Electrochemistry Flashcards
Define electrochemistry
Electrochemistry is the branch of chemistry that deals with the relation between chemical energy and electrical energy
Types of electrochemical reactions
Induced electrolytic reactions, spontaneous oxidation reduction reactions
What are induced electrolytic reaction
non-spontaneous reactions are forced to occur by the passage of electricity through the reactants
what are spontaneous oxidation reduction reactions
electricity is generated by the interaction of reactants
3 applications of electrochemical reactions
1) The electrolytic production of metals and their compounds
2) Study of industrial materials
3) Manufacture of batteries
4) Psychological studies of nervous systems in living organisms
What is a cell
Device which converts chemical energy to electrical energy. It is also known as voltaic cell or galvanic cell.
What is the arrangement of a cell
It is an arrangement of 2 electrodes and an electrolytic solution
What is a half cell
Part of cell which contains an electrode dipped in an electrolyte
Define an electrode
Metallic material in the form of a rod, wire or strip, which conducts electrons and it is in contact with electrolyte
Difference between anode and cathode
Anode is an electrode where oxidation takes place.
Cathode is an electrode where reduction takes place.
What is an electrolyte?
any substances which allow the passage of electric current through is, either in molten state or in solution.
eg. acids, bases, salts
Define electrolysis
Process of decomposition of an electrolyte by the passage of electricity
Define current
Flow of electric charge
Difference between anode and cathode compartment
Anode compartment is the half cell in which oxidation reaction takes place.
Cathode compartment is the half cell in which reduction reaction takes place.
What are electrochemical cells
devices in which redox reaction takes place indirectly and the decrease in potential energy is converted to electrical energy
What is a salt bridge
U shaped glass tube containing electrolytes like KCl, KNO3 etc. in semisolid agar agar
Advantages of salt bridge
1) Salt bridge maintains the electrical neutrality of each half cell and prevents the accumulation of ions in the electrode
2) Maintains the flow of current by completing the circuit
Different types of electrodes used in electrochemical cells
Metal-Metal ion electrode Metal-Metal insoluble salt electrode Gas electrode Redox electrode Ion selective electrode
Define Metal-metal ion electrode
a metal rod is immersed in its own salt solution. The electrode is represented as M/Mn+
eg. Zn electrode [Zn,Zn2+]
Define metal-metal insoluble salt electrode
a metal coated by its sparingly insoluble salt is immersed in a solution containing a common ion. The electrode is represented as M/MX//X-
Define Gas electrode
An inert metal (Au/Pt) is immersed in a solution containing ions of a gas. The gas is continuously bubbled through the solution. The electrode is represented as Pt/X2(gas at 1atm P)/X+
eg. Standard Hydrogen Electrode
Define Redox electrode
An inert metal is immersed in a solution containing ions of the same substance in 2 oxidation states. The electrode is represented as Pt/Mn1+/Mn2+(gas at 1atm P)/X+
eg. Quinhydron electrode
Define Ion Selective electrode
It is a sensor that converts the concentration of a specific ion dissolved in a solution into an electric potential, which can be measured.
It consists of an internal reference electrode (Pt/Ag, AgCl electrode) and a reference solution sealed in an ion selective membrane. The electrode is represented as Pt/0.1M HCl/Glass/Test H+ solution
eg. glass electrode
Define electrode potential
Tendency of a metallic electrode to lose or gain electrons when it is in contact with its own salt solution
Define oxidation and reduction reactions
Oxidation is the tendency of an electrode in a half cell to lose electrons
Reduction is the tendency of an electrode to gain electrons
define Helmholtz electrical double layer
The residual electrons in the metal and the metal cations in the solution create an electrical double layer known as Helmholtz electrical double layer.
The factors on which the value of electrode potential depends
1) Nature of the metal
2) Concentration of the solution
3) Temperature of the solution
Define standard electrode potential
equilibrium potential difference between metal electrode and its surrounding ions of unit concentration (1M or 1N) at 25oC
What is SHE
To determine the relative value of potential of an electrode, we will fix arbitrarily the potential of one of the electrodes as zero. For that the potential of reversible hydrogen electrode is taken as zero, hence it is known as standard hydrogen electrode
What are reference electrodes
Electrode which has a stable and well known electrode potential
Explain the 2 types of reference electrodes
1) Primary reference electrode: The best PRE used is SHE whose electrode potential at all temperatures is taken as 0. Due to its bulky size and difficulty in construction, other electrodes are used in normal case.
2) Secondary reference electrode: Electrodes with known constant potential which can be combined with other electrodes to find out unknown potentials. Eg. calomel electrode, glass electrode, quinhydron electrode
Advantages and Disadvantages of SHE
Advantages:
1) Fundamental electrode to which all the pH measurements are ultimately referred
2) shows no salt error, negligible internal resistance and negligible electrical leakage
3) can be used over the entire pH range
4) Highly accurate
Disadvantages:
1) Pt electrode is easily poisoned by the absorption of impurities from the solution and the gas
2) It can’t be used in a solution containing metal ions which are below in the electrochemical series
3) cant be used in solutions containing strong oxidising agents
How is an electrochemical cell constructed
It is constructed by connecting the given electrode and SHE through a salt bridge
Calomel electrode
Saturated calomel electrode is a secondary reference electrode. It consists of Hg, solid Hg2Cl2 and a solution of KCl. It produces a constant and reproducible electrode potential which will not change with small change in temperature
Glass electrode
It is a SRE which produce a constant and reproducible electrode potential. It is an ion selective electrode used with other ref electrodes to generate a PD
Advantages of Glass electrode
1) It can be used for the determination of pH of oxidising agents, reducing agents, viscous media, in presence of proteins which interfere the other electrodes etc
2) It can be used in the pH range of 0-12
3) It can be used in coloured, turbid and colloidal solutions
4) simple to operate
Applications of emf measurements
1) Determination of pH
2) Spontaneity of any reaction can be predicted
3) prediction of relative oxidising tendency
4) basic of electrochemical series
5) gives an idea about the replacement tendency of metals
electrochemical series
series which represents an arrangement of various metal electrodes in the increasing order of their standard reduction potentials or decreasing order of standard oxidation potentials
Applications of electrochemical series
1) Calculation of emf of a cell
2) Prediction of spontaneity of reaction (emf is positive)
3) Prediction of relative oxidising and reducing tendency
4) Replacement tendency of metals
5) Displacement of hydrogen from dilute acids
Nernst equation
E = Eo - (2.303RT/nF)log([M]/[Mn+])
Applications of Nernst equation
1) To study the effect of electrolyte concentration on electrode potential
2) For calculation of the potential of a cell under non standard conditions
3) Determine unknown concentration of one of the ionic species in a cell
4) The pH of a solution from the measurement of electrode potential
Maximum amount of work obtained from a cell
product of charge flowing per mole and maximum potential difference through which the charge is transferred
Potentiometric titrations
measurement of emf for recording end point of volumetric titrations by using a potentiometer
Advantages of potentiometric titrations
1) No indicators are required
2) can be used for coloured solutions
3) Prior info about the relative strength of the titrants is not required
4) Redox and ppt reactions can be followed
5) possible to carry out step by step titration of poly basic acid and mixtures of acids
Types of potentiometric titrations
1) Acid-Base
2) Oxidation-Reduction
3) Precipitation
Principle of potentiometric titrations
According to Nernst equation, the potential of an electrode depends upon the concentration of the electrolyte ions.
What happens on addition of KMnO4 to Fe2+ ion solution
Oxidation of Fe2+ to Fe3+ ions
What are fuel cells
primary electrochemical cell in which chemical energy of fuels is directly converted into electrical energy
Advantages of fuel cells
1) efficient and convert about 75%-83% of available chemical energy to electrical energy
2) electrode materials are supplied in the gaseous form.
eg. H2CO, hydrocarbons
3) whatever maybe the electrode material, CO2 and H2O should be the end products
4) The concentration of electrolyte remains invariant
5) Light, compact, simple and easy to maintain
6) Cell is used in earlier space vehicles and in underground works
7) less green house gas and noise pollution
8) Can be stacked and connected in series to generate higher voltage without making noise
9) Reduces thermal pollution from power generation
Storage cells
Secondary cells in which electrical energy is stored as chemical energy, when required, this electrical energy can be released from this stored chemical energy through an external circuit
eg. Lead storage cell, Nickel Cadmium cell , Edison cell, Lithium ion cell
Lithium ionic batteries
secondary rechargeable batteries
Most energetic rechargeable batteries available
varies from 3.5-4V, depending on the electrode material
3 primary functional components of a Li ion battery
Anode: lithiated graphite
Cathode: Mixed metal oxide
Electrolyte: Li salt
Characteristics of a Li ion battery
1) Depending on the choice of material for the anode, cathode and electrolyte, the voltage, capacity, life and safety of a Li ion battery can change drastically
2) Typically charged not greater than 80 to 90 percent of their max state of charge, and are allowed to discharge below some minimum SoC, perhaps 30% because operation at extremely high or low states of charge can dramatically reduce battery life
3) Li ion battery come in a variety of forms, but the most common for automotive applications are cylindrical cells and primitive cells
Advantages of Li ion batteries
1) formed into a wide variety of shapes and sizes so as to efficiently fill available space in the devices they power
2) lighter than other equivalent secondary batteries
3) produce a high open circuit voltage compared to other secondary batteries
Applications of Li ion batteries
1) Laptops
2) PDAs
3) Cell Phones
4) iPods
5) Cellular phones
6) Audio visual equipment
7) Office automation etc
electronic conduction
involves direct transfer of electrons through the conductor under the influence of an applied potential
2 conditions of electronic conduction
1) electrons flow from negative end to positive end
2) no change in the composition of the conductor
Conductivity
Ability of an ion in a solution to carry current
Specific resistance
resistance in ohms of a specimen of 1cm in length and 1cm2 in cross section
specific conductance
(reciprocal of specific resistance) Conductance of 1 cm cube of a solution
Equivalent conductance
conductivity of all the ions present in 1 gram equivalent of the electrolyte in solution
Molar conductance
Conductance of all the ions present in 1 mole
Types of conductivity cells
1) Cup type
2) Dip type
Why are electrodes in conductivity cell known as platinized Pt electrodes
The polarization effect is eliminated by coating the electrodes with a layer of Pt black
Cell constant
ratio of distance between the 2 electrodes and the area of the electrodes in the conductivity cell
Device used to measure conductivity
Conductometer
Importance of conductivity measurement
1) extremely useful and widespread method for quality control purposes
2) high reliability, sensitivity, relatively low cost make it a potential primary parameter of any good monitoring program
3) rapid and inexpensive way of determining ionic strength of a solution
4) Non specific technique, unable to distinguish different types of ions in the solution, but can give combined effect of all the ions present
applications of conductance measurements
1) study of strong and weak electrolytes
2) determination of degree of dissociation
3) Determination of solubility of sparingly soluble salts
4) Determination of conc of solns by conductometric titrations
