Lecture 15 CM1001

Standard Reduction Potential E° The relative oxidizing or reducing strength of redox couples are expressed in terms of their Standard Reduction Potential E (in Volts) The E° of a redox couple is measured with reference to the Standard Hydrogen Electrode (S.H.E.). The S.H.E. is assigned potential of 0V. This consists of a H2 gas electrode, a piece of platinum dipped in a solution of hydrogen ions (1M) with hydrogen gas (1atm) bubbling over the surface of the platinum. To find the potential to any other electrode relative to S.H.E., construct a galvanic cell.

Standard Reduction Potential E° Zn(s) is added to an aqueous solution of HNO3, Zn dissolves, H2 bubbles off Zn(s) + 2H3O+(aq)+2NO3- → H2(g) + Zn2+(aq) + 2NO3- (aq) + 2 H2O(l) oxidation./reduction reaction- can be expressed in 2 half reactions Zn(s)→Zn2+(aq) + 2e- Oxidation 2H3O+(aq) +2e- → H2(g) + 2 H2O(l) Reduction Allow these reactions to proceed but in two separate cells:

Standard Reduction Potential E°

Standard Reduction Potential E° Salt Bridge: tube containing an electrolyte solution e.g.,(KCl). Ions migrate from the salt bridge to keep solutions neutral. The two solutions cannot mix. 2 half reactions in two separate beakers: e- transfer occurs across the wire as an electric current convert chemical energy →electrical energy. Electrical current measured across the wire is the cell potential. E° measure of the electron attracting power Metals with large negative reduction potentials are good reducing agent, most easily oxidised themselves Elements with large reduction potentials are good oxidising agents. Reduced form of any element reduces the oxidised form of any element below it

Standard Reduction Potential E° By international agreement, the values of electrode potentials are given for the reduction process. The potential for the reverse reaction, an oxidation potential, is equal to the negative of the reduction potential. The reduction potential and the oxidation potential of an electrode have the same absolute value but are opposite in sign. Once a standard potential has been established relative to the standard Hydrogen electrode, it can be used to determine other potentials E.G., a galvanic cell at 25oC has an anode consisting of an Iorn strip immersed in 1M solution of Iorn (II) perchlorate (a Fe2+ / Fe electrode) and a cathode consisting of a copper strip immersed in a 1M solution of Copper (II) perchlorate (a Cu2+ / Cu electrode, Eored = 0.337 V)

Determination of Electrode Potential A potentiometer shows Eocell = 0.777 V The cell reaction is Fe (s) + Cu2+(aq) Fe2+ (aq) + Cu(s) What is the reduction potential of the Fe2+/Fe electrode? Answer: The standard cell potential (0.777 V) is the sum of the standard electrode potentials of the anode and cathode. Iron metal is oxidised in the anode half reaction Copper (II) ion is reduced in the cathode half-reaction.

Determination of Electrode Potential Anode: Fe (s) → Fe2+ (aq) + 2e- E = Eoox Cathode: Cu2+ (aq) + 2e- Cu (s) E = Eored = 0.337V _________________________________________________ Sum Fe (s) + Cu2+ (aq) → Fe2+ (aq) + Cu Eocell = Eoox+ Eored 0.777V = Eoox +0.337V Thus Eoox = Eocell – 0.337 V = 0.440 V The Iron electrode is the anode of the cell and is engaged in an oxidation half-reaction. In order to find the standard reduction potential for the electrode, we must reverse the sign of the oxidation potential

Determination of Electrode Potential Eored = - Eoox = -0.440V The standard reduction potential of the Fe2+/Fe electrode is -0.440 V Fe2+ (aq) + 2e-  Fe (s) Eored = -0.440 V

Corrosion Many metals, particularly Iron, undergo corrosion when exposed to air and water. Losses caused by corrosion of metals total billions of euro worldwide Corrosion is an electrochemical process. Iron in the presence of water tend to oxidise and give up electrons Anode Fe (s) → Fe2+ (aq) + 2e- The electrons pass through the Iron to the edge of the drop, where they reduce Oxygen from the air