Enduring Understanding 2.A: Macroscopic Physical Properties of Matter
- The physical properties of matter result from the structure of, arrangement of, and forces between the atoms, ions, and molecules that compose matter.
- The properties of solids, liquids and gases reflect the relative orderliness, freedom of motion, and the strength of interaction of the particles in those states.
- Solids are most orderly, with the least freedom of motion and the strongest interparticle bonds.
- Gases are the opposite, with the least order, greatest freedom of movement and weakest interparticle bonds.
- Liquids are intermediate, between solids and gases.
- Solids where the particles do not move much with respect to each other, can be crystalline, arranging themselves in a regular 3D lattice structure, or amorphous, with a more random arrangement. Solids have strong interparticle interactions.
- In liquids, the particles are also close to each other with relatively strong interparticle interactions, but they can move translationally.
- Physical properties, like viscosity and surface tension (in liquids) and hardness and malleability (in solids) depend on the strength of interparticle forces in the substance.
- Gases have particles that are separated from each other and free to move, and the forces between particles are minimal. Gases do not have a definite volume or a definite shape.
- The behavior of gases can be modeled by the Kinetic Theory of Gases. This 'ideal' behavior assumes tiny particles, and no interactions between the gas particles.
- No gas displays perfectly ideal behavior, but smaller, nonpolar atoms and molecules (e.g. H2, He) tend to be closer to ideal than large or polar gases (Ar, SO2)
- The Ideal Gas Law predicts the relationship between pressure, volume, and temperature for a given number (n) of particles: PV = nRT (R is a constant, the Gas Constant)
- Example: An ideal gas at a pressure of 4 atm in a rigid container is cooled from 400K to 200K. What is the expected new pressure in the container?
- By the ideal gas law, (PV/nT)1 = (PV/nT)2; n and V are constant so...
- (P/T)1 = (P/T)2, so 4/400 = P2/200
- P2 = 4 x 200/400 = 2 atm
- Because, at a given temperature and pressure, given number of particles will take up the same volume regardless of their mass, gases composed of particles with higher mass (like Ar, Kr) will have a higher density than gases composed of particles with a lower mass (H2, He), proportional to their relative masses.
- Example: At STP, hydrogen gas (H2 2.02 g/mol) has a density of 0.09 kg/m3. Assuming ideal behavior, what would an estimate the density of argon (Ar, 39.95 g/mol) be at STP?
- According to the ideal gas law, at the same pressure and temperature, a given volume will contain the same number of particles, n. Density (ρ) is mass/volume, so ρH2 = 0.09 kg/m3 = n (2.02 g/mol)/1 L and ρAr = n (39.95 g/mol)/1 L
- Rearranging: ρAr = 0.09 kg/m3 (39.95 g/mol)/(2.02 g/mol)
- ρAr = 0.09 kg/m3 x 20 = 1.8 kg/m3
- The estimate, 1.8 kg/m3, is quite close to the actual value of 1.78 kg/m3
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