Unit 3 Electrochemistry PDF

Unit Objectives Electrochemistry Electrochemistr ochemistr y...

Unit Objectives Electrochemistry Electrochemistr ochemistr y 3 After studying this Unit, you will be able to describe an electrochemical cell and differentiate between galvanic Chemical reactions can be used to produce electrical energy, conversely, electrical energy can be used to carry out chemical and electrolytic cells; reactions that do not proceed spontaneously. apply Nernst equation for calculating the emf of galvanic cell and define standard potential of the cell; Electrochemistry is the study of production of derive relation between standard electricity from energy released during spontaneous potential of the cell, Gibbs energy chemical reactions and the use of electrical energy of cell reaction and its equilibrium to bring about non-spontaneous chemical constant; transformations. The subject is of importance both define resistivity (ρ), conductivity for theoretical and practical considerations. A large (κ) and molar conductivity (L m) of number of metals, sodium hydroxide, chlorine, ionic solutions; fluorine and many other chemicals are produced by differentiate between ionic (electrolytic) and electronic electrochemical methods. Batteries and fuel cells conductivity; convert chemical energy into electrical energy and are describe the method for used on a large scale in various instruments and measurement of conductivity of devices. The reactions carried out electrochemically electrolytic solutions and can be energy efficient and less polluting. Therefore, calculation of their molar study of electrochemistry is important for creating new conductivity; technologies that are ecofriendly. The transmission of justify the variation of sensory signals through cells to brain and vice versa conductivity and molar and communication between the cells are known to conductivity of solutions with change in their concentration and have electrochemical origin. Electrochemistry, is therefore, a very vast and interdisciplinary subject. In define Λ°m (molar conductivity at this Unit, we will cover only some of its important zero concentration or infinite elementary aspects. dilution); enunciate Kohlrausch law and learn its applications; understand quantitative aspects of electrolysis; describe the construction of some primary and secondary batteries and fuel cells; explain corrosion as an electrochemical process. 2015-16(20/01/2015) 3. 1 Electrochemical In Class XI, Unit 8, we had studied the construction and functioning of Daniell cell (Fig. 3.1). This cell converts the chemical energy liberated Cells during the redox reaction Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) (3.1) to electrical energy and has an electrical potential equal to 1.1 V when concentration of Zn2+ and Cu2+ ions is unity (1 mol dm–3)*. Such a device is called a galvanic or a voltaic cell. If an external opposite potential is applied in the galvanic cell [Fig. 3.2(a)] and increased slowly, we find that the reaction continues to take place till the opposing voltage reaches the value 1.1 V [Fig. 3.2(b)] when, the reaction stops altogether and no current flows through the cell. Any further increase in the external potential again starts the reaction but in the opposite direction [Fig. 3.2(c)]. It now functions as an electrolytic cell, a device for using electrical energy to carry non-spontaneous chemical reactions. Both types of cells are Fig. 3.1: Daniell cell having electrodes of zinc and quite important and we shall study some of copper dipping in the solutions of their respective salts. their salient features in the following pages. Eext< 1.1V Eext = 1.1V (a) (b) e anode current cathode I=0 Zn salt Cu Zn Cu -ve bridge +ve When Eext = 1.1 V (i) No flow of electrons or current. (ii) No chemical ZnSO4 CuSO4 ZnSO4 CuSO4 reaction. When Eext < 1.1 V Eext >1.1 (i) Electrons flow from Zn rod to (c) Cu rod hence current flows from Cu to Zn. When Eext > 1.1 V (ii) Zn dissolves at anode and e– (i) Electrons flow copper deposits at cathode. Cathode Current Anode from Cu to Zn +ve –ve and current flows Zn Cu from Zn to Cu. Fig. 3.2 (ii) Zinc is deposited Functioning of Daniell at the zinc cell when external electrode and voltage Eext opposing the copper dissolves at cell potential is applied. copper electrode. *Strictly speaking activity should be used instead of concentration. It is directly proportional to concentration. In dilute solutions, it is equal to concentration. You will study more about it in higher classes. Chemistry 64 2015-16(20/01/2015) 3.2 Galvanic Cells As mentioned earlier (Class XI, Unit 8) a galvanic cell is an electrochemical cell that converts the chemical energy of a spontaneous redox reaction into electrical energy. In this device the Gibbs energy of the spontaneous redox reaction is converted into electrical work which may be used for running a motor or other electrical gadgets like heater, fan, geyser, etc. Daniell cell discussed earlier is one such cell in which the following redox reaction occurs. Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s) This reaction is a combination of two half reactions whose addition gives the overall cell reaction: (i) Cu2+ + 2e– → Cu(s) (reduction half reaction) (3.2) 2+ – (ii) Zn(s) → Zn + 2e (oxidation half reaction) (3.3) These reactions occur in two different portions of the Daniell cell. The reduction half reaction occurs on the copper electrode while the oxidation half reaction occurs on the zinc electrode. These two portions of the cell are also called half-cells or redox couples. The copper electrode may be called the reduction half cell and the zinc electrode, the oxidation half-cell. We can construct innumerable number of galvanic cells on the pattern of Daniell cell by taking combinations of different half-cells. Each half- cell consists of a metallic electrode dipped into an electrolyte. The two half-cells are connected by a metallic wire through a voltmeter and a switch externally. The electrolytes of the two half-cells are connected internally through a salt bridge as shown in Fig. 3.1. Sometimes, both the electrodes dip in the same electrolyte solution and in such cases we do not require a salt bridge. At each electrode-electrolyte interface there is a tendency of metal ions from the solution to deposit on the metal electrode trying to make it positively charged. At the same time, metal atoms of the electrode have a tendency to go into the solution as ions and leave behind the electrons at the electrode trying to make it negatively charged. At equilibrium, there is a separation of charges and depending on the tendencies of the two opposing reactions, the electrode may be positively or negatively charged with respect to the solution. A potential difference develops between the electrode and the electrolyte which is called electrode potential. When the concentrations of all the species involved in a half-cell is unity then the electrode potential is known as standard electrode potential. According to IUPAC convention, standard reduction potentials are now called standard electrode potentials. In a galvanic cell, the half-cell in which oxidation takes place is called anode and it has a negative potential with respect to the solution. The other half-cell in which reduction takes place is called cathode and it has a positive potential with respect to the solution. Thus, there exists a potential difference between the two electrodes and as soon as the switch is in the on position the electrons flow from negative electrode to positive electrode. The direction of current flow is opposite to that of electron flow. 65 Electrochemistry 2015-16(20/01/2015)  The potential difference between the two electrodes of a galvanic cell is called the cell potential and is measured in volts. The cell potential is the difference between the electrode potentials (reduction potentials) of the cathode and anode. It is called the cell electromotive force (emf) of the cell when no current is drawn through the cell. It is now an accepted convention that we keep the anode on the left and the cathode on the right while representing the galvanic cell. A galvanic cell is generally represented by putting a vertical line between metal and electrolyte solution and putting a double vertical line between the two electrolytes connected by a salt bridge. Under this convention the emf of the cell is positive and is given by the potential of the half- cell on the right hand side minus the potential of the half-cell on the left hand side i.e., Ecell = Eright – Eleft This is illustrated by the following example: Cell reaction: Cu(s) + 2Ag+(aq) → Cu2+ (aq) + 2 Ag(s) (3.4) Half-cell reactions: Cathode (reduction): 2Ag+(aq) + 2e– → 2Ag(s) (3.5) Anode (oxidation): Cu(s) → Cu2+(aq) + 2e– (3.6) It can be seen that the sum of (3.5) and (3.6) leads to overall reaction (3.4) in the cell and that silver electrode acts as a cathode and copper electrode acts as an anode. The cell can be represented as: Cu(s)|Cu2+(aq)||Ag+(aq)|Ag(s) and we have Ecell = Eright – Eleft = EAg Ag – ECu + 2+ Cu (3.7) 3.2.1 The potential of individual half-cell cannot be measured. We can Measurement measure only the difference between the two half-cell potentials that of Electrode gives the emf of the cell. If we arbitrarily choose the potential of one Potential electrode (half-cell) then that of the other can be determined with respect to this. According to convention, a half-cell called standard hydrogen electrode (Fig.3.3) represented by Pt(s) H2(g) H+(aq), is assigned a zero potential at all temperatures corresponding to the reaction 1 H+ (aq) + e– → H (g) 2 2 The standard hydrogen electrode consists of a platinum electrode coated with platinum black. The electrode is dipped in an acidic solution and pure hydrogen gas is bubbled through it. The concentration of both the reduced and oxidised forms of hydrogen is maintained at unity (Fig. 3.3). This implies that the pressure of hydrogen gas is one bar and the concentration of hydrogen ion in the Fig. 3.3: Standard Hydrogen Electrode (SHE). solution is one molar. Chemistry 66 2015-16(20/01/2015)  At 298 K the emf of the cell, standard hydrogen electrode second half-cell constructed by taking standard hydrogen electrode as anode (reference half-cell) and the other half-cell as cathode, gives the reduction potential of the other half-cell. If the concentrations of the oxidised and the reduced forms of the species in the right hand half-cell are unity, then the cell potential is equal to standard electrode potential, E⊖R of the given half-cell. E⊖ = E⊖R – E⊖ L As E⊖L for standard hydrogen electrode is zero. E⊖ = E⊖R – 0 = E⊖R The measured emf of the cell: + 2+ Pt(s)  H2(g, 1 bar)  H (aq, 1 M)  Cu (aq, 1 M)  Cu is 0.34 V and it is also the value for the standard electrode potential of the half-cell corresponding to the reaction: Cu2+ (aq, 1M) + 2 e– → Cu(s) Similarly, the measured emf of the cell: + 2+ Pt(s)  H 2(g, 1 bar)  H (aq, 1 M)  Zn (aq, 1M)  Zn is -0.76 V corresponding to the standard electrode potential of the half-cell reaction: Zn2+ (aq, 1 M) + 2e– → Zn(s) The positive value of the standard electrode potential in the first 2+ + case indicates that Cu ions get reduced more easily than H ions. The reverse process cannot occur, that is, hydrogen ions cannot oxidise Cu (or alternatively we can say that hydrogen gas can reduce copper ion) under the standard conditions described above. Thus, Cu does not dissolve in HCl. In nitric acid it is oxidised by nitrate ion and not by hydrogen ion. The negative value of the standard electrode potential in the second case indicates that hydrogen ions can oxidise zinc (or zinc can reduce hydrogen ions). In view of this convention, the half reaction for the Daniell cell in Fig. 3.1 can be written as: 2+ – Left electrode: Zn(s) → Zn (aq, 1 M) + 2 e Right electrode: Cu2+ (aq, 1 M) + 2 e– → Cu(s) The overall reaction of the cell is the sum of above two reactions and we obtain the equation: Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s) 0 emf of the cell = Ecell = E0R – E 0L = 0.34V – (– 0.76)V = 1.10 V Sometimes metals like platinum or gold are used as inert electrodes. They do not participate in the reaction but provide their surface for oxidation or reduction reactions and for the conduction of electrons. For example, Pt is used in the following half-cells: Hydrogen electrode: Pt(s)|H2 (g)| H+(aq) With half-cell reaction: H+ (aq)+ e– → ½ H 2(g) Bromine electrode: Pt(s)|Br2(aq)| Br–(aq) 67 Electrochemistry 2015-16(20/01/2015)  With half-cell reaction: ½ Br2(aq) + e– → Br– (aq) The standard electrode potentials are very important and we can extract a lot of useful information from them. The values of standard electrode potentials for some selected half-cell reduction reactions are given in Table 3.1. If the standard electrode potential of an electrode is greater than zero then its reduced form is more stable compared to hydrogen gas. Similarly, if the standard electrode potential is negative then hydrogen gas is more stable than the reduced form of the species. It can be seen that the standard electrode potential for fluorine is the highest in the Table indicating that fluorine gas (F2) has the maximum tendency to get reduced to fluoride ions (F–) and therefore fluorine gas is the strongest oxidising agent and fluoride ion is the weakest reducing agent. Lithium has the lowest electrode potential indicating that lithium ion is the weakest oxidising agent while lithium metal is the most powerful reducing agent in an aqueous solution. It may be seen that as we go from top to bottom in Table 3.1 the standard electrode potential decreases and with this, decreases the oxidising power of the species on the left and increases the reducing power of the species on the right hand side of the reaction. Electrochemical cells are extensively used for determining the pH of solutions, solubility product, equilibrium constant and other thermodynamic properties and for potentiometric titrations. Intext Questions 3.1 How would you determine the standard electrode potential of the system Mg 2+|Mg? 3.2 Can you store copper sulphate solutions in a zinc pot? 3.3 Consult the table of standard electrode potentials and suggest three substances that can oxidise ferrous ions under suitable conditions. 3. 3 Nernst We have assumed in the previous section that the concentration of all Equation the species involved in the electrode reaction is unity. This need not be always true. Nernst showed that for the electrode reaction: Mn+(aq) + ne– → M(s) the electrode potential at any concentration measured with respect to standard hydrogen electrode can be represented by: RT [M] E( = E V( n + ) – ln n+ Mn+ / M) M /M nF [M ] but concentration of solid M is taken as unity and we have V RT 1 E( n+ ) = E ( n + ) – ln n+ (3.8) M /M M /M nF [M ] V E( n+ ) has already been defined, R is gas constant (8.314 JK–1 mol–1 ), M /M F is Faraday constant (96487 C mol–1), T is temperature in kelvin and [Mn+] is the concentration of the species, Mn+. Chemistry 68 2015-16(20/01/2015)  Table 3.1: Standard Electrode Potentials at 298 K Ions are present as aqueous species and H 2O as liquid; gases and solids are shown by g and s. Reaction (Oxidised form + ne– → Reduced form) E⊖/V F2(g) + 2e– → 2F– 2.87 Co 3+ +e – → Co2+ 1.81 H 2O2 + 2H + 2e + – → 2H 2O 1.78 MnO4– + 8H+ + 5e – → Mn2+ + 4H2O 1.51 Au 3+ + 3e – → Au(s) 1.40 Cl2(g) + 2e – → 2Cl– 1.36 Cr2O 7 2– + 14H + 6e + – → 2Cr3+ + 7H2O 1.33 O 2(g) + 4H + 4e + – → 2H 2O 1.23 MnO2(s) + 4H + 2e + – → Mn2+ + 2H2O 1.23 Br2 + 2e – → 2Br– 1.09 NO3– + 4H+ + 3e– → NO(g) + 2H2O 0.97 2Hg 2+ + 2e – → Hg22+ 0.92 Increasing strength of oxidising agent Increasing strength of reducing agent + Ag + e – → Ag(s) 0.80 Fe 3+ +e – → Fe2+ 0.77 O 2(g) + 2H + 2e + – → H2O 2 0.68 I2 + 2e – → 2I– 0.54 Cu + e + – → Cu(s) 0.52 Cu2+ + 2e– → Cu(s) 0.34 AgCl(s) + e – → Ag(s) + Cl– 0.22 AgBr(s) + e – → Ag(s) + Br– 0.10 + 2H + 2e – → H2(g) 0.00 Pb 2+ + 2e – → Pb(s) –0.13 Sn 2+ + 2e – → Sn(s) –0.14 Ni 2+ + 2e – → Ni(s) –0.25 Fe2+ + 2e– → Fe(s) –0.44 Cr 3+ + 3e – → Cr(s) –0.74 Zn 2+ + 2e – → Zn(s) –0.76 – – 2H2O + 2e → H2(g) + 2OH (aq) –0.83 Al 3+ + 3e – → Al(s) –1.66 2+ – Mg + 2e → Mg(s) –2.36 Na + e + – → Na(s) –2.71 Ca 2+ + 2e – → Ca(s) –2.87 K+ + e– → K(s) –2.93 + Li + e – → Li(s) –3.05 1. A negative E⊖ means that the redox couple is a stronger reducing agent than the H+/H2 couple. 2. A positive E⊖ means that the redox couple is a weaker reducing agent than the H+/H2 couple. 69 Electrochemistry 2015-16(20/01/2015)  In Daniell cell, the electrode potential for any given concentration of Cu and Zn2+ ions, we write 2+ For Cathode: E( V RT 1 Cu2 + /Cu ) = E (Cu2 + /Cu ) – ln (3.9) 2F Cu ( aq )  2+ For Anode: RT 1 E( 2 + ) = E V 2 + Zn / Zn ( Zn / Zn ) – 2F ln Zn2+ ( aq )  (3.10)   The cell potential, E(cell) = E ( 2 + – E( 2 + ) Cu /Cu ) Zn / Zn V RT 1 V RT 1 = E( – ln – E( Zn 2 + / Zn ) + ln Cu2 + /Cu ) Zn (aq) 2+ 2F Cu (aq)  2+ 2F V V RT 1 1 = E( – E( Zn 2 + / Zn ) – ln – ln Cu2 + /Cu ) 2F Cu 2+ ( aq )  Zn 2+ ( aq )  V RT [ Zn 2 + ] E(cell) = E( cell) – 2F ln (3.11) [Cu2 + ] It can be seen that E(cell) depends on the concentration of both Cu 2+ and Zn2+ ions. It increases with increase in the concentration of Cu2+ ions and decrease in the concentration of Zn2+ ions. By converting the natural logarithm in Eq. (3.11) to the base 10 and substituting the values of R, F and T = 298 K, it reduces to V 0.05 9 [Zn 2 + ] E(cell) = E( ce ll) – log (3.12) 2 [ Cu2 + ] We should use the same number of electrons (n) for both the electrodes and thus for the following cell Ni(s) Ni2+(aq)  Ag+(aq) Ag The cell reaction is Ni(s) + 2Ag+(aq) →Ni2+(aq) + 2Ag(s) The Nernst equation can be written as V RT [Ni 2+ ] E(cell) = E( cell) – 2F ln [Ag + ]2 and for a general electrochemical reaction of the type: ne a A + bB → cC + dD Nernst equation can be written as: V RT E(cell) = E( cell) – nF 1nQ V RT [C] c [D]d = E( cell) – nF ln [A]a [B]b (3.13) Chemistry 70 2015-16(20/01/2015)  Represent the cell in which the following reaction takes place Example 3.1 Mg(s) + 2Ag+(0.0001M) → Mg 2+(0.130M) + 2Ag(s) V Calculate its E (cell) if E( cell) = 3.17 V. The cell can be written as Mg Mg2+ (0.130M) Ag+ (0.0001M) Ag Solution V RT  Mg 2+  E (cell) = E (cell) – ln 2 2F  Ag +  0.059V 0.130 = 3.17 V – log = 3.17 V – 0.21V = 2.96 V. 2 (0.0001)2 3.3.1 Equilibrium If the circuit in Daniell cell (Fig. 3.1) is closed then we note that the reaction Constant Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) (3.1) from Nernst takes place and as time passes, the concentration of Zn 2+ keeps Equation on increasing while the concentration of Cu 2+ keeps on decreasing. At the same time voltage of the cell as read on the voltmeter keeps on decreasing. After some time, we shall note that there is no change in the concentration of Cu 2+ and Zn2+ ions and at the same time, voltmeter gives zero reading. This indicates that equilibrium has been attained. In this situation the Nernst equation may be written as: V 2.30 3RT [Zn 2+ ] E (cell) = 0 = E( cell) – log 2F [Cu 2+ ] V 2.303 RT [Zn2 + ] or E( cell) = log 2F [Cu 2+ ] But at equilibrium, [Zn 2 + ] [Cu2 + ] = Kc for the reaction 3.1 and at T = 298K the above equation can be written as V 0.059 V E( cell) = log K C = 1.1 V ( E(Vcell) = 1.1V) 2 (1.1V × 2) log K C = = 37.288 0.059 V K C = 2 × 1037 at 298K. In general, V 2.303RT E( cell) = log KC (3.14) nF Thus, Eq. (3.14) gives a relationship between equilibrium constant of the reaction and standard potential of the cell in which that reaction takes place. Thus, equilibrium constants of the reaction, difficult to measure otherwise, can be calculated from the corresponding E⊖ value of the cell. 71 Electrochemistry 2015-16(20/01/2015)  Example 3.2 Calculate the equilibrium constant of the reaction: Cu(s) + 2Ag+(aq) → Cu 2+(aq) + 2Ag(s) V E (cell) = 0.46 V 0.059 V Solution E(Vcell) = log KC = 0.46 V or 2 0. 46 V × 2 log KC = 0.059 V = 15.6 KC = 3.92 × 1015 3.3.2 Electro- Electrical work done in one second is equal to electrical potential chemical multiplied by total charge passed. If we want to obtain maximum work Cell and from a galvanic cell then charge has to be passed reversibly. The Gibbs reversible work done by a galvanic cell is equal to decrease in its Gibbs Energy of energy and therefore, if the emf of the cell is E and nF is the amount the Reaction of charge passed and ∆ rG is the Gibbs energy of the reaction, then ∆rG = – nFE(cell) (3.15) It may be remembered that E(cell) is an intensive parameter but ∆ rG is an extensive thermodynamic property and the value depends on n. Thus, if we write the reaction Zn(s) + Cu2+ (aq) → Zn2+(aq) + Cu(s) (3.1) ∆rG = – 2FE(cell) but when we write the reaction 2 Zn (s) + 2 Cu2+ (aq) →2 Zn2+(aq) + 2Cu(s) ∆rG = – 4FE(cell) If the concentration of all the reacting species is unity, then E(cell) = E(Vcell) and we have V ∆rG⊖ = – nF E (cell) (3.16) V Thus, from the measurement of E we can obtain an important ( cell) thermodynamic quantity, ∆ rG , standard Gibbs energy of the reaction. ⊖ From the latter we can calculate equilibrium constant by the equation: ∆rG⊖ = –RT ln K. Example 3.3 The standard electrode potential for Daniell cell is 1.1V. Calculate the standard Gibbs energy for the reaction: Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s) ⊖ V Solution ∆rG = – nF E(c ell) n in the above equation is 2, F = 96487 C mol –1 and E V(cell) = 1.1 V Therefore, ∆ rG⊖ = – 2 × 1.1V × 96487 C mol –1 = – 21227 J mol–1 = – 212.27 kJ mol–1 Chemistry 72 2015-16(20/01/2015)  Intext Questions 3.4 Calculate the potential of hydrogen electrode in contact with a solution whose pH is 10. 3.5 Calculate the emf of the cell in which the following reaction takes place: Ni(s) + 2Ag+ (0.002 M) → Ni2+ (0.160 M) + 2Ag(s) Given that E V(cell) = 1.05 V 3.6 The cell in which the following reaction occurs: 2Fe 3+ ( aq ) + 2I− ( aq ) → 2Fe 2+ ( aq ) + I 2 ( s ) has Ecell 0 = 0.236 V at 298 K. Calculate the standard Gibbs energy and the equilibrium constant of the cell reaction. 3. 4 Conductance It is necessary to define a few terms before we consider the subject of of Electrolytic conductance of electricity through electrolytic solutions. The electrical resistance is represented by the symbol ‘R’ and it is measured in ohm (Ω) Solutions which in terms of SI base units is equal to (kg m2)/(S3 A2). It can be measured with the help of a Wheatstone bridge with which you are familiar from your study of physics. The electrical resistance of any object is directly proportional to its length, l, and inversely proportional to its area of cross section, A. That is, l l R ∝ or R = ρ (3.17) A A The constant of proportionality, ρ (Greek, rho), is called resistivity (specific resistance). Its SI units are ohm metre (Ω m) and quite often its submultiple, ohm centimetre (Ω cm) is also used. IUPAC recommends the use of the term resistivity over specific resistance and hence in the rest of the book we shall use the term resistivity. Physically, the resistivity for a substance is its resistance when it is one metre long and its area of cross section is one m2. It can be seen that: 1 Ω m = 100 Ω cm or 1 Ω cm = 0.01 Ω m The inverse of resistance, R, is called conductance, G, and we have the relation: 1 G = = A =κ A (3.18) R ρl l The SI unit of conductance is siemens, represented by the symbol ‘S’ and is equal to ohm–1 (also known as mho) or Ω–1. The inverse of resistivity, called conductivity (specific conductance) is represented by the symbol, κ (Greek, kappa). IUPAC has recommended the use of term conductivity over specific conductance and hence we shall use the term conductivity in the rest of the book. The SI units of conductivity are S m–1 but quite often, κ is expressed in S cm–1. Conductivity of a material in S m–1 is its conductance when it is 1 m long and its area of cross section is 1 m2. It may be noted that 1 S cm–1 = 100 S m–1. 73 Electrochemistry 2015-16(20/01/2015)  Table 3.2: The values of Conductivity of some Selected Materials at 298.15 K Material Conductivity/ Material Conductivity/ S m–1 S m–1 Conductors Aqueous Solutions Sodium 2.1×103 Pure water 3.5× 1 0 – 5 3 Copper 5.9×10 0.1 M HCl 3.91 Silver 6.2×103 0.01M KCl 0.14 Gold 4.5×103 0.01M NaCl 0.12 3 Iron 1.0×10 0.1 M HAc 0.047 Graphite 1.2×10 0.01M HAc 0.016 Insulators Semiconductors Glass 1.0× 1 0–16 CuO 1 × 1 0–7 Teflon 1.0× 1 0–18 Si 1.5× 1 0 – 2 Ge 2.0 It can be seen from Table 3.2 that the magnitude of conductivity varies a great deal and depends on the nature of the material. It also depends on the temperature and pressure at which the measurements are made. Materials are classified into conductors, insulators and semiconductors depending on the magnitude of their conductivity. Metals and their alloys have very large conductivity and are known as conductors. Certain non-metals like carbon-black, graphite and some organic polymers* are also electronically conducting. Substances like glass, ceramics, etc., having very low conductivity are known as insulators. Substances like silicon, doped silicon and gallium arsenide having conductivity between conductors and insulators are called semiconductors and are important electronic materials. Certain materials called superconductors by definition have zero resistivity or infinite conductivity. Earlier, only metals and their alloys at very low temperatures (0 to 15 K) were known to behave as superconductors, but nowadays a number of ceramic materials and mixed oxides are also known to show superconductivity at temperatures as high as 150 K. Electrical conductance through metals is called metallic or electronic conductance and is due to the movement of electrons. The electronic conductance depends on (i) the nature and structure of the metal (ii) the number of valence electrons per atom (iii) temperature (it decreases with increase of temperature). * Electronically conducting polymers – In 1977 MacDiarmid, Heeger and Shirakawa discovered that acetylene gas can be polymerised to produce a polymer, polyacetylene when exposed to vapours of iodine acquires metallic lustre and conductivity. Since then several organic conducting polymers have been made such as polyaniline, polypyrrole and polythiophene. These organic metals, being composed wholly of elements like carbon, hydrogen and occasionally nitrogen, oxygen or sulphur, are much lighter than normal metals and can be used for making light-weight batteries. Besides, they have the mechanical properties of polymers such as flexibility so that one can make electronic devices such as transistors that can bend like a sheet of plastic. For the discovery of conducting polymers, MacDiarmid, Heeger and Shirakawa were awarded the Nobel Prize in Chemistry for the year 2000. Chemistry 74 2015-16(20/01/2015)  As the electrons enter at one end and go out through the other end, the composition of the metallic conductor remains unchanged. The mechanism of conductance through semiconductors is more complex. We already know (Class XI, Unit 7) that even very pure water has small amounts of hydrogen and hydroxyl ions (~10–7M) which lend it very low conductivity (3.5 × 10–5 S m–1). When electrolytes are dissolved in water, they furnish their own ions in the solution hence its conductivity also increases. The conductance of electricity by ions present in the solutions is called electrolytic or ionic conductance. The conductivity of electrolytic (ionic) solutions depends on: (i) the nature of the electrolyte added (ii) size of the ions produced and their solvation (iii) the nature of the solvent and its viscosity (iv) concentration of the electrolyte (v) temperature (it increases with the increase of temperature). Passage of direct current through ionic solution over a prolonged period can lead to change in its composition due to electrochemical reactions (Section 3.4.1). 3.4.1 Measurement We know that accurate measurement of an unknown resistance can be of the performed on a Wheatstone bridge. However, for measuring the resistance Conductivity of an ionic solution we face two problems. Firstly, passing direct current of Ionic (DC) changes the composition of the solution. Secondly, a solution cannot Solutions be connected to the bridge like a metallic wire or other solid conductor. The first difficulty is resolved by using an alternating current (AC) source of power. The second problem is solved by using a specially designed vessel called conductivity cell. It is available in several designs and two simple ones are shown in Fig. 3.4. Connecting Connecting wires wires Platinized Pt Fig. 3.4 electrodes Two different types of conductivity cells. Platinized Pt electrode Platinized Pt electrode Basically it consists of two platinum electrodes coated with platinum black (finely divided metallic Pt is deposited on the electrodes electrochemically). These have area of cross section equal to ‘A’ and are separated by distance ‘l’. Therefore, solution confined between these electrodes is a column of length l and area of cross section A. The resistance of such a column of solution is then given by the equation: l l R = ρ = (3.17) A κA 75 Electrochemistry 2015-16(20/01/2015)  The quantity l/A is called cell constant denoted by the symbol, G*. It depends on the distance between the electrodes and their area of cross-section and has the dimension of length–1 and can be calculated if we know l and A. Measurement of l and A is not only inconvenient but also unreliable. The cell constant is usually determined by measuring the resistance of the cell containing a solution whose conductivity is already known. For this purpose, we generally use KCl solutions whose conductivity is known accurately at various concentrations (Table 3.3) and at different temperatures. The cell constant, G*, is then given by the equation: l G* = = R κ (3.18) A Table 3.3: Conductivity and Molar conductivity of KCl solutions at 298.15K Concentration/Molarity Conductivity Molar Conductivity mol L–1 mol m–3 S cm–1 S m–1 S cm2mol –1 S m2 mol–1 1.000 1000 0.1113 11.13 111.3 111.3× 1 0 –4 0.100 100.0 0.0129 1.29 129.0 129.0× 1 0 –4 0.010 10.00 0.00141 0.141 141.0 141.0× 1 0 –4 Once the cell constant is determined, we can use it for measuring the resistance or conductivity of any solution. The set up for the measurement of the resistance is shown in Fig. 3.5. It consists of two resistances R3 and R4, a variable resistance R1 and the conductivity cell having the unknown resistance R2. The Wheatstone bridge is fed by an oscillator O (a source of a.c. power in the audio frequency range 550 to 5000 cycles per second). P is a suitable detector (a headphone or other electronic device) and the bridge is balanced when no current passes through the detector. Under these conditions: Fig. 3.5: Arrangement for measurement R1 R 4 of resistance of a solution of an Unknown resistance R2 = (3.19) R3 electrolyte. These days, inexpensive conductivity meters are available which can directly read the conductance or resistance of the solution in the conductivity cell. Once the cell constant and the resistance of the solution in the cell is determined, the conductivity of the solution is given by the equation: cell constant G* κ= = (3.20) R R The conductivity of solutions of different electrolytes in the same solvent and at a given temperature differs due to charge and size of the Chemistry 76 2015-16(20/01/2015)  ions in which they dissociate, the concentration of ions or ease with which the ions move under a potential gradient. It, therefore, becomes necessary to define a physically more meaningful quantity called molar conductivity denoted by the symbol Λm (Greek, lambda). It is related to the conductivity of the solution by the equation: κ Molar conductivity = Λ m = (3.21) c In the above equation, if κ is expressed in S m–1 and the concentration, c in mol m–3 then the units of Λm are in S m2 mol–1. It may be noted that: 1 mol m –3 = 1000(L/m3 ) × molarity (mol/L), and hence κ (S cm −1 ) Λm(S cm2 mol–1) = −3 1000 L m × molarity (mol L−1 ) If we use S cm–1 as the units for κ and mol cm–3, the units of concentration, then the units for Ëm are S cm2 mol–1. It can be calculated by using the equation: 2 –1 κ (S cm −1 ) × 1000 (cm 3 / L) Λm(S cm mol ) = molarity (mol /L) Both type of units are used in literature and are related to each other by the equations: 1 S m2mol–1 = 104 S cm2mol–1 or 1 S cm 2mol–1 = 10–4 S m2 mol–1. Resistance of a conductivity cell filled with 0.1 mol L–1 KCl solution is Example 3.4 100 Ω. If the resistance of the same cell when filled with 0.02 mol L–1 KCl solution is 520 Ω, calculate the conductivity and molar conductivity of 0.02 mol L–1 KCl solution. The conductivity of 0.1 mol L–1 KCl solution is 1.29 S/m. Solution The cell constant is given by the equation: Cell constant = G* = conductivity × resistance = 1.29 S/m × 100 Ω = 129 m–1 = 1.29 cm–1 Conductivity of 0.02 mol L–1 KCl solution = cell constant / resistance G* 129 m –1 = = = 0.248 S m–1 R 520 Ω Concentration = 0.02 mol L–1 = 1000 × 0.02 mol m–3 = 20 mol m–3 κ Molar conductivity = Λm = c 248 × 10 – 3 S m –1 = –3 = 124 × 10–4 S m2mol–1 20 mol m 1.29 cm –1 Alternatively, κ = = 0.248 × 10–2 S cm–1 520 Ω 77 Electrochemistry 2015-16(20/01/2015)  and Λm = κ × 1000 cm3 L–1 molarity–1 0.248×10 –2 S cm –1 ×1000 cm3 L–1 = 0.02 mol L– 1 = 124 S cm2 mol–1 Example 3.5 The electrical resistance of a column of 0.05 mol L–1 NaOH solution of diameter 1 cm and length 50 cm is 5.55 × 103 ohm. Calculate its resistivity, conductivity and molar conductivity. Solution A = π r2 = 3.14 × 0.52 cm2 = 0.785 cm2 = 0.785 × 10–4 m2 l = 50 cm = 0.5 m ρ l RA 5.55 × 103 Ω × 0.785 cm2 R= or ρ= = = 87.135 Ω cm A l 50 cm 1  1  Conductivity = κ = =  87.135  S cm–1 ρ   = 0.01148 S cm –1 κ × 1000 Molar conductivity , Λm = cm3 L–1 c 0.01148 S cm –1 ×1000 cm3 L–1 = 0.05 mol L –1 = 229.6 S cm2 mol–1 If we want to calculate the values of different quantities in terms of ‘m’ instead of ‘cm’, ρ = RA l 5.55 × 103 Ω × 0.785×10–4 m 2 = × 0 –2 Ω m = 87.135 1 0.5 m 1 100 κ = = Ω m = 1.148 S m–1 ρ 87.135 κ 1.148 S m –1 and Λm = = = 229.6 × 10 –4 S m 2 mol–1. c 50 mol m –3 3.4.2 Variation of Both conductivity and molar conductivity change with the Conductivity concentration of the electrolyte. Conductivity always decreases with and Molar decrease in concentration both, for weak and strong electrolytes. Conductivity This can be explained by the fact that the number of ions per unit with volume that carry the current in a solution decreases on dilution. Concentration The conductivity of a solution at any given concentration is the conductance of one unit volume of solution kept between two Chemistry 78 2015-16(20/01/2015)  platinum electrodes with unit area of cross section and at a distance of unit length. This is clear from the equation: G = κ A = κ (both A and l are unity in their appropriate units in l m or cm) Molar conductivity of a solution at a given concentration is the conductance of the volume V of solution containing one mole of electrolyte kept between two electrodes with area of cross section A and distance of unit length. Therefore, κA Λm = =κ l Since l = 1 and A = V ( volume containing 1 mole of electrolyte) Λm = κ V (3.22) Molar conductivity increases with decrease in concentration. This is because the total volume, V, of solution containing one mole of electrolyte also increases. It has been found that decrease in κ on dilution of a solution is more than compensated by increase in its volume. Physically, it means that at a given concentration, Λm can be defined as the conductance of the electrolytic solution kept between the electrodes of a conductivity cell at unit distance but having area of cross section large enough to accommodate sufficient volume of solution that contains one mole of the electrolyte. When concentration approaches zero, the molar conductivity is known as limiting molar Fig. 3.6: Molar conductivity versus c½ for acetic conductivity and is represented by the acid (weak electrolyte) and potassium symbol Ë°m. The variation in Λm with chloride (strong electrolyte) in aqueous concentration is different (Fig. 3.6) for solutions. strong and weak electrolytes. Strong Electrolytes For strong electrolytes, Λm increases slowly with dilution and can be represented by the equation: Λm = Ë m° – A c ½ (3.23) It can be seen that if we plot (Fig. 3.12) Ë m against c1/2, we obtain a straight line with intercept equal to Ëm° and slope equal to ‘–A’. The value of the constant ‘A’ for a given solvent and temperature depends on the type of electrolyte i.e., the charges on the cation and anion produced on the dissociation of the electrolyte in the solution. Thus, NaCl, CaCl2, MgSO4 are known as 1-1, 2-1 and 2-2 electrolytes respectively. All electrolytes of a particular type have the same value for ‘A’. 79 Electrochemistry 2015-16(20/01/2015)  Example 3.6 The molar conductivity of KCl solutions at different concentrations at 298 K are given below: c/mol L–1 Λ m/S cm 2 mol–1 0.000198 148.61 0.000309 148.29 0.000521 147.81 0.000989 147.09 Show that a plot between Ëm and c1/2 is a straight line. Determine the values of Ëm° and A for KCl. Solution Taking the square root of concentration we obtain: c1/2/(mol L –1 )1/2 Λ m/S cm 2mol–1 0.01407 148.61 0.01758 148.29 0.02283 147.81 0.03145 147.09 A plot of Λm( y-axis) and c1/2 (x-axis) is shown in (Fig. 3.7). It can be seen that it is nearly a straight line. From the intercept (c1/2 = 0), we find that Ëm° = 150.0 S cm2 mol–1 and A = – slope = 87.46 S cm 2 mol–1/(mol/L–1) 1/2. Fig. 3.7: Variation of Ëm against c½. Chemistry 80 2015-16(20/01/2015)  Kohlrausch examined Ëm° values for a number of strong electrolytes and observed certain regularities. He noted that the difference in Ë m° of the electrolytes NaX and KX for any X is nearly constant. For example at 298 K: Ë° – Ë° m (KCl) = Ë° m (NaCl) – Ë° m (KBr) m (NaBr) = Ë m° ° (KI) – Ë m (NaI) 2 ≃ 23.4 S cm mol –1 and similarly it was found that Ëm° (NaBr)– Ë m° (NaCl)= Ëm° (KBr) – Ë m° (KCl) ≃ 1.8 S cm2 mol–1 On the basis of the above observations he enunciated Kohlrausch law of independent migration of ions. The law states that limiting molar conductivity of an electrolyte can be represented as the sum of the individual contributions of the anion and cation of the electrolyte. Thus, if λ°Na+ and λ°Cl– are limiting molar conductivity of the sodium and chloride ions respectively, then the limiting molar conductivity for sodium chloride is given by the equation: Ë° = λ0 + + λ0 – m (NaCl) Na Cl (3.24) In general, if an electrolyte on dissociation gives ν+ cations and ν– anions then its limiting molar conductivity is given by: Ëm° = ν+ λ0+ + ν – λ0– (3.25) Here, λ0+ and λ 0− are the limiting molar conductivities of the cation and anion respectively. The values of λ0 for some cations and anions at 298 K are given in Table 3.4. Table 3.4: Limiting Molar Conductivity for some Ions in Water at 298 K Ion λ 0/(S cm2mol–1) Ion λ 0/(S cm2 mol–1) H+ 349.6 OH– 199.1 Na + 50.1 Cl– 76.3 K+ 73.5 Br– 78.1 Ca2+ 119.0 CH3COO– 40.9 2− Mg 2+ 106.0 SO 4 160.0 Weak Electrolytes Weak electrolytes like acetic acid have lower degree of dissociation at higher concentrations and hence for such electrolytes, the change in Λm with dilution is due to increase in the degree of dissociation and consequently the number of ions in total volume of solution that contains 1 mol of electrolyte. In such cases Ë m increases steeply (Fig. 3.12) on dilution, especially near lower concentrations. Therefore, Ëm° cannot be obtained by extrapolation of Λm to zero concentration. At infinite dilution (i.e., concentration c → zero) electrolyte dissociates completely (α =1), but at such low concentration the conductivity of the solution is so low that it cannot be measured accurately. Therefore, Ëm° for weak electrolytes is obtained by using Kohlrausch law of independent migration of ions (Example 3.8). At any concentration c, if α is the degree of dissociation 81 Electrochemistry 2015-16(20/01/2015)  then it can be approximated to the ratio of molar conductivity Ëm at the concentration c to limiting molar conductivity, Ëm°. Thus we have: Λm α= (3.26) Λm° But we know that for a weak electrolyte like acetic acid (Class XI, Unit 7), c α2 c Λm2 cΛ2 K = = = ο οm a (1 − α ) 2 Λ  Λm ( Λm − Λm ) Λmο 1 − mο  (3.27)  Λm  Applications of Kohlrausch law Using Kohlrausch law of independent migration of ions, it is possible to calculate Ëm° for any electrolyte from the λo of individual ions. Moreover, for weak electrolytes like acetic acid it is possible to determine the value of its dissociation constant once we know the Ëm° and Λm at a given concentration c. Example 3.7 Calculate Ë m° for CaCl2 and MgSO4 from the data given in Table 3.4. Solution We know from Kohlrausch law that Λmο ( CaCl 2 ) ο ο = λCa 2+ + 2λCl– = 119.0 S cm2 mol–1 + 2(76.3) S cm2 mol–1 = (119.0 + 152.6) S cm2 mol–1 = 271.6 S cm2 mol–1 Λmο ( MgSO ) = λοMg + λοSO = 106.0 S cm2 mol–1 + 160.0 S cm2 mol–1 4 2+ 2– 4 = 266 S cm2 mol–1. Example 3.8 Ëm° for NaCl, HCl and NaAc are 126.4, 425.9 and 91.0 S cm2 mol –1 respectively. Calculate Λo for HAc. Solution Λmο ( HA c) = λοH + λοAc = λοH + λοC l + λοAc + λοNa − λοC l − λοN a + – + – – + – + ο ο ο = Λ m ( HCl) +Λ m (NaAc ) −Λ m ( NaCl) = (425.9 + 91.0 – 126.4 ) S cm2 mol –1 = 390.5 S cm2 mol–1. Example 3.9 The conductivity of 0.001028 mol L–1 acetic acid is 4.95 × 10–5 S cm–1. Calculate its dissociation constant if Ë m ° for acetic acid is 390.5 S cm2 mol–1. κ 4. 95 × 10 −5 S cm − 1 1000 cm 3 Solution Λm = = × = 48.15 S cm3 mol–1 c 0. 001028 mol L− 1 L Λ 48.15 Scm 2mol−1 α = Λmο = 390.5 Scm 2 mol−1 = 0.1233 m cα 2 0. 001028mol L– 1 × (0. 1233) 2 k = = = 1.78 × 10–5 mol L–1 (1 − α ) 1− 0.1233 Chemistry 82 2015-16(20/01/2015)  Intext Questions 3.7 Why does the conductivity of a solution decrease with dilution? 3.8 Suggest a way to determine the Λ ° value of water. m 3.9 The molar conductivity of 0.025 mol L–1 methanoic acid is 46.1 S cm2 mol–1. Calculate its degree of dissociation and dissociation constant. Given λ0(H +) = 349.6 S cm2 mol–1 and λ0 (HCOO–) = 54.6 S cm2 mol –1. 3. 5 Electrolytic In an electrolytic cell external source of voltage is used to bring about a chemical reaction. The electrochemical processes are of great importance Cells and in the laboratory and the chemical industry. One of the simplest electrolytic Electrolysis cell consists of two copper strips dipping in an aqueous solution of copper sulphate. If a DC voltage is applied to the two electrodes, then Cu 2+ ions discharge at the cathode (negatively charged) and the following reaction takes place: Cu2+(aq) + 2e– → Cu (s) (3.28) Copper metal is deposited on the cathode. At the anode, copper is converted into Cu2+ ions by the reaction: Cu(s) → Cu 2+(s) + 2e– (3.29) Thus copper is dissolved (oxidised) at anode and deposited (reduced) at cathode. This is the basis for an industrial process in which impure copper is converted into copper of high purity. The impure copper is made an anode that dissolves on passing current and pure copper is deposited at the cathode. Many metals like Na, Mg, Al, etc. are produced on large scale by electrochemical reduction of their respective cations where no suitable chemical reducing agents are available for this purpose. Sodium and magnesium metals are produced by the electrolysis of their fused chlorides and aluminium is produced (Class XII, Unit 6) by electrolysis of aluminium oxide in presence of cryolite. Quantitative Aspects of Electrolysis Michael Faraday was the first scientist who described the quantitative aspects of electrolysis. Now Faraday’s laws also flow from what has been discussed earlier. Faraday’s Laws of Electrolysis After his extensive investigations on electrolysis of solutions and melts of electrolytes, Faraday published his results during 1833-34 in the form of the following well known Faraday’s two laws of electrolysis: (i) First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt). (ii) Second Law: The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic Mass of Metal ÷ Number of electrons required to reduce the cation). 83 Electrochemistry 2015-16(20/01/2015)  There were no constant current sources available during Faraday’s times. The general practice was to put a coulometer (a standard electrolytic cell) for determining the quantity of electricity passed from the amount of metal (generally silver or copper) deposited or consumed. However, coulometers are now obsolete and we now have constant current (I) sources available and the quantity of electricity Q, passed is given by Q = It Q is in coloumbs when I is in ampere and t is in second. The amount of electricity (or charge) required for oxidation or reduction depends on the stoichiometry of the electrode reaction. For example, in the reaction: Ag +(aq) + e– → Ag(s) (3.30) One mole of the electron is required for the reduction of one mole of silver ions. We know that charge on one electron is equal to 1.6021× 10–19C. Therefore, the charge on one mole of electrons is equal to: N A × 1.6021 × 10 –19 C = 6.02 × 1023 mol –1 × 1.6021 × 10–19 C = 96487 C mol–1 This quantity of electricity is called Faraday and is represented by the symbol F. For approximate calculations we use 1F ≃ 96500 C mol–1. For the electrode reactions: Mg2+ (l) + 2e– → Mg(s) (3.31) Al3+ (l) + 3e– → Al(s) (3.32) 2+ 3+ It is obvious that one mole of Mg and Al require 2 mol of electrons (2F) and 3 mol of electrons (3F) respectively. The charge passed through the electrolytic cell during electrolysis is equal to the product of current in amperes and time in seconds. In commercial production of metals, current as high as 50,000 amperes are used that amounts to about 0.518 F per second. Example 3.10 A solution of CuSO4 is electrolysed for 10 minutes with a current of 1.5 amperes. What is the mass of copper deposited at the cathode? Solution t = 600 s charge = current × time = 1.5 A × 600 s = 900 C According to the reaction: Cu2+(aq) + 2e– = Cu(s) We require 2F or 2 × 96487 C to deposit 1 mol or 63 g of Cu. For 900 C, the mass of Cu deposited = (63 g mol–1 × 900 C)/(2 × 96487 C mol–1) = 0.2938 g. 3.5.1 Products of Products of electrolysis depend on the nature of material being Electrolysis electrolysed and the type of electrodes being used. If the electrode is inert (e.g., platinum or gold), it does not participate in the chemical reaction and acts only as source or sink for electrons. On the other hand, if the electrode is reactive, it participates in the electrode reaction. Thus, the products of electrolysis may be different for reactive and inert Chemistry 84 2015-16(20/01/2015) electrodes.The products of electrolysis depend on the different oxidising and reducing species present in the electrolytic cell and their standard electrode potentials. Moreover, some of the electrochemical processes although feasible, are so slow kinetically that at lower voltages these do not seem to take place and extra potential (called overpotential) has to be applied, which makes such process more difficult to occur. For example, if we use molten NaCl, the products of electrolysis are sodium metal and Cl2 gas. Here we have only one cation (Na+) which is reduced at the cathode (Na+ + e– → Na) and one anion (Cl –) which is oxidised at the anode (Cl– → ½Cl2 + e– ). During the electrolysis of aqueous sodium chloride solution, the products are NaOH, Cl2 and H2. In this case besides Na+ and Cl– ions we also have H+ and OH– ions along with the solvent molecules, H2O. At the cathode there is competition between the following reduction reactions: V Na+ (aq) + e– → Na (s) E( cell) = – 2.71 V V H+ (aq) + e – → ½ H2 (g) E( cell) = 0.00 V The reaction with higher value of E⊖ is preferred and therefore, the reaction at the cathode during electrolysis is: H+ (aq) + e – → ½ H2 (g) (3.33) but H+ (aq) is produced by the dissociation of H2O, i.e., H2O (l ) → H + (aq) + OH– (aq) (3.34) Therefore, the net reaction at the cathode may be written as the sum of (3.33) and (3.34) and we have H2O (l ) + e – → ½H2(g) + OH– (3.35) At the anode the following oxidation reactions are possible: V Cl– (aq) → ½ Cl2 (g) + e– E( cell) = 1.36 V (3.36) V 2H2O (l ) → O2 (g) + 4H+(aq) + 4e– E( ce ll) = 1.23 V (3.37) The reaction at anode with lower value of E ⊖ is preferred and therefore, water should get oxidised in preference to Cl– (aq). However, on account of overpotential of oxygen, reaction (3.36) is preferred. Thus, the net reactions may be summarised as: H O NaCl (aq)   2 → Na+ (aq) + Cl– (aq) Cathode: H2O(l ) + e– → ½ H2 (g) + OH– (aq) Anode: Cl– (aq) → ½ Cl2 (g) + e– Net reaction: NaCl(aq) + H2O(l ) → Na +(aq) + OH– (aq) + ½H2(g) + ½Cl2 (g) The standard electrode potentials are replaced by electrode potentials given by Nernst equation (Eq. 3.8) to take into account the concentration effects. During the electrolysis of sulphuric acid, the following processes are possible at the anode: V 2H2O(l) → O2(g) + 4H+(aq) + 4e– E( cell) = +1.23 V (3.38) 85 Electrochemistry 2015-16(20/01/2015)  V 2SO42– (aq) → S2O 82– (aq) + 2e – E( ce ll) = 1.96 V (3.39) For dilute sulphuric acid, reaction (3.38) is preferred but at higher concentrations of H2SO 4, reaction (3.39) is preferred. Intext Questions 3.10 If a current of 0.5 ampere flows through a metallic wire for 2 hours, then how many electrons would flow through the wire? 3.11 Suggest a list of metals that are extracted electrolytically. 3.12 Consider the reaction: Cr2 O72– + 14H+ + 6e– → 2Cr3+ + 7H2O What is the quantity of electricity in coulombs needed to reduce 1 mol of Cr2O 72–? 3.6 Batteries Any battery (actually it may have one or more than one cell connected in series) or cell that we use as a source of electrical energy is basically a galvanic cell where the chemical energy of the redox reaction is converted into electrical energy. However, for a battery to be of practical use it should be reasonably light, compact and its voltage should not vary appreciably during its use. There are mainly two types of batteries. 3.6.1 Primary In the primary batteries, the reaction occurs only once and after use Batteries over a period of time battery becomes dead and cannot be reused again. The most familiar example of this type is the dry cell (known as Leclanche cell after its discoverer) which is used commonly in our transistors and clocks. The cell consists of a zinc container that also acts as anode and the cathode is a carbon (graphite) rod surrounded by powdered manganese dioxide and carbon (Fig.3.8). The space between the electrodes is filled by a moist paste of ammonium chloride (NH4Cl) and zinc chloride (ZnCl2 ). The electrode reactions are complex, but they can be written approximately as follows : Anode: Zn(s) → Zn2+ + 2e– Cathode: MnO2+ NH4++ e–→ MnO(OH) + NH3 In the reaction at cathode, manganese is reduced fro