Thursday, February 11, 2010

Section 4

Phase Diagrams

• A phase diagram is a graph of pressure versus temperature that shows the conditions under which the phases of a substance exists
• The triple point of a substance indicates the temperature and pressure conditions at which the solid, liquid, and vapor of the substance can coexist at equilibrium
• The critical point of a substance indicates the critical temperature and critical pressure
• The critical temperature (t sub c) is the temperature above which the substance cannot exist in the liquid state
• Above this temperature, water cannot be liquefied, no matter how much pressure is applied
• The critical pressure( P sub c) is the lowest pressure at which the substance can exist as a liquid at the critical temperature

Section 5

Water

Structure of Water

• Water molecules consist of two atoms of hydrogen and one atom of oxygen united by polar-covalent bonds
• The molecules in solid or liquid water are linked by hydrogen bonding
• The number of linked molecules decreases with increasing temperature
• Ice consists of water molecules in the hexagonal arrangement
• The hydrogen bonds between molecules of liquid water at 0 degrees C are fewer and more disordered than those between molecules of ice at the same temperature
• Liquid water is denser than ice
• As the temperature approaches the boiling point, groups of liquid water molecules absorb enough energy to break up into separate molecules

Physical Properties of Water

• At room temperature pure liquid water is transparent, odorless, tasteless, and almost colorless
• The molar enthalpy of fusion of ice relatively large compared with the molar enthalpy of fusion of other solids
• Water expands in volume as it freezes, because its molecules form an open rigid structure
• This lower density explains why ice floats in liquid water
• Both the boiling point and molar enthalpy of vaporization of water are high compared with those of non polar substances of comparable molecular mass
• The values are high because of the strong hydrogen bonding that must be overcome for boiling point to occur
• Steam (vaporized water) stores a great deal of energy as heat

Which will burn you worsts

(triangle) H sub f = 6.009 KJ/mole

(triangle) H sub v= 40. 79 KJ/mole