Presentation is loading. Please wait.

Presentation is loading. Please wait.

Electrochemistry & Solutions 1. Solutions and Mixtures

Similar presentations

Presentation on theme: "Electrochemistry & Solutions 1. Solutions and Mixtures"— Presentation transcript:

1 Electrochemistry & Solutions 1. Solutions and Mixtures
Department of Chemistry Electrochemistry & Solutions 1. Solutions and Mixtures Year 1 – Module 3 8 Lectures Dr Adam Lee

2 Aims To: Understand physical chemistry of solutions and their thermodynamic properties  predict/control physical behaviour  improve chemical reactions Link electrochemical properties to chemical thermodynamics  rationalise reactivity.

3 Synopsis Phase rule Clapeyron & Clausius-Clapeyron Equations
Chemical potential Phase diagrams Raoults law (Henry’s law) Lever rule Distillation and Azeotropes Osmosis Structure of liquids Interactions in ionic solutions Ion-ion interactions Debye-Huckel theory Electrodes Electrochemical cells Electrode potentials Nernst Equation Electrode types Recommended Reading R.G. Compton and G.H.W. Sanders, Electrode Potentials Oxford 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.

4 Phase Diagrams


6 Vacuum Pump



9 No. of degrees of freedom No. of components No. of phases



12 Gibbs Free Energy Josiah Willard Gibbs
Josiah Willard Gibbs Gibbs Free Energy American mathematical physicist developed theory of chemical thermodynamics. First US engineering PhD…later Professor at Yale.

13 Benoit Paul Emile Clapeyron Parisian engineer and mathematician. Derived differential equation for determining heat of melting of a solid




17 Modified eqn. for Liquid/Gas and Solid/Gas Lines: Clausius -
Clapeyron Eqn. Maths dx/x = dlnx dp/p = dlnp = ò dx x n 1 + 2 T dT dp V H D Clapeyron Equation Rudolf Julius Emmanuel Clausius


19 Clausius-Clapeyron Equation

20 99





25 GA = GoA + nART ln pA, GB = GoB + nBRT ln pB
Consider ideal gas at constant temperature: dG = Vdp – SdT = Vdp Since pV = nRT, If initial state 1 = STP (1 atm) In general for a mixture AB: GA = GoA + nART ln pA, GB = GoB + nBRT ln pB since GA = nAA A = Ao + RT ln pA

26 p = pA + pB

27 nA nB nA+ nB Ginitial = nAA + nBB = nA[ + RTlnp] + nB[B + RTlnp]
yA yA yB yB Ginitial = nAA + nBB = nA[ + RTlnp] + nB[B + RTlnp] Gfinal = nA[ + RTlnyAp] + nB[B + RTlnyBp] G = Gfinal - Ginitial = RT[nAlnyAp – nAlnp + nBlnyBp - + nBlnp] nA nB nA+ nB A B Initial Final ya yb Gmixing ya yb

28 yA yA yB yB yA yA yB yB Gmixing Smixing ya ya 1 yb 1 yb

29  Chemical Potential (in English!) G  ln(pressure)
Molecules acquire more spare energy Gibbs Free Energy Greater “chemical potential” G  ln(pressure) Pressure Low Pressure High Pressure Constant Temperature Effect of environment on this free energy Energy free for molecules to “do stuff”at STP Gmolar = G molar + RT lnp

30 Why do we use Chemical Potential?
Gibbs Free Energy (G) is total energy in entire system available to “do stuff” - includes all molecules, of all substances, in all phases G = nAA + nBB For single component e.g. pure H2O For mixtures e.g. H2O/C2H5OH No real need to use  Free energy from 2 sources G = nH2OH2O Free energy only comes from H2O G = nH2OH2O+nEtOHEtOH  tells us how much from H2O versus C2H5OH

31 Why do we different molecules have different Chemical Potentials?
Involatile Volatile Ethanol can soak up much more energy in extra vibrational modes and chemical bonds - will respond differently to pressure/temperature increases Chemical Potential : 1. A measure of "escaping tendency" of components in a solution 2. A measure of the reactivity of a component in a solution Free Energy (G) &  Gas-phase molecule Liquid-phase Solid-state Pressure

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 lnxA lnxA Mixing always lowers  Pure B Pure A Dilution xA 1


35 Volatile Involatile

36 Volatile Involatile



39 Case 2: -ve deviation A more attracted by B (e.g. CHCl3 + acetone)
mixH = < 0 > ideal A B Case 3: +ve deviation A less attracted by B (e.g. EtOH + water) mixH = > 0 < ideal

40 Summary: Raoult’s Law for Solvents
Proportionality constant pA = xA . pAΘ Total pressure Volatile high vapour pressure Liquid p o A 1 x Partial pressure of A Involatile low vapour pressure p pBΘ 1 x o B Partial pressure of B pB = xB . pBΘ A B

41 = tendancy of system to increase S
High p0ө(A) Low p0ө(A) High order: low S Less order: higher S A A A A Strong desire to  S Less need to  S Boiling of A favoured A happier in liquid p0ө = vapour pressure = tendancy of system to increase S

42 Proportionality constant
Amount in solution

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:
pH2O pO2 Solvent: H2O Solute: O2 H2O O2 Henry'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 noticeable Consider O2 dissolution in water: Important in Green Chemistry for selective oxidation Cinnamic Acid Cinnamaldehyde Cinnamyl Alcohol Similar 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 Oxidation Adam F. Lee et al, Green Chemistry 2000, 6, 279


46 A yA xA


48 Liquid-Gas Distribution
Volatile Involatile B A LIQUID GAS Lever Rule Tie-line LIQUID GAS A B A-B Composition Liquid-Gas Distribution


50 Example Problem The 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 equilibrium The 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) = 0.25 2. x(O) = 0.25 Boiling/ Condensation Temperature Liquid Vapour x(T) = 1, T = C x(O) = 1, x(T) = 0, T = C P.W. Atkins, Elements of Phys.Chem. page 141

51 y1 boiling x1 x2 cool y2


53 Case 2: -ve deviation A more attracted by B (e.g. CHCl3 + acetone)
mixH = < 0 > ideal A B Case 3: +ve deviation A less attracted by B (e.g. EtOH + water) mixH = > 0 < ideal

54 azeotropic composition

55 A B Residue Distlllate B A

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 phases Clapeyron Equation H = enthalpy of phase change V = volume change associated with phase change Clausius-Clapeyron Equation or Mol fractions xA = nA / nA+nB ni = mols of i yA = pA / pA + pB pi = partial pressure of i Raoults Law pA = poA xA and pB = poB xB Lever Rule (for tie-line joining phases via point a) nl =no. moles in liquid phase nv =no. moles in liquid phase

58 A(solution) < A(solvent)
Contains solute (e.g. NaCl, glucose) WHY?!!! Solvent A A(l) = A(l) + RTlnxA


Download ppt "Electrochemistry & Solutions 1. Solutions and Mixtures"

Similar presentations

Ads by Google