Electrochemistry :an Introduction Electrochemistry is the branch of chemistry deals with the chemical changes produced by electricity and the production of electricity by chemical changes. Electricity is the movement of electrons and is measured in Amps. The substances which allow an electric current to flow through them are called electrical conductors; while those which do not allow any electric current to flow through them are called non-conductors(insulators). Electrical conductors are of two types: (A) Metallic conductors or electronic conductors: is an object or type of material that allow the flow of electrical conductors in one or more directions. A metal wire is a common electrical conductor. In general, metals belong to this category. The metals remain unchanged during the flow of current except warming. Here transfer of electric current is due to transfer of free electrons of outer shells without any transfer of matter. Example: Cu, Ag, Al,Au,Cr,Co etc. In metals, the mobile charged particles are electrons. Positive charges may also be mobile, such as the cationic electrolytes of a battery, or the mobile protons of the proton conductors of a fuel cell.Graphite also conducts electricity due to presence of free e in its hexagonal sheet like structure. (B) Electrolytic conductors: Also called electrolytic conductor. a conducting medium in which the flow of current is accompanied by the movement of matter in the form of ions. any substance that dissociates into ions when dissolved in a suitable medium or melted and thus forms a conductor of electricity The substances which in fused state or in aqueous solution allow the electric current to flow accompanied by chemical decomposition are called electrolytes. On the other hand substances which in fused state or in aqueous solution do not allow the electric current to flow are called non-electrolytes. Usually, electrovalent compounds are good electrolytes and covalent compounds are non-electrolytes (Glycerol, glucose etc.).The metal rod or foil dipped in electrolyte, through which the electric current is conveyed, is called electrode. The electrode on which negative ions (anions) arrive are called positive electrode or anode. Similarly the electrode on which positive ions (cations) arrive are called negative electrode or cathode. Major differences between metallic and electrolytic conduction Metallic conduction Electrolytic conduction 1. Conduction is due to movement of 1. Conduction is due to movement of electrons ions(Flow of electricity takes place by the decomposition of substance) 2. No chemical change takes place(flow of electricity takes place without the 2. Ions get oxidized or reduced at decomposition of substance electrodes 3. No transfer of matter Occurs 3. Transfer of Matter occurs in the form of ions 4. Resistance Temperature(conduction decreases with increase of temperature) 4. Resistance (conduction 5-Ohm,s law is followed increases with increase of temperature) 6- Faraday’s law is not followed 5-Ohm,s law is followed 6. Faraday’s law is followed Classification of electrolytes Examples of strong and weak electrolytes are given below: Strong Electrolytes strong acids HCl, HBr, HI, HNO3, HClO3, HClO4, and H2SO4 strong bases NaOH, KOH, LiOH, Ba(OH)2, and Ca(OH)2 salts NaCl, KBr, MgCl2, and many, many more Weak Electrolytes weak acids HF, HC2H3O2 (acetic acid), H2CO3 (carbonic acid), H3PO4 (phosphoric acid), and many more weak bases NH3 (ammonia), C5H5N (pyridine), and several more, all containing "N Semiconductor is a material that has a resistivity value between that of a conductor and an insulator. The conductivity of a semiconductor material can be varied under an external electrical field. Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. Semiconductor devices include the transistor, many kinds of diodes including the light-emitting diode, the silicon controlled rectifier, and digital and analog integrated circuits. Solar photovoltaic panels are large semiconductor devices that directly convert light energy into electrical energy. In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current can be carried either by the flow of electrons or by the flow of positively-charged "holes" in the electron structure of the material. There are many examples of semiconductors such as Si,Ge,SnO2,TiO2,ZnO,Cds,CdSe, and many more Electrical insulator is a material whose internal electric charges do not flow freely, and therefore make it nearly impossible to conduct an electric current under the influence of an electric field. This contrasts with other materials, semiconductors and conductors, which conduct electric current more easily. The property that distinguishes an insulator is its resistivity; insulators have higher resistivity than semiconductors or conductors. The common examples of insulators are paper and mica glass. Comparison between conductor, insulator and semiconductor by Band Gap Energy Difference between conductor, insulator and semiconductor Conductor Insulator Semiconductor The conductivity of The conductivity of The conductivity of conductor is very high insulator is very low semiconductor is moderate It has very low resistivity It has very high resistivity It has moderate resistivity. It has no forbidden gap It has large forbidden It has small forbidden gap gap. Conductor has positive Insulator has negative Semiconductor has temperature coefficient of temperature coefficient of negative temperature resistance resistance coefficient of resistance In conductor, both the In insulator, effect of In semiconductor, effect effect of resistance and resistance is decreases of resistance is decreases temperature are and effect of temperature and effect of temperature increases is increases is increases. There is large number of There is small number of There is moderate electrons available for electrons available for number of electrons conduction conduction. available for conduction. Some practical units used in electrochemistry: The units obtained in electrochemistry are derived from System International(SI).The important units are listed as following: Joule(J): It is the unit of energy and may defined as It is equal to the energy transferred (or work done) to an object when a force of one newton acts on that object in the direction of its motion through a distance of one meter (1 newton metre or N·m) Coulomb(C): It is the unit of electrical charge(electricity )and may be defined as, t is the charge (symbol: Q or q) transported by a constant current of one ampere in one second: it is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt 1 Faraday (F) = 96500 Coulomb e-= electro charge=F Faraday (F): is defined as the charge on one mole of electrons Ampere(A or amp): It is the unit of electrical current ( measuring the rate of flow of electrons) and it defined as" The ampere is equivalent to one coulomb, i.e.,, ampere =(charge/time) 1A= 1 C /1 Ohm (Ω): It is the unit of electrical resistance and it defined as, an electrical resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromotive force Volt(V): It is the unit of electric potential, electric potential difference(voltage), and electromotive force electrical(EMF) potential and may be defined as, One volt is defined as the difference in electric potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It is also equal to the potential difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, it is the potential difference between two points that will impart one joule of energy per coulomb of charge that passes through it. It can be expressed in terms of SI base units (m,kg, s, and A) as . It can also be expressed as amperes times ohms (current times resistance, Ohm<s law), watts per ampere (power per unit current, Joule law), or joules per coulomb (energy per unit charge) which is also equivalent to electron volts per elementary charge: (v) Watt(W): It is the unit of electric power and may be defined as, “The rate of supply of electric power when one joule of electrical energy is made available per second”. Also it may be defined as, one watt is the rate at which work is done when one ampere (A) of current flows through an electrical potential difference of one volt (V). Two additional unit conversions for watt can be found using the above equation and Ohm,s law. Electrode: one of two poles of the cell and it is defined as is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air. Anode(-): is an electrode through which conventional current flows into a polarized electrical device .This, it an electrode where oxidation occur. Cathode(+): is the electrode from which a conventional current leaves a polarized electrical device. Thus, it a electrode where reduction occur. Half reaction: the ox or red reaction measured against SHE(Standard Hydrogen electrode) V=0.0..