BMT 474-241 Electrode Potenial (1) PDF

Electrochemis Electrodes try C Potentials O U R S E NAME: Electrochemistry C O U R S E C O D E : 4022143-3  Redox Reactions Oxidation: A reaction which is brought about by loss of electrons Zn...

Electrochemis Electrodes try C Potentials O U R S E NAME: Electrochemistry C O U R S E C O D E : 4022143-3  Redox Reactions Oxidation: A reaction which is brought about by loss of electrons Zn Zn2  2e Reduction: A reaction which is brought about by gain of electrons Cu  2e Cu 2 Oxidation reaction produces electrons Reduction reaction consumes electrons 2  Redox Dire t Redox on c Reactions Reacti What happens if a bar of zinc is immersed into copper sulfate? Zn rod CuSO4(aq ) (Cu2+) Deposit of 3 Cu metal  Redox Reactions Total reaction (Redox reaction) Zn (s) + Cu2+ (aq) Reduction Zn2+ (aq) + Cu (s) Oxidation 4  Redox Half- Reactions 2Mg (s) + O 2MgO 2 (g) (s) half-two reactions Oxidation half-reaction (lose 2Mg 2Mg2+ + 4e- e- ) O2 + Reduction half-reaction (gain 2O2- e- ) 4e- 2Mg + O2 + 2Mg2+ + 2O2- + 4e- 4e- 2Mg + 2Mg O2 O 5  Oxidation-Reduction Reactions Oxidizing Agent- a substance that accepts electrons from another substance, causing the other substance to be oxidized. Reducing Agent- a substance that donates electrons to another substance, 6  Training questions 1 Determine the oxidizing and reducing agents in the following reaction: a) 2Fe3+ + Sn2+ → 2Fe2+ + Sn4+ b) 2Cu2+ + Sn2+ → 2Cu+ + Sn4+ 7 Elec ochemical ll t C s 3  Electrochemical Cell Components Anode: the electrode at which oxidation occurs. It sends electrons into the cathode Cathode: the electrode at which reduction occurs. It receives electrons from the anode Electrical wires: transfer the electrons as an electrical current from anode to cathode Electrolyte: an aqueous or molten substance that conducts electricity by the movement of ions Salt bridge: interconnects the solutions in the anode compartment and the cathode compartment 4 Daniell Electrochemical Cell 5  Salt Bridge Salts: saturated solutions of KCl, KNO3, etc. in agar jelly or gelatin Functions of the salt bridge: 1. To prevent two half cell solutions from mixing and only allow the flow of charges in the circuit. 2. To connects the two compartments to complete the electrical circuit 3. To balance the charges of two half cell solutions. 6  Galvanic Electrochemical Cell Galvanic (Voltaic) Cell The chemical reaction is used to produce electrical energy  a spontaneous redox reaction chemical energy electrical energy 7 Galvanic Cell 8  Cell Representations Zn Zn2  2e Cu2  2e Cu line notation Oxidation Reduction Zn/Zn+2 (aq, 1 // Cu+2 (aq, 1 M) / M) Cu A line between Junction between State, electrode and its two half-cells concentration solution Salt bridge 9  Cell Representations Porous membrane Zn/Zn+2 (aq, 1 M) Cu+2 (aq, 1 M) / Cu 10  Electrode Potentials An Electrode potential:  a measure of its ability to lose electrons (reduction potential)  a measure of its ability to gain 12  Half-cell voltage Half-cell potential cannot be measured without a second electrode. It is physically impossible to measure the potential of a single electrode: only the difference between the potentials of two electrodes can be measured. The half-cell potential of the standard hydrogen electrode has been arbitrarily set to zero. Other half cell potentials are expressed as a potential difference with this electrode.  Half-cell voltage : measurement The standard hydrogen electrode (SHE) is universally used for reference and is assigned a standard potential of 0 V. The [H+] in solution is in equilibrium with H2 gas at a pressure of 1 atm at the Ptsolution interface. One especially attractive feature of the SHE is that the Pt metal electrode is not consumed during the reaction. Half-cell voltage : measurement  Standard Potentials How do we measure the potential of an electrode? By using a standard electrode Standard electrode its potential is zero Potentiometer: measures the difference (hypothetical between cathode and anode potentials value) An electrode to be measured * standard conditions ( 1 atm, 25 oC and 1 M) 13  Standard Hydrogen Potential Standard Hydrogen Electrode (SHE) Oxidatio H2 (g) 2H+ + n: (aq) 2e- Reductio 2H+ (aq) + n: 2e- H2 (g) 14  Standard Hydrogen Potential The SHE is seldom used in routine electrochemical measurements because:  It is more difficult to prepare.  There is need for a supply of hydrogen gas which makes it somewhat inefficient and hazardous.  Reference Silver-Silver Chloride Reference Electrode E = + 0.205 V 0 Electrodes Ag/AgCl is a practical electrode that approaches the characteristics of a perfectly nonpolarizable electrode Can be easily fabricated in the laboratory. Consists of a metal coated with a layer of a slightly soluble ionic compound of that metal with a suitable anion. Whole structure is immersed in an electrolyte containing the anion in relatively high concentrations.  Electrolytic process - Ag/AgCl electrode fabrication An electrochemical cell with the Ag electrode on which the AgCl layer is to be deposited serves as anode and another piece of Ag—having a surface area much greater than that of the anode—serves as cathode. The reactions begin to occur as soon as the battery is connected, and the current jumps to its maximal value. As the thickness of the deposited AgCl layer increases, the rate of reaction decreases and the current drops. This situation continues, and the current approaches zero asymptotically  Cell potential Electromotive Force (emf) E cell   E cathode  E anode    If E˚ > 0 ,(+); it’s means the cell will be spontaneous (produce a voltage).  If E˚ < 0 (-); it’s means the cell must be supplied that voltage from an external power supply in order for the cell reaction to run Standard Reduction Potentials  Cell potential Electromotive Force (emf) Calculate the standard cell potential of the following: 1- Zn(s) + Zn2+(aq) + 2H+ (aq) H2(g) 2H+ (aq)+ 2e- → H2 (g) Eº cell = Eº cathode – Eº anode = 0-(-0.76)= + 0. 76 Zn → Zn2+ + 2e 2- Cd/Cd2+// Ag+/ Ag Ag+ (aq)+ 1e- →Ag x2  Cell potential Electromotive Force (emf) Calculate the standard cell potential of the following: The cell contains two electrode, the first is Mg electrode immersed in its ions and another is Ni electrode immersed in its ions. Eº Ni = -0.25V Eº Mg = -2.363V Ni+2 (aq)+ 2e- → Ni Mg → Mg2+ + 2e Eº cell = Eº cathode – Eº anode = -0.25-(- 2.363)= + 2. 113 V  Exercise A Galvanic cell is made by connecting a standard Cu/Cu2+ electrode to a S.H.E. The cell potential is 0.34 V. The Cu electrode is the cathode. What is the standard reduction potential of the Cu/Cu2+ electrode?