Thermochemical Equations• Thermochemical equations are

balanced chemical equations that include the physical states of all reactants and products and the energy change.

• 2H2O(l) 2H2(g) + O2(g) ΔH = 572 kJ

• CH4(g) + 2O2(g) CO2(g) + 2H2O(g) ΔH = -802 kJ

Phase Changes• Occurs when energy is added or

removed from a system and the substance can go from one physical phase to another

Enthalpy of Combustion• Enthalpy heat of combustion

(ΔHcomb) is the enthalpy change for the complete burning of one mole of the substance.

-- carried out under standard conditions which are one atmospheric pressure (1 atm) and 298K (250C)

- C6H1206(s) + 6O2(g)6CO2(g)+ 6H2O(l)ΔHo

comb = -2808kJ

Enthalpy of CombustionSubstance Formula H comb

Sucrose C12H22O11 -5644

Octane C82H18(l) -5471

Glucose C6H12O6(s) -2808

Propane C3H8(g) -2219

Ethanol C2H5OH(l) -1367

Methane CH4(g) -891

carbon C(s) -394

Phase Changes• Occurs when energy is added or

removed from a system and the substance can go from one physical phase to another

Changes of State• Molar enthalpy (heat) of

vaporization (ΔHvap) is the heat required to vaporize one mole of liquid.

-- think of water vaporizing from your skin after you take a hot shower. Your skin provides the heat needed to vaporize the water and as the water absorbs the heat you feel cool (shiver)

ΔHvap = -ΔHcond (condensation)

Changes of State • Molar enthalpy (heat) of fusion

(ΔHfus) is the heat required to melt one mole of a solid substance.

--think of ice in a drink. The drink cools as it provides the heat for the ice to melt

ΔHfus = - ΔHsolid (solidification—freezing)

Changes of StateSubstance Formula Hvap kJ/mol Hfus kJ/mol

Water H2O 40.7 6.01

Ethanol C2H5OH 38.6 4.92

Methanol CH3OH 35.2 3.22

Ammonia NH3 23.3 5.66

??What do you notice about the magnitude of the molar enthalpy of vaporization versus the molar enthalpy of fusion?

The molar enthalpy of vaporization for a substance is much larger than the molar enthalpy of fusion for the same substance. It takes much more energy to change a substance from a liquid to a gas than it does to change a solid to a liquid.

Endothermic Phase ChangesMelting• The energy absorbed to melt a solid

is not used to raise the temperature of that solid

• The energy instead disrupts the bonds holding the solid’s molecules together and cause the molecules to move into the liquid phase

Endothermic Phase Changes• The amount of energy required to

melt one mole of a solid depends on the strength of the forces that hold the solid together

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

Vaporization• Particle that escape from the liquid enter

the gas phase and those liquids at room temperature the gas phase is called vapor

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

• Once the solid becomes a liquid then and only then does the temperature of the substance begin to increase

Endothermic Phase Changes

• When vaporization takes place only at the surface of the liquid it is called evaporation

• Evaporation is the method by which the human body maintains and controls its temperature

Endothermic Phase Changes

Sublimation• Is the process by which a

solid changes directly to a gas without first becoming a liquid• Dry ice (CO2) and snow are

the most common examples

Endothermic Phase Changes

• If ice cubes are left in the freezer for extended periods of time, they will eventually sublime and become smaller–This process is also helpful in freeze drying foods for hikers and astronauts

Endothermic Phase Changes

Exothermic Phase ChangesCondensation• When a vapor molecule loses energy

its velocity is reduced therefore colliding more with other molecules to form a liquid

• Condensation is the process by which a gas or vapor becomes a liquid and it is the reverse action of vaporization

Deposition• Is the process by which a substance

changes from a gas or vapor to a solid without first becoming a liquid

• It is the reverse action of sublimation• The formation of snow crystals high

up in the atmosphere is an example

Exothermic Phase Changes

Freezing Point• Is the temperature at which a

liquid is converted into a crystalline solid• The same temperature as the

melting point of a given substance

Exothermic Phase Changes

Phase Change Graph

• Graph shows the energy required to go from one phase to the other• Where the graph inclines,

potential energy is at its greatest and temperature is increasing• Where the graph plateaus (flat

region) kinetic energy is at its greatest but the temperature remains constant

Phase Change Graph

Phase Diagrams• A phase diagram is a graph of

pressure versus temperature that shows in which phase a substance exists under different conditions of temperature and pressure

The triple point is the point on a phase diagram that represents the temperature and pressure at which three phases of a substance can coexist

The critical point is the point that indicates critical pressure and temperature above which water cannot exist as a liquid

Phase Diagrams

Phase Diagrams• Different for each substance

because of the different boiling/freezing points

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