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KU CHEM 170 - What Affects Solubility

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Chem 170 1st Edition Lecture 13Outline of Last Lecture I. SolutionII. Concentration UnitsIII. EntropyOutline of Current Lecture IV. Last Lecture: EntropyV. What Affects SolubilityCurrent LectureChapter 13 – SolutionsLast Lecture: Entropy (S) is a measure of disorder. The greater the degree of randomness or disorder in a system, the greater the entropy of a system.ex. NaCl a crystalline solid is dissolved in water. −¿+¿+Cl¿NaCl(s )→ Na¿endothermic: ΔH>0−¿+ H2O+¿+Cl¿Na¿endothermic: ΔH>0NaCl settles down by solvation exothermic ΔH<0total is endothermic ΔH<0ΔH= ΔHNaC l+ ΔHwater + ΔHsolventTherefore the dissolution of NaCl in water requires energy. This seems counterintuitive, but NaCl(s) has a crystalline structure (high order) while Na+(aq) + Cl-(aq) is disorderly. Therefore the dissolution of NaCl uses energy and increases the disorder of the universe.What Affects Solubility?Since the reactions olute(s)+so lvent(l)→ solutionis endothermic (ΔH>0)it can be rewritten as∆ H +solute+solvent → solutionLe Chatelier’s principle states that if a system at equilibrium is placed under stress (temperature, pressure or volume) then the system will respond to find a new equilibrium. In this case if heat is added to the reaction, then to relieve the stress the solid solute will dissolve in the solvent.These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.On the other had one would cool down gasses to dissolve them into the solvent. An example of this would be CO2 in soda. The solubility increase as temperature decreases, because as temperature decreases the molecules will get closer together, thereby lowering the energy of the system and lowering the entropy. This explains why gas bubbles appear when the temperature increases. According the Le Chatelier’s principle temperature stress is relieved through gas bubbles.s olute(g)+so lvent(l)→ solution is exothermic (ΔH<0)aka. solute+solvent→ solution+∆ HHenry’s Law states that the solubility of a gas increases as pressure increases (Concentration=K Pgas , where K is aa constant of a gas). This makes sense because as the vapor pressure above the solution increase then there will be more gas in the solution, which is shown by an increased boiling point. ex. The solubility of a natural gas at 20°C and 1 atm is 0.02 g/kg of water. If the pressure is increased to 20 atm while the temperature remains 20°Cabove 1000 kg of water, then what mass of the natural gas will dissolve into the water?k =CP=0.02 gkg of water1 atm=0.02 gkg of water ×atmC=kP=0.02 gkg(20 atm)=0.4 gkg of water(0.4 gkg of water)(1000 kg of water)=400 g of natrual gasFrench chemist F. M. Rault, discovered that a solute added to a solvent always reduced the vapor pressure of a solution and increased the boiling point.Let χB=mole fractionof solutePA0=vapor pressure of pure solventPA=vapor pressure of A above the solutionΔ P=PA0−PA= χBPA0note: if there were 2 components then χA+ χB=1and χA=mol Amol A−mol BPA0−PA=(1− χA)PA0PA= χAPA0ex. There was a mixture of benzene (C6H6, P0=95.1 mmHg) and toluene (C7H8,P0=28.4 mmHg) and the mole ratio of each is 0.5.χtoluene=0.5χbenzene=0.5Benzene has a higher vapor pressure, because there is more benzene in the gas phase than toluene.Pbenzene= χAPA0=0.5× 95.1=47.6 mmHgPtoluene= χBPB0=0.5 ×28.4=14.2 mmHgtotal P=47.6 mmHg+14.2 mmHg=61.8


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