The equilibrium phenomena that does not involve any chemical change such as phase changes Unit 5 Physical Equilibria and solubility Phase Equilibria Thermodynamics o Free Energy Entropy makes everything wander away all over the universe enthalpy via IMFs holds everything together At any given temperature the most stable phase is the substance with the lowest free energy o IMFs lower enthalpy Energy is lower when molecules are together than when they are apart Lowest enthalpy solids Highest enthalpy gases Heating Curves o No phase transition energy flowing into system leads to temperature change o Phase transition energy flowing into system and temperature stays constant Energy results in higher potential energy to break IMFs between molecules o Slope of line during heating is heat capacity o Length of line during phase transition is amount of heat required to melt solid Phase Diagram Shows regions of stability of different phases as a function of temperature and pressure o Water s solid liquid line has a negative slope Most substances have a positive solid liquid line o Triple point Temperature and pressure at which three phases are in equilibrium Typically the solid liquid gas triple point Many substances have more than one solid phase and therefore can have more than one triple point o Critical point vaporization curve the substance exists as a super critical fluid At temperatures and pressures greater than last point of Super critical fluid can vary between the two extremes of the liquid and the gas A fluid like a liquid and a gas but has a density that Going to a SCF is not a phase transition Vapor pressure The pressure of the vapor over a liquid at equilibrium partial pressure of a substance in equilibrium with its condensed phase o Depends only on temperature and IMF o High vapor pressure easiest to get into gas phase Think easiest to evaporate o Open container molecules diffuse into atmosphere and eventually all liquid o Closed container molecules that evaporate will diffuse but some will collide with evaporates liquid and condense again Rate of evaporation depends on temperature and IMF Rate of condensation depends on temperature IMF and concentration in gas phase Vapor Pressure and Free Energy G H TS o The lower the free energy the more stable the state o If IMFs are strong the vapor pressure is low Only few molecules can overcome the attractions of molecules for each other o At vapor pressure the liquid and the vapor have the same free energy Vapor Pressure and Temperature o Vapor pressure of all liquids increases with increasing temperature As temperature increases we favor a higher entropy state gas phase Hvap RT P Ke Where K vapor pressure in the limit of infinite temperature Claussius Clapeyron Equation Derivation of the Claussius Clapeyron equation assume that the vapor behaves as an ideal gas such that its molar free energy chemical potential is simply given as P G G RTln P Solutions homogenous mixtures typically composed of a major component that is a liquid solvent and a minor component that is dissolved in the liquid solute o Solvent is the majority of mixture IMFs are very similar to pure substance o Solute completely surrounded by solvent IMFs for solute extremely different Mixtures o Gases mix spontaneously at constant temperature and pressure because G 0 No IMFs no potential energy constant enthalpy H 0 Caused by entropy increase o A solution is ideal if there is no change in IMFs H 0 for forming the solution Enthalpy of Solution Hsolution Hlatticeenergy Hsolvation o Two key parts to changes in the forces Largest changes are for solute from losing solute solute interaction and gaining solute solvent interactions o H solution is the change in enthalpy on forming the solution H lattice energy energy required to pull solute apart Positive number energy in H solvation energy released when the solute interacts with the solvent AKA H hydration when talking about solutions with water as solvent Negative number energy out Requires energy to dissolve something Generally the lattice energy is larger than the solvation energy and the enthalpy of solution is positive Like Dissolves Like What Will Dissolve o Dissolution is spontaneous when G 0 T Ssolution Hsolution Can be made possible by raising temperature Can be made possible by making enthalpy term very small o Make change in IMFs from unmixed solute and solvent to solution are minimal What Won t Dissolve o The larger the charge density of two ions the stronger the forces holding together the solute and the less likely they are to dissolve Charge density how compact the charge of an ion is Small ion with large charge High charge density Large ion with low charge Low charge density o When high charge density ions dissolve into solution they interact very strongly with the water molecules Water clings to ion Reduces entropy of water Entropy of solution for high charge density ions is negative o Enthalpy is now large because different IMFs and entropy is negative therefore Miscibility nothing is in favor of dissolving making a solution of liquids o If two liquids mix in any proportions we call them miscible o If they don t they are immiscible Henry s Law Mixtures of liquids and gases the mole fraction of gas dissolved in the liquid is directly proportional to the pressure of the gas over the liquid Pgas KXgas Cgas kHPgas Where K is a constant that depends on IMFs between the solute and the solvent And Xgas is the mole fraction o For a gas the pure substance has a concentration that is extremely dependent on the conditions Most important factor for a gas dissolving into a liquid is the concentration of the gas above the liquid partial pressure o The solubility of gas depends on the partial pressure of the gas above the solvent Double the pressure Double the concentration mole fraction Other differences for gases dissolving into liquids o Enthalpy of gases dissolving in a liquid is negative Don t have to overcome IMFs since they are in the gas phase and already pulled apart thus lattice energy is zero Hsolution Hsolvation which will be a negative term Dissolution of gases is exothermic Gas solubility decreases with increasing temperature o Entropy of gases dissolving in a liquid is negative Solute goes from gas phase to a liquid Solution Properties Concentration measure of the amount of solute that is dissolved in the solvent molarity mol solute L solution molality mol solute kg solvent The Van t Hoff Factor i the
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