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Phase Equilibrium
Reading, Interpreting, and Drawing Phase Diagrams
Review: Energy and Phase
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
• 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
• 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
• 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.
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