Presentation on theme: "Electrochemistry & Solutions 1. Solutions and Mixtures"— Presentation transcript:
1 Electrochemistry & Solutions 1. Solutions and Mixtures Department of ChemistryElectrochemistry & Solutions1. Solutions and MixturesYear 1 – Module 38 LecturesDr Adam Lee
2 AimsTo:Understand physical chemistry of solutions and their thermodynamic properties predict/control physical behaviour improve chemical reactionsLink electrochemical properties to chemical thermodynamics rationalise reactivity.
3 Synopsis Phase rule Clapeyron & Clausius-Clapeyron Equations Chemical potentialPhase diagramsRaoults law (Henry’s law)Lever ruleDistillation and AzeotropesOsmosisStructure of liquidsInteractions in ionic solutionsIon-ion interactionsDebye-Huckel theoryElectrodesElectrochemical cellsElectrode potentialsNernst EquationElectrode typesRecommended ReadingR.G. Compton and G.H.W. Sanders, Electrode PotentialsOxford Chemistry Primers No 41.P. W. Atkins, The Elements of Physical Chemistry,OUP, 3rd Edition, Chapters 5, 6 & 9.P. W. Atkins, Physical Chemistry,OUP, 7th Edition, Chapters 7, 8 & 10 OR 8th Edition, Chapters 4, 5, 6 & 7.
12 Gibbs Free Energy Josiah Willard Gibbs Josiah Willard GibbsGibbs Free EnergyAmerican mathematical physicist developed theory of chemical thermodynamics. First US engineering PhD…later Professor at Yale.
13 Benoit Paul Emile ClapeyronParisian engineer and mathematician. Derived differential equation for determining heat of melting of a solid
29 Chemical Potential (in English!) G ln(pressure) Molecules acquiremore spare energyGibbsFreeEnergyGreater “chemical potential”G ln(pressure)PressureLow PressureHigh PressureConstant TemperatureEffect of environmenton this free energyEnergy free for moleculesto “do stuff”at STPGmolar = G molar + RT lnp
30 Why do we use Chemical Potential? Gibbs Free Energy (G) is total energy in entire systemavailable to “do stuff”- includes all molecules, of all substances, in all phasesG = nAA + nBBFor single componente.g. pure H2OFor mixturese.g. H2O/C2H5OHNo real need to use Free energy from 2 sourcesG = nH2OH2OFree energy only comesfrom H2OG = nH2OH2O+nEtOHEtOH tells us how much fromH2O versus C2H5OH
31 Why do we different molecules have different Chemical Potentials? InvolatileVolatileEthanol can soak up much more energy in extra vibrational modes and chemical bonds- will respond differently to pressure/temperature increasesChemical Potential :1. A measure of "escaping tendency" of components in a solution2. A measure of the reactivity of a component in a solutionFreeEnergy(G)& Gas-phase moleculeLiquid-phaseSolid-statePressure
32 Raoult's law Volatility eqns. of straight lines passing thru origin Total pressure above boiling liquid
33 lnxA xA runs between 0 (none present) to 1 (pure solution) Mixing (diluting a substance) always lowers xA this means mixing ALWAYS gives a negative lnxAlnxAMixing alwayslowers Pure BPure ADilutionxA1
39 Case 2: -ve deviation A more attracted by B (e.g. CHCl3 + acetone) mixH = < 0b.pt. > idealABCase 3: +ve deviationA less attracted by B(e.g. EtOH + water)mixH = > 0b.pt. < ideal
40 Summary: Raoult’s Law for Solvents Proportionality constantpA = xA . pAΘTotal pressureVolatilehigh vapour pressureLiquidpoA1xPartial pressure of AInvolatilelow vapour pressureppBΘ1xoBPartial pressure of BpB = xB . pBΘAB
41 = tendancy of system to increase S High p0ө(A)Low p0ө(A)High order: low SLess order: higher SAAAAStrong desire to SLess need to SBoiling of AfavouredA happier in liquidp0ө = vapour pressure= tendancy of system to increase S
43 Dissolution is EXOTHERMIC For dissolution of oxygen in water, O2(g) O2(aq), enthalpy change under standard conditions is kJ/mole.
44 Consider O2 dissolution in water: pH2OpO2Solvent: H2OSolute: O2H2OO2Henry's law accurate for gases dissolving in liquids when concentrations and partial pressures are low.As conc. and partial pressures increase, deviations from Henry's law become noticeableConsider O2 dissolution in water:Important in Green Chemistry for selective oxidationCinnamic AcidCinnamaldehydeCinnamyl AlcoholSimilar to behavior of gases- deviate from the ideal gas law at high P and low T.Solutions obeying Henry's law are therefore often called ideal dilute solutions.Aspects of Allylic Alcohol OxidationAdam F. Lee et al, Green Chemistry 2000, 6, 279
50 Example ProblemThe following temperature/composition data were obtained for a mixture of octane (O) and toluene (T) at 760 Torr, where x is the mol fraction in the liquid and y the mol fraction in the vapour at equilibriumThe boiling points are C for toluene and C for octane. Plot the temperature/composition diagram of the mixture. What is the composition of vapour in equilibrium with the liquid of composition:1. x(T) = 0.252. x(O) = 0.25Boiling/CondensationTemperatureLiquidVapourx(T) = 1, T = Cx(O) = 1, x(T) = 0, T = CP.W. Atkins, Elements of Phys.Chem. page 141
56 Topics Covered (lectures 2-4) Chemical Potential- A(l) = A(l) + RTlnxA- A(g) = A(g) + RTlnpA Mol fractions- A = nA / nA+nB Raoult’s Law- pA = poA xA pB = poB xB- ideal solutions- +ve/-ve deviations Vapour-pressure diagrams- Tie-lines- Lever Rule
57 Solutions Equations Phase Rule F = c - p + 2 c = components degrees of freedom p = no. of phasesClapeyron EquationH = enthalpy of phase changeV = volume change associated with phase changeClausius-Clapeyron EquationorMol fractionsxA = nA / nA+nB ni = mols of iyA = pA / pA + pB pi = partial pressure of iRaoults LawpA = poA xA and pB = poB xBLever Rule (for tie-line joining phases via point a)nl =no. moles in liquid phasenv =no. moles in liquid phase