium

• The Solid-Liquid Equilibrium is nearly vertical.. Why??

• Phase diagrams are unique to a particular substance.

• The general structure will be roughly the same, but each substance will have its own BP and MP (Temp and Press)

Carbon Dioxide, CO2

Effects of Pressure• Remember, solids and liquids are nearly incompressible.

• Most substances, such as carbon dioxide, experience a slight increase in melting point when the pressure increases.

• However, the effect of pressure on boiling point is MUCH greater… Why??

Drawing a Phase Diagram• The triple point of sulfur dioxide is at -73°C and 0.17 kPa. The critical

point is at 158°C and 7.87x103kPa. The normal boiling point of sulfur dioxide is -10°C. Solid sulfur dioxide is denser than liquid sulfur dioxide. Sketch the phase diagram of sulfur dioxide.

• What state is sulfur dioxide in at 200 kPa and -100°C?• What state is sulfur dioxide in at 1 kPa and 80°C?• What happens as you increase the pressure on a sample of sulfur

dioxide at -11°C from 150 kPa to 300kPa?

More Practice

• The triple point of benzene is at 5.5°C and 4.8 kPa. The critical point is at 289°C and 4.29 x 103 kPa. Vapor pressure above solid benzene is 101.3 kPa at 80.1°C. Solid benzene is denser than liquid benzene. Sketch the phase diagram of benzene.