Kinetic Molecular Theory(Do not take notes) • Composition & structure of molecules affect the chemical & physical properties of matter. • Solids & liquids have a lot of variation between both physical & chemical properties. • Gases, however, have very similar physical and chemical properties.
Kinetic Molecular Theory • Explains the behavior of gases in terms of particles in motion • Assumptions: • Particles are very small with huge amounts of empty space between them (so no attractive or repulsive forces between them). • Gas particles are in constant, random motion. Move in straight lines until they collide with each other or container walls. Collisions are elastic – no energy is lost. • Mass & velocity affect the kinetic energy of individual gas particles (K.E. = ½ mv2)
Using KMT to explain gases • Low density: gases have extremely low densities because of the large spaces between gas molecules • Compression & expansion: gases can be compressed if you decrease the space between gas molecules; the random motion of gas particles will cause gases to expand to any space available • Diffusion & effusion: both of these properties occur because of the constant random motion of gas particles and the lack of attractive or repulsive forces among gas particles.
Gas Pressure • Pressure is the force exerted compared to the area so lying down on ice spreads out your weight. • Air pressure or atmospheric pressure is exerted in all directions since air is all around us moving randomly • Measured with a barometer – one end is a vacuum; the other is open to the air • Many units are used for pressure: pascal, torr, psi and atmospheres are all common
Intermolecular Forces of Attraction • Occur between identical particles & help explain how solids, liquids and gases exist at the same temperature • Three main types: dispersion, dipole-dipole and hydrogen bonds. • All intermolecular forces are weaker than intramolecular forces (they have to be)
Dispersion Forces • The weakest intermolecular force • Significant in nonpolar substances because no other intermolecular forces exist • Explains why non-polar liquids are hard to pour and evaporate quickly (alcohols, gasoline, etc)
Dipole-Dipole Forces • Occur in polar molecules because permanent dipoles exist • Molecules line up head to tail or positive region near a negative region • Stronger than dispersion forces but weaker than hydrogen bonds
Hydrogen bonds • A special type of dipole-dipole bond • Occur only between H and fluorine, oxygen or nitrogen • Explains why water (18.00 g/mol) has such a huge surface tension and high boiling point while methane (16.05 g/mol) has very low surface tension and is a gas at room temperature
Properties of liquids • Liquid particles have random motion but greater attractive forces than gases, so liquids have a volume but no shape • Density is also greater than gases while compression is much less since liquid particles are already close • Liquids are less fluid than gases because they diffuse much slower due to the intermolecular forces interfering
Properties of liquids • Viscosity is the measure of the resistance of a liquid to flowing • Viscosity is influenced by intermolecular forces, shape of the particle and temperature • Year round oils actually change shape from spheres in cold weather to long strands when hot to increase viscosity
Properties of liquids – surface tension • Particles at the surface have a greater downward pull than particles in the middle • Greater intermolecular forces usually means greater surface tension • Water forms a drop because of its high surface tension • Surfactants are compounds that lower surface tension in a substance, like detergent in water
Properties of liquids – capillary action • Occurs when water is placed in a container or substance that it’s highly attracted to • If the attraction (adhesion) is greater than water’s attraction to itself (cohesion), capillary action occurs. • Like the downward curve you see when a liquid is in a glass • When water is drawn up between the cellulose fibers of paper towels or the crystals in a diaper
Properties of Solids • According to KMT solids have as much kinetic energy at room temp as gases or liquids, but attractive forces are so great that particles in a solid move around a fixed point • Most solids are more dense than liquids or gases (water is the exception)
Phase Changes • Most substances can exist in 1 of 3 states on Earth: solid, liquid or gas depending on the pressure and temperature • When energy is added or removed a substance may change from one phase to another • KMT predicts this because as temperature increases, motion increases and forces of attraction can be overcome.
Phase Changes that Require NRG • Includes melting, vaporizing (boiling), evaporating and sublimation • For melting, boiling and sublimation, the temperature plateaus where the phase change occurs because all incoming energy is being used to break bonds • Stronger bond = more energy = higher boiling point or lower freezing point
Evaporation • When a liquid changes to a gas but only molecules at the surface escape to become vapor • Occurs at a lower temperature than boiling point and is also a slower process than vaporization • Explains why sweating cools us or other animals down
Sublimation • Process in which a solid goes directly to a gas without a liquid phase • Iodine, dry ice, moth balls and air fresheners all tend to sublime • Water (ice) will sublime if pressure is decreased – like when ice cubes shrivel up in your freezer • Freeze dried food is when food is frozen in a vacuum so the water sublimes. Makes food lighter and unable to grow bacteria.
Phase Changes that release NRG • Includes condensation, freezing, & deposition • Occurs when a vapor or liquid comes into contact with a cooler substance. The gas or liquid loses heat and if enough heat is lost, the forces of attraction become great enough to form a liquid or solid. • This is why clouds form and why it warms up when it starts snowing • Deposition is when a gas goes right to a solid like when it snows or frost forms
Phase Diagrams • Graphs pressure versus temperature • Lines on diagram indicate where more than one phase exists • Triple point is the temperature and pressure where all 3 states would exist or all 6 phase changes can occur • Critical point = pressure and temperature where water can not exist as a liquid; regardless of how much pressure you put on the vapor beyond this point it will be a gas