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R = 0.08206 atm L/mol K Ideal gas equation predicts gas behavior 2
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Ideal gas law describes what gases do, but not why. Kinetic Molecular Theory of Gases (KMT): model that explains gas behavior. developed in mid-1800s based on concept of an ideal or perfect gas 3
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Tiny particles in constant, random, straight-line motion Molecules collide w/ each other & w/ walls of container Gas molecules are points; gas volume is empty space between molecules Molecules independent of each other (no attractive or repulsive forces between them). 4
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Moving molecule has kinetic energy KE depends on mass (m) and speed (u) Temperature (in K) proportional to average molecular KE At higher T, average speed higher At lower T, average speed lower At T = 0, speed = 0 (molecules stop moving) 5
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Different gases at same temperature All have same average KE (same temperature) Heavier gases are slower; lighter gases are faster 6
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7 Molecules colliding with container → gas pressure What if there are more molecules? More collisions → higher pressure
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8 Molecules colliding with container → gas pressure What if the container is smaller? More collisions → higher pressure
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9 Molecules colliding with container → gas pressure What if the molecules are moving faster? Harder, more frequent collisions → higher pressure
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10 Moving molecules fill the container Light molecules escape faster, heavy molecules more slowly Large spaces between molecules allow gas to be compressed
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11 Ideal gas remains a gas when cooled, even to 0 K Real gases condense to liquid state when cooled How do we explain condensation? Pressure (atm) Temperature (K) 0 0 Ideal gas pressure decreases steadily & becomes zero at absolute zero Real gas pressure decreases abruptly to zero when gas condenses to liquid
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12 KMT ignores attractions between gas molecules Gas molecules are too far apart & too fast for attractions to act BUT... attractive forces do exist between all molecules! At low enough T, attractions overcome kinetic energy & molecules stick together to form a liquid
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For every substance there are 2 opposing tendencies: Kinetic energy of the molecules, which tend to make them move apart from each other (gas-like) Attraction between molecules, which tends to make them stick together (liquid-like) At any given temp., kinetic energy is the same for all molecules, so the attractive forces between molecules determines whether something is a liquid, solid, or gas.
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At same temp. (same KE), molecules are gases, liquids, solids. This suggests that some molecules have stronger attractions between molecules. How do you judge the strength of the attractive forces between molecules? Look at the boiling point. Low Boiling point = weak intermolecular forces (molecules are not sticky) High Boiling point = strong intermolecular forces (molecules are sticky)
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Molecules of similar structure: Boiling points increase as molar mass increases Suggests that intermolecular attractions increase as molecular mass increase FormulaMolar Mass BPFormulaMolar Mass BP F2F2 38 g/mol85 KCH 4 16 g/mol111 K Cl 2 71 g/mol239 KC2H6C2H6 30 g/mol184 K Br 2 160 g/mol332 KC3H8C3H8 44 g/mol231 K I2I2 254 g/mol457 KC 4 H 10 58 g/mol273 K
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Predict which would have a higher boiling point, and why? Na or K F 2 or Br 2
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As you heat a solid, the temp. increases until the solid melts. Temp. will remain constant until solid melts completely. You observe the same pattern when a gas is cooled until it changes into a solid Temp. will remain constant until gas changes into solid
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18 Add energy Temperature
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19 melting/freezing point Add energy Temperature (s) melting freezing boiling condensing (l) (g) boiling point
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20 Melting & boiling are ENDOTHERMIC Freezing & condensing are EXOTHERMIC
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21 Changing temperature changes KE (#1, 3, 5) Changing state changes potential energy (#2, 4) energy to melt or freeze = heat of fusion (∆H fusion ) energy to vaporize or condense = heat of vaporization (∆H vaporization )
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24 An H atom is covalently bonded (red-white) to its own O and weakly bonded (dotted line) to the neighboring O This weak bond to a neighboring O is called a hydrogen bond The O–H bond in water is very polar, and the atoms are very small The dipoles are close together, so their attraction is very strong
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25 Hydrogen bonding occurs only between molecules containing N–H, O–H, and F–H bonds Hydrogen bonding is much stronger than ordinary intermolecular attractions ⇒ very high boiling points for their mass Hydrogen bonds are not as strong as covalent bonds (15-40 kJ/mol, vs >150 kJ/mol)
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Heat of Fusion- change in energy when a solid substance melts Heat of Vaporization- change in energy when liquid substance vaporizes (evaporates)
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Given that the heat of fusion (ice) is 6.0 kj/mol, how much energy is needed to melt an ice cube with a mass of 42 g? Given that the heat of vaporization (water) is 41 kj/mol, how much energy is need to convert 24 g of water into steam?
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A quick look into the past… At any given temp. molecules have the same avg. KE Light molecules = fast Heavy molecules = slow Having the same avg. KE means some molecules are moving faster than avg., and some slower.
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Consider if you will… A container of water at 25 o C (room temp.) All the water molecules have the same avg. KE Water is not hot, but a few fast molecules will leave and become water vapor. Vapor molecules exert pressure = vapor pressure
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Now, consider this… Water molecules can escape from open container, but not a closed container. Energy is transferred by collisions w/ other vapor molecules. Some vapor molecules will be slow enough to return to liquid. Eventually, rate of evaporation = rate of condensing This is called DYNAMIC EQUILIBRIUM
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Molecules continue to evaporate and condense. # of vapor molecules and vapor pressure are constant. A liquid in a closed container has a constant equilibrium vapor pressure. Changing temp. changes equilibrium, and vapor pressure remains constant at any given temp.
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Imagine this… Open container and heat it. KE increases, more molecules evaporate Bubbles of water form throughout liquid, rise to surface and break… boiling begins
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As temp. increases, vapor pressure increases A liquid boils when the vapor pressure matches the external pressure. In an open container, external pressure is atmospheric pressure
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