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Electrochemistry Electrochemistry …is concerned with the properties of solutions of electrolytes and with processes that occur at electrodes …..all of which involve the transfer of electrons. THE CHARGE AND CURRENT •Electrochemistry studies the processes which involve charge •The charge is a source of electric field Element of charge: 1.602·10-19 C The energy change is ±1.602·10-19 J if we move the charge across the potential drop of 1V If we do the same with 1 mol of charges, we obtain… A B 1V + - - dQ The current is the change of charge per time I dt Transport of charge Q and matter ( n (mol)) Q Nze N n.N A Q n.z.N A.e 1 Faraday’s constant F N A.e 96484Cmol m Q n.z.F Q I.t n M FARADAY’S LAW (1834) “The chemical power of a current of electricity is in direct proportion to the absolute quantity of electricity which passes” “Electrochemical Equivalents coincide, and are the same, with ordinary chemical equivalents” m Q M zF MIt m zF Solutions of electrolytes dissociation and solvation of ions electric conductivity of electrolytes strong electrolytes weak electrolytes activity coefficients ionic strength solubility product Acids and bases Ionic product of water pH Dissociation constants of weak acids and bases Electrochemical cells and electrodes EMF and electrode potentials Ion-selective electrodes Solutions of Electrolytes: Electrolytes Strong Weak The degree of dissociation: c' c HCl CH3COOH c’ – dissociated part c - total concentration Electrolyte – solution of cations and anions resulting from the substance dissociation Strong electrolyte – fully (almost) dissociated strong acids, bases or their salts (e.g. NaCl, HCl, KOH,…) HCl H 2O H3O Cl 0.5 Weak electrolyte – partly (slightly) dissociated substance (e.g. CH3COOH, NH4OH) CH 3COOH H 2O CH3COO H3O 0.01 0.5 Charge number of ions z : Cations: Na+ z = 1 Anions: Cl- z = -1 Mg2+ z = 2 -2 SO4 z = -2 Zwitterions: z = 0 Ampholytes are molecules containing both acidic and basic groups (e.g. aminoacids) The principle of electroneutrality: in an electrolytic solution the concentrations of all the ionic species are such that the solution as a whole is neutral positive charge = negative charge Svante August Arrhenius (19 February 1859 – 2 October 1927) Nobel prize for Chemistry in 1903 1887 – theory of electrolyte solutions AB A B An equilibrium in solution between undissociated molecules AB and the ions A+ and B- Dissociation: -binary electrolyte (2 ions): NaCl Na Cl 2 2 -ternary (3 ions): CaCl2 Ca 2Cl 3 3 -quaternary (4 ions): FeCl3 Fe 3Cl 4 Stepwise dissociation: H3PO4 H H 2 PO4 2 H 2 PO4 H HPO4 2 3 HPO4 H PO4 Na2 HPO4 ,CaHPO4 , K3PO4 The colligative properties of electrolytes Freezing point depression T iKk .cm Boiling point elevation T iKe.cm icRT Osmotic pressure i – van’t Hoff’s coefficient Nonelectrolytes: i - osmotic coefficient Electrolytes: i . - number of ions In diluted solutions: = 1 Electrolytes e.g.: Na2SO4 2Na SO4 3 Dissociation of electrolytes, due to disintegration of molecules to ions, affects significantly colligative properties of solutions Solvents and Solvation o solvent r (25 C) N-methyl-formamide 187 formamide 109 H2O 78,3 CH3OH 32,6 Anhydrid acetic acid 20 pyridine 12,3 CH3COOH 6,15 etylether 4,3 benzene 2,3 N-hexane 2,0 1 e2 Columb’s law: F . 0 r 4 R2 - permitivity 0 r NaCl: Energy is required to pull apart atoms, ions, or molecules that are attracted to each other. But when atoms, ions, or molecules come together, energy is released. One way to say it is, “It takes energy to break bonds, and energy is released when bonds are formed.” 1. Dissociation and Solvation of ions: Na g Cl g 1 1 kr H 779kJmol solv H 775kJmol H 2Ol endothermic exothermic H Ol NaClcrystal2 Na aq Cl aq 1 dis H 3,87kJmol dis H kr H solv H Enthalpy of dissolution a) Salts: NaCl, KCl, NH4Cl, etc. Endothermic dissolution dis H 0 b) Dissolution of non-ionic substances (gas, liquid) - electrolytic dissociation and hydration of ions: HClg H 2Ol H3O aq Cl aq H 2SO4 l H 2O H3O aq HSO4 aq 2 HSO4 aq H 2Ol H3O aq SO4 aq Strongly exotermic!!! Enthalpy of dissolution dis H 0 Enthalpy of dilution dil H 0 Electric conductivity of electrolytes: strong electrolyte weak electrolyte Non-electrolyte Electrical conductivity of electrolytes: Physics: l electrical resistivity [.m] Ohm’s law: U RI R l length of conductor [m] S S cross-section [m2] 1 Conductance: G [S] R 1 Electrical conductivity: [S.m-1] (specific) l G. G.C C – resistive capacity [m-1] S Conductivity of the electrolyte i: H 2Oi Ridistilled water: 1 H 2O 2 4Sm Water for pharmaceutical use H O 0,3mSm1 (Pharmacopeia): 2 Molar conductivity: [Sm2mol-1] c [mSm2mol-1] Molar conductivity of the AB electrolyte: B A +, - conductivity of ions +, - stoichiometric coefficients Limiting molar conductivity:0 lim c0 (molar conductivity at infinite dilution) Kohlraush’s law of independent migration of ions: 0 B A 0 0 0 0 Limiting molar conductivities (S.cm2.mol-1): 0 KCl 149,86 0 (KI) 150,38 0 (KClO4 ) 140,04 0 (NaCl) 126,45 (NaI) 126,94 0 0 (NaClO4 ) 116,48 23,41 23,44 23,56 An example of limiting molar conductivities of selected salts composed of same cations and anions. Note the difference in conductivity (expressed by ) due to cations is higher. Calculation of limiting molar conductivity: 2 1 0 ZnCl2 Zn0Zn Cl0Cl 10,56 2.7,63 25,82mSm mol In view of Kohlraush’s law of independent migration of ions, limiting molar conductivities of individual ions can be combined to calculate limiting molar conductivities of electrolytes. Dissociation: Strong electrolyte: Weak electrolyte: Note the difference in dissociation of strong and weak electrolytes Strong electrolyte – at low concentration depends on c linearly: Kohlraush’s law 0 k c k – coefficient which depends on the nature of the specific salt in solution Weak electrolyte 0: -additive quantity 0 CH3COOH 0 CH3COONa 0 HCl 0 NaCl 2 1 91,0 426,0 126,0 391,0Scm mol 2 -1 0 [Scm mol ] For strong electrolytes we can determine limiting molar HCl 426,0 conductivity 0 experimentally from the dependence of = fc. NaCl 126,0 Due to extremal growth of at very low concentrations of weak electrolytes, the value of 0 is not possible determine from an CH3COONa 91,0 experiment. Frequently are these values calculated, as shown for CH3COOH. Weak electrolyte – the conductivity depends on the degree of dissociation : c' c’ – concentration of the dissociated part c c – total concentration 0 Weak electrolyte e.g. CH3COOH, NH3: a a K H3O A HAaq H 2Ol H3O aq A aq A aHA At the dissociation degree : H3O c A c HA 1 c 2c The dissociation constant KA: K A 1 c2 K A Ostwald’s dilute law: 0 0 The degree of dissociation of a weak electrolyte depends on its dissociation constant Problem solving: Concentration of a weak acid HA in aqueous solution is c=0.015 mol/dm3 and at this concentration the found degree of dissociation of the acid = 0.09. Calculate dissociation constant KA and pKA of the acid HA, and pH of the solution. Activity and activity coefficients of strong electrolyte K+A- : 0 RT ln a Chemical potential of the solution: i i i ai ci i + - Electrolyte K A : KA At standard condition: 0 0 0 KA 0 0 RT ln a RT ln a 0 KA KA RT ln aa A mean activity of the electrolyte: a aa c c c A mean activity coefficient: a c a c a c The activity of ions: K K A A We do not know to resolve an activity of anions and cations in a solution, respectively. Due to that, a mean activity of electrolyte and a mean activity coefficient were introduced. Debye – Hückel limiting law log z z A I For water, 25 oC: A 0,509 The ionic strength I of a solution is a function of the concentration of all ions present in that solution : 1 2 I ci zi 2 i Debye, Hückel, 1923 Debye-Huckel limiting law defines a mean activity coefficient. I is the ionic strength of a solution. It characterizes total electrostatic effect of all ions in solution. Problem solving Solutions of two salts K2SO4 and MgCl2 were mixed together. Final concentration in the solution was 3 found: K2SO4 c=0.001 mol/dm , and MgCl2 c=0.002 mol/dm3. Calculate the ionic strength of the solution. The solubility product Ks . - solubility of a slightly soluble salt AgCl, CaF2,... MX s M aq X aq T, p konst. a a M X K aMX s 1 aMX (pure solid) The solubility product Ks K a .a M X s M X 2 K s M X In water solution, an equilibrium between a solid (not-dissolved) salt MX and its ions (M+ and X-) will be created. Note, aMX in the denominator of the fraction of equilibrium constant (K) is an activity of pure solid. aMX = 1 for pure solid under standard conditions. Numerator, aM . aX is the + - solubility product, Ks. [M ], [X ] express concentrations of the ions, respectively. The solubility product Ks of selected salts: AgCl : AgCls Ag aqCl aq 2 K s aAg .aCl Ag .Cl . BaSO sBa2 aq SO 2 aq BaSO4 : 4 4 2 2 2 Ks aBa.aSO Ba .SO4 . 4 2 CaF 2: CaF2 sCa aq 2F aq 2 2 2 3 Ks aCa .aF Ca .F .
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