3-4-03
Nature of Solids
• Hydrogen chloride (a molecular compound) has a mp=-112 deg. C
• Not all solids melt- wood and sugar tend to decompose when heated.
• Most solid substances are crystalline in structure.
• In a crystal, such as Fig. 16.1, page 397, the particles (atoms, ions, or molecules) are arranged in an orderly, repeating, three-dimensional pattern called a crystal lattice.
• All crystals have a regular shape, which reflects their arrangement.
• Some solid substances can exist in more than one form.
• Elemental carbon is an example, as shown in Fig 16.12, page 403
• 1. Diamond, formed by great pressure
• 2. Graphite, which is your pencil
• 3. Buckminsterfullerene (also called "buckyballs") arranged in hollow cages like a soccer ball
3-6-03
• These are called allotropes of carbon, because all are made of carbon, and all are solid.
•Allotropes are two or more different molecular forms of the same element in the same physical state.
•Not all solids are crystalline, but instead are amorphous.
•Amorphous solids lack an ordered internal structure
•Rubber, plastic, and asphalt are all amorphous solids- their atoms are randomly arranged.
•Another example is glasses- substances cooled to a rigid state without crystallizing.
•Glasses are sometimes called supercooled liquids.
•The irregular internal structures of glasses are intermediate between those of a crystalline solid and a free-flowing liquid.
•Do not melt at a definite mp, but gradually soften when heated.
•When a crystalline solid is shattered, the fragments tend to have the same surface angles as the original solid.
•By contrast, when amorphous solids such as glass is shattered, the fragments have irregular angles and jagged edges.
3-10-03
The Properties of Gases
Objectives
-Describe the properties of gas particles
-Explain how the kinetic energy of gas particles relates to Kelvin temperature.
Kinetic Theory Revisited
-1. Gases- consist of hard, spherical particles (usually molecules or atoms)
-2. Small- so the individual volume is considered to be insignificant.
-3. Large empty space between them.
-4. Easily compressed and expanded.
-5. No attractive or repulsive forces
-6. Move rapidly in constant motion
-Recall that the average kinetic energy of a collection of gas particles is directly proportional to the Kelvin temperature of the gas.
Variables that describe a gas
-1. pressure (P) in kilopascals
-2. volume (V) in Liters
-3. temperature (T) in Kelvin
-4. number of moles (n)
Factors that affect gas pressure
-1. Amount of Gas
•When we inflate a balloon, we are adding gas molecules
•Increasing the number of gas particles increases the number of collisions
-thus, the pressure increases.
•If temp. is constant- doubling the number of particles doubles pressure.
Pressure and the number of molecules are directly related.
•More molecules means more collisions
•Fewer molecules means fewer collisions.
•Gases will naturally move from areas of high pressure to low pressure because there is an empty space to move in- spray can is example.
Common Use?
•Aerosol (spray) cans
-gas moves from higher pressure to lower pressure
-a propellant forces the product out
-whipped cream, hair spray, paint
-2. Volume of Gas
•In a smaller container, molecules have less room to move.
•Hit the sides of the container more often
•As volume decreases, pressure increases (think of a syringe)
P/(1/v)= constant, PV= constant (inverse relationship)
-3. Temperature of Gas
•Raising the temperature of a gas increases the pressure, if the volume is held constant.
•The molecules hit the walls harder, and more frequently.
•The only way to increase the temperature at constant pressure is to increase the volume.
5. Combined Gas Law
•The combined gas law deals with the situation where only the number of molecules stays constant.
•Formula: (P1 x V1)/T1=(P2 x V2)/T2
•This lets us figure out one thig when two of the others change
•A 15L cylinder of gas at 4.8 atm pressure and 25 C is heated to 75 C and compressed to 17 atm. What is the new volume?
Dalton's Law of Partial Pressures
•The total pressure inside a container is equal to the partial pressure due to each gas.
•The partial pressure is the contribution by that gas.
Ptotal=P1+P2+P3
