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Changes of State

Objectives:

    1.    Relate changes in state to changes in energy.

    2.    Compare the different states of matter in terms of potential and kinetic energy

    3.    Describe the major events  that occur along a heating curve.

    4.    Interpret a phase diagram.

Key Terms:

    vaporization    condensation    evaporation    equilibrium vapor pressure    boiling point    heat of vaporization    freezing point    heat of fusion    sublimation    deposition    heating curve    phase diagram

Notes: (14-4)

Matter on Earth exists as either liquid, solid, or gas.  Other than water, most matter exists in a single phase (liquid, solid, or gas) but can be made to change phase by adding or deleting pressure or temperature.

Energy and Phase Changes

According to the kinetic-molecular theory, a substances phase is determined by the balance of its kinetic and intermolecular forces.  What this means is that you can convert matter from one phase to another by simply adding or deleting kinetic energy (heat).  When attempting to understand the phase changes it is important to remember what is occurring in endo and exothermic reactions.
Endothermic - Energy is being absorbed - bonds are breaking - products are less ordered
Exothermic - Energy is being released - bonds are being formed - products are more ordered

(server.chem.ufl.edu/~itl/2045_s99/ lectures/FG11_024.GIF)

Changes in State:

liquid to gas - vaporization, by process of evaporation
solid to gas - sublimation
liquid to solid - freezing
gas to liquid - condensation
gas to solid - deposition

Evaporation

Evaporation is again explained by the Kinetic-molecular theory.  When the kinetic energy of the molecules in a liquid become greater than the intermolecular attractions between them, evaporation occurs.

within a liquid, evaporation is always occurring to some extent at the surface of the liquid
Temperature and evaporation rate are directly related
Liquids with high evaporation rates are said to be volatile - usually flammable due to the presence of organic materials and oxides
Evaporation cools a surface or solution by removing the molecules with the highest kinetic energy
Most solutions in a closed container are in liquid-vapor equilibrium
        condensation and evaporation rates are equal
        rate of molecules leaving the solution are equal to the molecules entering the liquid
        The solution is at equilibrium vapor pressure 

 

 

Boiling Point:

The boiling point of a liquid is directly related to the vapor pressure of the liquid.  Vapor pressure is a measure of the vapor present in a liquid.  When the vapor pressure equals the atmospheric pressure boiling occurs.  It is important to remember that vaporization is an endothermic process as heat is removed from the liquid through boiling.

vapor pressure of a liquid is directly related to the temperature applied to the liquid
the boiling point is directly related to the atmospheric pressure exerted on the solution
boiling occurs when the vapor pressure = atmospheric pressure
the temperature of a liquid will remain constant at the boiling point until all of the liquid is vaporized.

Freezing and Melting:

The freezing and melting points of a liquid exist at the same temperature and represent an equilibrium between the liquid and solid phases.  

melting and freezing points are not greatly affected by atmospheric pressure
melting depends on the heat of fusion which like the boiling is an endothermic process
freezing uses the same amount of energy as the heat of fushion bu is an exothermic process

Heating Curves:

((server.chem.ufl.edu/~itl/2045_s99/ lectures/FG11_024.GIF))

The heating curve is a representation of the phases of a substance compared to the temperature and internal energy of a substance.  The blue lines represent the different phases of a substance while the red lines represent the heats of fusion and vaporization. 

endothermic reactions move from left to right as the products increase in kinetic energy and become less organized

Phase Diagrams

((server.chem.ufl.edu/~itl/2045_s99/ lectures/FG11_024.GIF))

The phase diagram is another representation of the internal energy of a substance compared with the pressure and temperature of its surroundings.  

temperature and pressure are inversely related to the process of vaporization
pressure has little effect on the melting and freezing of a substance
the triple point is a point where all three phases occur in equilibrium
 

 

Last modified: March 19, 2003