What is electrochemistry?

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Donald Gregory
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3/1/217 A quote of the week (or cael of the week): The work of science has nothing whatever to do with consensus. Consensus is the business of politics.... There is no such thing as consensus science.

1 3/1/217 A quote of the week (or cael of the week): The work of science has nothing whatever to do with consensus. Consensus is the business of politics.... There is no such thing as consensus science. If it's consensus, it isn't science. If it's science, it isn't consensus. Period. Michael Crichton Physical Cheistry GTM/17 1 What is electrocheistry? This is a branch of cheistry dealing with cheical reactions, accopanied by a transfer of electric charge. As a atter of fact, these reactions include the following phenoena: flow of electricity (current), foration of gradients of electric potential (voltage), accuulation of electric charge (capacitance) As well as all effects typical for cheical reactions (release or absorption of heat, changes in concentrations of reactants and products, foration of concentration gradients, etc.). Physical Cheistry GTM/17 2 What is electrocheistry? (2) The aforeentioned phenoena ay be either results of spontaneous reactions or ust be forced (induced) externally. The quantities being easures of these phenoena (current, potential or voltage, charge, capacity) ay be either causes or effects (independent or dependent variables) in electrocheical experients. If we adopt tie as an additional variable (always independent one), therefore all the experients in electrocheistry ay be considered as easureents of three variables (i, V, t). Different cobinations of these three quantities produce a whole range of electrocheical techniques. Physical Cheistry GTM/17 3 1

2 3/1/217 etallic conductors ionic conductors What is electrocheistry? (3) crossing the interface? DC source flowof electr etal etal Physical Cheistry GTM/17 flowof ions ions 4 solut What is electrocheistry? (4) conventional cheical reaction Cu 2+ + Zn = Zn 2+ + Cu redox reaction Cu Zn covered plate Zn is a reductant Cu 2+ is an oxidant CuSO Zn 4 solution Physical Cheistry GTM/17 5 Electrocheical process electrons Liquid junction salt bridge ZnSO 4 solution Zn plate CuSO 4 solution Cu plate Anode( ): Zn =Zn e Cathode(+): Cu e =Cu Cobined: Cu 2+ + Zn = Zn 2+ + Cu Physical Cheistry EPM/6 6 2

3 3/1/217 Types of electrolytes Liquid electrolytes: solutions based on ionizing (inducing dissociation) solvents ost frequently aqueous and we restrict our considerations to these olten salts (esp. ionic liquids) Solid electrolytes (frequently of ixed conductance echanis): conducting polyers, seicondustors Physical Cheistry EPM/6 7 Ionic conductance R1 R2 R3 When the bridge is balanced: R1 R R2 R 3 R Physical Cheistry EPM/6 8 Ionic conductance (2) Resistance (passive) is given by the II Oh law: Conductance is expressed as: 1 s G R l l R s Conductoetric cell constant: And conductivity as: 1 k kg R l k s Physical Cheistry EPM/6 9 3

4 3/1/217 Ionic conductance (3) Typical conductivities: of etals: of aqueous electrolytes: order of 1 8 S/ to 1 6 S/ or 1 6 S/c to 1 4 S/c order of 1-4 S/ to 1 S/ or 1-6 S/c to 1-2 S/c KCl serves as a standard for deterination of the conductoetric cell constant, its,1 M solution conductivity is,14823 S -1 at 298,15K. Physical Cheistry EPM/6 1 Ionic conductance (4) Molar conductivity (forerly equivalent conductivity) is defined as: strictly in SI units: practically: c 1 c Allowing for the fact that oles of different substances can carry different charges, one should indicate the substance in the subscript. Physical Cheistry EPM/6 11 Ionic conductance (5) Kohlrausch law: c 1/ 2 Physical Cheistry EPM/6 12 4

5 3/1/217 Ionic conductance (6) Law of independent obility of ions in solution applies to the olar conductivity at infinite dilution: general case KCl v v KBr KCl NaCl NaCl Kohlrausch noticed that: NaBr KBr LiCl NaBr 1:1 electrolyte LiBr... Cl Br KI NaI Physical Cheistry EPM/ Ionic conductance (7) Law of independent obility of ions in solution is soeties applied to the olar conductivity at finite concentrations: If a solution contains several ions, its concuctivity ay be calculated according to the forula (always correct): c i i 1 Physical Cheistry EPM/6 14 Exceptional obility of H + and OH ions Molar conductivities at infinite dilution of selected inorganic ions [S c 2 /ol] at 25 o C. Ion H + OH Na + Li + K + ClO 4 F NO 3 Cl λ Physical Cheistry EPM/6 15 5

6 3/1/217 Exceptional obility of H + and OH ions (2) Physical Cheistry EPM/6 16 What is titration? Titration is a ethod of quantitative analysis consisting of gradual addition to the saple solution a reagent (known as titrant, ost frequently as a solution of known concentration titre fro a graduated tube with a stopcock, known as a burette) reacting with the substance under deterination the analyte, with parallel tracing of the reaction progress utilizing an additional easuring technique, aiing at the detection of the point when stoichioetric ratio of titrant to analyte is achieved. This point (oent) is known as the equivalence point. Actual point is known as the end point of titration. The saller the difference between the two points, the saller the titration error. Physical Cheistry EPM/6 17 (2) Titrations ay be classified according to: type of analytical reaction (acid-base, redox, coplexoetric, precipitation titrations) ethod eployed for tracing the reaction progress end point detection (visual titrations with color indicators, potentioetric, conductoetric, spectrophotoetric, etc.) way of adding the titrant (classic, standard solution + burette or couloetric titrations). Other classifications are possible, relying on details of the titration procedure. Physical Cheistry EPM/6 18 6

7 3/1/217 (3) Titration curve: Titration curve (experiental or calculated odelled) is a plot presenting changes in certain quantity, being a property of the titrated solution, as a function of nuber of oles of the titrant added (it ay be volue of standard solution added, electric charge, etc.). In the case of conductoetric titration, the possible plots are: R=f(V t ) or G=f(V t ) or κ=f(v t ) or λ =f(v t ) Titration curve perits finding the titration end point, as the quantity in question changes near it in a special way (revealing an inflection, a_break, axiu or iniu, etc.) Physical Cheistry EPM/6 19 (4) strong acid with strong base weak acid with weak base Physical Cheistry EPM/6 2 (5) strong acid with weak base weak acid with weak base Physical Cheistry EPM/6 21 7

8 3/1/217 (6) NH 4 Cl with strong base (NaOH) weak dicarboxylic acid with strong base Physical Cheistry EPM/6 22 (7) precipitation titration; chlorides with silver nitrate Physical Cheistry EPM/6 23 Howgh!!! 8

ELEC NCERT. 1. Which cell will measure standard electrode potential of copper electrode? (g,0.1 bar) H + (aq.,1 M) Cu 2+ (aq.

ELEC NCERT. 1. Which cell will measure standard electrode potential of copper electrode? (g,0.1 bar) H + (aq.,1 M) Cu 2+ (aq. I. Multiple Choice Questions (Type-I) 1. Which cell will easure standard electrode potential of copper electrode? Pt (s) H 2 (g,0.1 bar) H + (aq.,1 M) Cu 2+ (aq.,1m) Cu Pt(s) H 2 (g, 1 bar) H + (aq.,1