energy and phase changes

Energy and Phase Changes

• Three states or phases of matterSolidLiquidGas

• Energy is involved in every phase change.– Some require energy to occur (endothermic)– Some release energy (exothermic)– Based on the breaking or formation of bonds

between the particles (atoms, molecules, fun)

Energy and Phase Changes

Intermolecular Forces

• Differing attractive forces between molecules cause some materials to be solids, some to be liquids, and some to be gases at the same temperature.

Gases

• Kinetic Molecular Theory• Constant random motion

High energy movement spreads molecules throughout container.

• Large space between gas molecules• Elastic collisions• Little or no intermolecular attraction

or repulsion.

Liquids

• Kinetic molecular theory still involved, but intermolecular forces are strong enough to greatly limit movement.

• Forces of attraction keep molecules closely packed in a fixed volume, but not in a fixed position. Molecules can flow past each other.

• Liquids are much denser than gases because of the stronger intermolecular forces holding the particles together.

Solids

• Solids: particles’ attractive intermolecular forces are greater than their kinetic energy of motion.

• Particles in a solid vibrate in a fixed position.• Most solids are more dense than their liquid.• Note: Ice is not more dense than liquid water.

It FLOATS!

Solids (cont.)

• Crystalline solids are solids with atoms, ions, or molecules arranged in an orderly, geometric shape.

Heating Curve for Water

http://media.wwnorton.com/college/chemistry/chemtours/interface.asp?chapter=chapter_05&folder=heating_curves

Phase Changes

• Phase changes that require energy to occur: Melting Vaporization Sublimation

• Phase changes that release energy: Freezing Condensation Deposition

Phase Changes Involve Heat Transfer

• Heat is the transfer of energy from an object at a higher temperature to an object at a lower temperature.

Phase Changes That Require Energy (cont.)

• When ice is heated, the ice eventually absorbs enough energy to break the hydrogen bonds that hold the water molecules together.

• When the bonds break, the particles move apart and ice melts into water.

• The melting point of a crystalline solid is the temperature at which the forces holding the crystalline structure together are broken and it becomes a liquid.

Phase Changes That Require Energy (cont.)

• Particles with enough energy escape from the liquid and enter the gas phase.

Phase Changes That Require Energy (cont.)

• Vaporization is the process by which a liquid changes to a gas or vapor.

• Evaporation is vaporization only at the surface of a liquid.

Phase Changes That Require Energy (cont.)

• In a closed container, the pressure exerted by a vapor over a liquid is called vapor pressure.

Phase Changes That Require Energy (cont.)

• The boiling point is the temperature at which the vapor pressure of a liquid equals the atmospheric pressure.

Phase Changes That Require Energy (cont.)

• Sublimation is the process by which a solid changes into a gas without becoming a liquid.

• Dry ice = solid CO2 changing directly to gas.

Phase Changes That Release Energy

• As heat flows from water to the surroundings, the particles lose energy.

• The freezing point is the temperature at which a liquid is converted into a crystalline solid.

• The bonds that are formed create a more stable situation and excess energy is released.

Phase Changes That Release Energy (cont.)

• As energy flows away (cooling) from water vapor, the velocity decreases.

• The process by which a gas or vapor becomes a liquid is called condensation.

• Deposition is the process by which a gas or vapor changes directly to a solid, and is the reverse of sublimation.

• Heat of fusion (Hf) = heat absorbed when a solid melts– Energy needed to break bonds (endothermic)

• Heat of solidification or crystallization (Hs) = heat released when a liquid freezes (= - Hf )– Energy released from the formation of bonds (exothermic)

• Heat of vaporization (Hv) = heat absorbed when a liquid changes into a gas– Energy needed to break bonds

• Heat of condensation (Hc) = heat released when a gas condenses to become a liquid (= - Hv )– Energy released from the formation of bonds

Heat Energy of Phase Changes

• Heat of vaporization:

• Heat of fusion

Heating Curves - Where is the energy going?

q = c x m x ∆TLeads to:

q = m x Hf (or Hv, Hs, Hc)

For water (memorize):Hf = 80 cal/g or 6.02 kJ/mol (melting/freezing)

Hv = 540 cal/g or 40.6 kJ/mol (boiling/condensing)

C liquid water = 1.00 cal/g*deg C or 4.184J/ g*deg CC ice = 0.50 cal/g*deg C or 2.1 J/ g*deg CC water vapor = 0.50 cal/g*deg C or 2.1 J/ g*deg C

Heat Energy of Phase Changes

Phase Diagrams• A phase diagram is a graph of pressure versus

temperature that shows in which phase a substance will exist at various T and P.

Phase Diagrams (cont.)

• The triple point is the temperature and pressure at which all three phases of a substance can coexist.

• Above the critical point temperature the substance can no longer be found in a liquid state at any P.

Phase Diagrams (cont.)

• The phase diagram for different substances are different from water.