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BU CHEM 108 - States of Matter

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CHEM 108 1st Edition Lecture 3Kinetic – Molecular TheoryAt constant pressure, the state of matter depends largely on two competing factors:1. The amount of thermal energy the particles possess, making the particles move apart from one another 2. The strength of attraction between the particles, making the particles condenseThese two factors are in competition with one another:1. Low temperature: thermal energy low compared to attraction – matter is in condensed state (liquid or solid) 2. High temperature: thermal energy high compared to attraction – matter is in gaseous stateStates and Degrees of Freedom- The molecules in a gaseous state have complete freedom of motion they have enough thermal energy overcomes the attractive forces between molecules - The molecules in a solid state are locked in place molecules and don’t have enough thermal energy to overcome the attractive forces- The molecules in a liquid state have limited freedom • they have enough kinetic energy toovercome some of the attractive forces, but not enough to escape each other and form a gasPhase Changes- Increasing thermal energy (i.e., heating) of a substance can cause a change of state- Decreasing thermal energy (i.e., cooling) of a substance can cause a change of state.- Six possible phase changes:Boiling point- The boiling point is the temperature at which the vapor pressure is equal to the pressure on the liquid, usually atmospheric pressure.- When the pressure on the liquid increases, as is the case with a pressure cooker, the boiling point increases. - Conversely, when the pressure on the liquid decreases, as is the case at high altitude, the boiling point decreases. - The stronger the intermolecular forces, the higher the boiling point. Thats why water has such a high boiling point due to its hydrogen bonding.SolubilitySolubility depends on relative strength of solute-solvent interactions, which also means that solubility will increase as polarity increases. Like dissolves Like: - Ionic/Polar solutes will be soluble in polar solvents.For example only polar substances will dissolve in water because water is a polar substance. - Nonpolar solutes will be soluble in nonpolar solvents.- Solubility decreases as relative hydrogen bonds decrease and also as dispersion forcesincreasesHenry’s Law: - The solubility of a sparingly soluble chemically unreactive gas is proportional to the partial pressure of the gas: Cgas = kH*PgasWhere C is the concentration of the gas in solution, kH is Henry’s Law constant for the gas, Pgas is partial pressure of gas.- The solubility of a gas (Sgas) is directly proportional to its partial pressure, (Pgas) Sgas = kH*Pgas- In general, pressure has little or no effect on the solubility of solids or liquids in water. But the solubility of a gas increases as pressure increases.- From Henry’s law we can deduce that S1P1=S2P2where S is solubility and P is pressure.Solubility Behavior-Hydrophobic (“water-fearing”) Interaction that repels water diminishes water solubility.Ex: Hydrocarbon compounds, any non-polar substance - Hydrophilic (“water-loving”) Interaction that attracts water, promotes water solubility.Ex: Hydroxyl compounds, any polar


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