Electrochemistry - chapter 3
Specific conductance
1/specific resistance Resistance of conductor = specific resistance x (distance between electrodes/area of cross section
In sparingly soluble salts what is the equation of Λm
1000κ/solubility
1 S m² mol⁻¹
10⁴ S cm² mol⁻¹
1 S cm² mol⁻¹
10⁻⁴ S m² mol⁻¹
Lead storage cell reaction during recharging
2PbSO₄ + 2H₂O → Pb + PbO₂ + 4H⁺ + 2SO₄²⁻ It acts as electrolytic cell while recharging
Hydrogen economy
Hydrogen has the highest calorific value among all the fuels. It is used as fuel in rockets. It can be used as fuel in vehicles also so that pollution can be prevented from petrol and diesel. Hydrogen is present in natural gas. It can also be obtained by photoelectrolysis of water on a large scale. Hydrogen is used in fuel cell. Fuel cells are used in space vehicles.
Specific resistivity
If a solution is placed between two parallel electrodes having cross-sectional area A and distance l apart then R = p x l/A p = specific resisitivity (ohm cm)
Galvanisation
Coating zinc over iron so as to protect it from rusting
Preferential discharge theory
If more than one type of iron is attracted towards a particular electrode, then the ion discharged is one which requires the least energy. The decreasing order of the discharge potential/ the increasing order of the deposition of cations and anions: Cations: K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, H⁺, Cu²⁺, Ag⁺, Au³⁺ Anions: SO₄²⁻, NO₃⁻, OH⁻,Cl⁻,Br⁻,I⁻
Standard hydrogen electrode (SHE)
It's used a reference electrode. Its electrode potential is taken as 0 volts. Consists of platinum wire coated with finely divided platinum black containing pure hydrogen gas at 1 atm and solution of 1M of HCl to maintain equilibrium between H⁺ ions and H₂. Platinum black catalyses the reaction.
Electrode potential
The tendency of an elemnt when it is placed in contact with its ions, to become positively or negatively charged by losing or gaining electrons
Voltage equation
V = IR
specific conductance of electrolyte increases with...
an increase in conc.
conductance of a metal decreases with..
an increase in temperature
Salt bridge
U-shaped tube filled with agar-agar paste containing inert electrolyte like KCl or KNO₃ which does not react with solutions. It prevents accumulation of charges in the half-cell
Concentration cells type 1 - electrode concentration cells
consists of two cells were identical electrodes are dipped in the same solution with different levels of concentration. If p₁>p₂ oxidation occurs at LHS electrode and reduction occurs at RHS electrode
degree of ionisation equation
molar conductance/molar conductivity at infinite dilution
maximum work equation
number of electrons x EMF of cell x 96500 C
electrical energy
product of potential difference and quantity of charge. Electrical energy = potential difference x quantity of charge or potential difference x current x time
Efficiency of cells
ratio of useful work obtainable from the cell to the total work.
Λm equation
specific conductance/molarity or specific conductance/number of moles÷1000 or 1000κ/Molarity or molar conductivity = molar conductivity at infinite dilution - constant√concentration
Daniell Cell; when external voltage = 1.1V
No flow of current or electrons No chemical reaction
Lead storage cell reaction during discharging
Pb + PbO₂ + 4H⁺ + 2SO₄²⁻ → 2PbSO₄ + 2H₂O
weak electrolytes
Those electrolytes which do not ionise completely on aqueous solutions or in molten state. CH₃COOH for example.
Acid dissociation equation
(concentration of electrolyte x degree of ionisation²) / (1 - degree of ionisation)
Learn how to find products of electrolysis at cathode and anode
***TO DO
Products of electrolysis
- the products depend on the nature of the electrolyte - nature of the electrodes. - If electrode is inert then it does not take part in the reaction - some of the electrochemical processes although feasible but slow in their rates at lower voltage - these do not take place as they need extra voltage. - The products of electrolysis differ in molten state and aqueous solution of electrolyte
displacement in non metals
A non-metal higher in series (towards the bottom) having high value of reduction potential will displace another non-metal with lower reduction potential
Kohlrausch's Law of independent migration of ions
According to this law, molar conductivity of an electrolyte, at infinite dilution can expressed as the sum of contributions from its individual ions. If the molar conductivity of the cations is denoted by λ₊°° (should be infinity) and that of the anions by λ_°° then the law of independent migration of ions is λm°° = v₊λ₊°° + v_λ_°° or Λ° = v₊λ₊° + v_λ_° where v₊ and v_ are the number of cations and anions per formula of electrolyte. For example, NaCl has 1 v₊ and 1 v_ and CaCl₂ has 1 v₊ and 2 v_
Electrochemical equivalent
Amount of substance deposited when 1 ampere current is passed for 1 second, ie, when 1 coloumb of charge is passed. unit - g C⁻¹
Why is a lower standard reduction potential preferred?
Because it means a greater reducing strength
Strong electrolytes
Those electrolytes which dissociate completely into ions - NaCl, HCl, NaOH
Variation of conductivity and molar conductivity with concentration
Conductivity always decreases with decrease in concentration for both weak and strong electrolytes. This is because the number of ions per unit volume that carry the current in a solution decreases on dilution. Molar conductivity increases with decrease in concentration. This is because both number of ions as well mobility of ions increase with dilution Molar conductivity = (1000 x specific conductance)/ Molarity At infinite dilution the electrolye dissociates completely but at such a low concentration the conductivity of the solution is so low that it cannot be measured accurately.
Daniell Cell
Consists of a zinc rod dipped in zinc sulphate solution and a copper rod in copper sulphate solution. Both the solutions are kept apart by taking zinc sulphate in a porous pot and putting it in a bath of copper sulphate solution. If we increase the external voltage more than 1.10V the reaction takes place in opposite direction.
Mercury cell
Consists of zinc mercury amalgam as anode, a paste of HgO and carbon as cathode. The electrolyte is a paste of KOH and ZnO. The reaction of the cell is The net reaction = Zn (amalgam) + HgO (s) →ZnO(s) + Hg(l) It gives constancy in voltage over long period because no ions are involved in net cell reaction. It is used in watches and hearing aids
Fuel cells
Electrical cells that are designed to convert the energy of the combustion of fuels such as hydrogen, carbon monoxide, methane, etc. directky into electrical energy. One of the most successful cells uses the reaction of hydrogen with oxygen to form water. It has been used for electric power in the Apollo space programme. The water vapours produced are condensed and added to drinking water supplies for astronauts. Very efficient. In the cell, hydrogen and oxygen are bubbled through a porous carbon electrodes into concentrated aqueous solution of sodium hydroxide. Catalysts are incorporated into the electrodes: Overall reaction = 2H₂(g) + O₂(g) → 2H₂O(l)
Daniell Cell; when external voltage > 1.1V
Electrons flow from Cu to Zn Current flows from Zn to Cu Zinc is deposited at the zinc electrode and copper dissolves at copper electrode
Daniell Cell; when external voltage < 1.1V
Electrons flow from Zn rod (negative) to Cu rod (positive) so the current flows from Cu to Zn. Zn dissolves at anode and copper deposits at cathode.
Cell potential equation for oxidation potential
E°cell = E°anode - E°cathode
Cell potential equation for reduction potential
E°cell = E°cathode - E°anode
Faraday's second law of electrolysis
If same charge is passed through different electrolytes, the mass of substance deposited will be proportional to their equivalent weights. (mass of substance 1/ equivalent weight of 1) = (mass of substance 2/ equivalent weight of 2) etc
concentration cells
If two electrodes of the same metal are dipped separately into two solutions of the same electrolyte at different concentrations and solutions are connected through a salt bridge.
Leclanche cell (Dry cell)
In this cell, the anode consists of a zinc container and the cathode is a graphite electrode surrounded by powdered MnO₂ and carbon. The paste between electrodes is filled with a moist paste of NH₄Cl and ZnCl₂. The net reaction = Zn + NH₄⁺(aq) + MnO₂(s) → Zn²⁺ + MnO(OH) + NH₃
cathodic protection
Instead of coating more reactive metal metal or iron, the use of such metal is made as sacrificial anode. In this method, a plate of more reactive metal (Zn or Mg) is buried in the earth.The iron pipe or tank is connected to it by wires. Here the iron becomes cathode and more reactive metal as anode. The minerals present in soil act as electrolyte. The reactive metal is sacrificed to protect the iron. These reactive metal plates are oxidised quickly, they are replaced from time to time which is easy to do.
Lead storage battery
It consists of a lead anode and a grid of lead packed with lead dioxide as cathode. A solution of sulphuric acid (38% by mass) is used as electrolyte. On recharging the battery, the reaction is reversed.
Nernst equation
It gives the relation between electrode potential and concentration of metal ions. The standard electrode reaction can be witten as : Mⁿ⁺(aq) + ne⁻ → M(s) The electrode potential for the above reaction can be written as E(Mⁿ⁺/M) = E°(Mⁿ⁺/M) + (RT/nF) x 1n[Mⁿ⁺] E°(Mⁿ⁺/M) = standard electrode potential for a molar solution of metal ion R = the gas constant = 8.31 JK⁻¹ mol⁻¹ T = temperature in Kelvin n = number of moles of electrons F = faraday's constant = 96500 C mol⁻¹ Mⁿ⁺ = at 298 K
Nickel-cadmium cell
It is another type of secondary cell which has a longer life than Lead Storage cell but more expensive to manufacture .
EMF (electromotive force)
It is energy per unit of charge passing through a cell. units = volts It is the potential difference when the circuit draws no current. Measured by potentiometer.
corrosion
It is the process in which metal is oxidised by loss of electrons to other electro-negative elements like oxygen and sulphur. Corrosion of iron occurs in the presence of moisture and oxygen.
Faraday constant
It's equal to charge on 1 mol of electrons. 96487 C mol⁻¹
Primary cells
Primary cell is a cell in which electrical energy is produced by the reaction occurring in the cell. It cannot be recharged. For e.g. Daniell cell, dry cell, mercury cell.
degree of dissociation
Ratio of molar conductivity at a specific concentration to the molar conductivity at infinite dilution. It is denoted by α α = molar conductivity to the power of the specific concentration/ Molar conductivity °°
Faraday's first law of electrolysis
The amount of substance deposited during electrolysis is directly proportional to quantity of electricity passed. mass is directly proportional to quantity of electricity Z = a constant called electrochemical equivalent of the substance I = current T = time mass = ZIt
Galvanic cell
The cell in which electricity is produced as a result of chemical reaction - aka voltaic cell
Electrolytic cell
The cell in which electrolysis of electrolytes is carried out in aqueous solution or in molten state by passing electric current through it. In this cell the electrical energy is converted into chemical energy.
Molar conductivity
The conducting power of all the ions produced by one gram mole of an electrolyte in a solution. It is denoted by Λm. Λm = S cm² mol⁻¹ or ohm⁻¹ cm² mol⁻¹
Conductivity
The conductivity of solutions of different electrolytes in the same solvent and at a given temperature differ due to difference in charge and size of the ions in which they dissociate, the concentration of ions or ease with which the ions move under potential gradient.
Potential difference
The difference in potentials of two electrodes of a cell. Measured in volts by a voltmeter
Cell potential
The difference in standard reduction potential of cathode and standard reduction potential of anode.
Conductance
The ease with which current flows through a conductor is called its conductance. It is reciprocal of the resistance. C = 1/Resistance or I/V The unit of conductance is Siemens, S 1S = 1Ω⁻¹
Electrolytic conduction
The flow of electric current through an electrolytic solution is called electrolytic conduction
Equivalent weight
The number of parts by weight of a given substance which combines or displaces directly or indirectly 1 part by weight of hydrogen, 8 parts by weight of oxygen or 35.5 parts by weight of chlorine. Eq. Wt. = atomic weight/ valency
Standard electrode potential
The pd in volts developed in a cell when the given electrode which is in contact with its ions having a concentration of 1 mol L⁻¹ is coupled with a standard hydrogen electrode.
Standard reduction electrode potential
The potential difference developed in a cell in volts when a given electrode in contact with its ions having 1 molar concentration undergoes reduction when it is coupled with standard hydrogen electrode.
Standard oxidation electrode potential
The potential difference when given electrode in contact with its ions having 1 molar concentration undergoes oxidation when coupled with standard hydrogen electrode
cell constant
The ratio of distance between electrodes to the cross sectional area between electrodes. cell constant = distance/cross sectional area
Specific conductivity (electrolytic conductivity)
The reciprocal of specific resisitvity of an electrolytic solution. It is denoted by κ (kappa) κ = Conductance of solution x distance/cross sectional area κ = 1/specific resistance = (1/R) x (distance between electrodes/cross sectional area)
Electrochemical series/ activity series
The series of elements which have been arranged on the basis of their electrode potential. They are arranged in increasing or decreasing order of their standard reduction potential
Ohm's Law
The strength of current (I) flowing through a conductor is directly proportional to the potential difference (V) applied across the conductor and inversely proportional to resistance (R) of the conductor. V = IR
Electrochemistry
This deals with the relationship between chemical energy and electrical energy and how one can be converted into another.
Secondary cells
Those cells which are used for storing electricity - lead storage battery, nickel-cadmium cell. They can be recharged.
displacement
a metal higher in electrochemical series will displace the metal from its solution which is lower in series
Property of a metal having -ve reduction potential
a stronger reducing agent than H₂
Property of a metal having +ve reduction potential
a weaker reducing agent than H₂
Cell constant equation
distance between electrodes/area of cross section
Λm increases with an...
increase in temperature.
Degree of ionisation
the ratio of number of ions produced to the total number of molecules of electrolyte
Property of the oxides of metal having high reduction potentials
they are thermally not stable
Concentration cells type 2 - electrolytic concentration cells
two same electrodes that are dipped in an electrolyte which possesses different concentration level. An electrolyte has a tendency to diffuse from a solution of higher concentration level to a solution of lower concentration level. With a period of time, the two concentrations tend to be equal.
Coulomb
unit of charge. = to charge of 1C when 1 ampere current is passed for 1 second Q = IT
