Physical chemistry vol II dr j n gurtu, aayushi gurtu (pragati prakashan, 2008)

p~~ ~- PHYSICAL CHEMISTRY I Vol II) Dr J N Gurtu M.Sc., Ph D Former Principal Meerut College, MEERUT Aayushi Gurtu ~ PRAGATIPRAKASHAN PRAGATIPRAKASHAN Heat! Office: Educatiollal Publishers Second Edition 2008 PRAGATl BHAWAN, 240, W K Road, MeerUl-250 001 Tele Faxe' 0121-2643636, 2640642 SMS/Ph()ne~: 0121-6544642 6451644 Regd Office: New Market, Begum Bridge, Meerut-250 001 Phone:0121-2651907 ISBN: 978-81-8398-511-6 Published by K.K Mittal for Pragati Prakashan, Meerut - 250 00 I and Photocomposing by : Pragati Laser Type Setters Pvt Ltd., Meerut (Phone 2(i(i 1657) Pri'lted at Urvashl Offset Press, Meerut CONTENTS THERMODYNAMICS-I Basic definitions Energy 11 Internal energy 11 1- 38 12 Zeroth law of thermodynam:cs 13 First law of thermodynamics Heat changes 14 Heat content or enthalpy Heat capacity 15 15 17 Applications of first law of thermodynamics Joule-Thomson effect 26 Miscellaneous numerical problems Exercises 31 35 THERMOCHEMISTRY 39-73 Heat of reaction or enthalpy of reaction 39 Variation of enthalpy of reaction with temperature, i.e., Kirchoffls equation 45 Heat of formation or enthalpy of formation Enthalpies of compounds 46 Heat of combustion or enthalpy of combustion 47 Heat of neutralisation or enthalpy of neutralisation Heat of transition or enthalpy of transition 50 50 Heat of solution and heat of dilution or enthalpy of solution and dilution Intrinsic energy Laws of thermochemistry Flame temperature Resonance energy 55 55 58 Bond energy and dissociation energy 59 61 Miscellaneous numerical problems 52 54 Exothermic and endothermic reactions and compounds Exercises 42 62 71 74-134 THERMODYNAMICS-II Spontaneous and non-spontaneous processes Reversible processes Carnotls cycle 74 74 75 Second law of thermodynamics 80 Thermodynamic or Kelvin scale of temperatu~e 81 Concept of entropy 85 (vi) Criteria of spontaneity (irreve,jibility) and conditions of equilibrium Prediction of direction or occurrence of a process 102 104 105 Nernst's heat theorem 11 Third law of thermodynamics 11 Free energy and work function 123 Exercises 129 Concept of residual entropy CHEMICAL EQUILIBRIUM Chemical equilibrium 35 Law of mass action 36 Le Chatelier's principle 135-164 41 van't Hoff isotherm or maximum work obtained from gaseous reactions van't Hoff isochore or van't Hoff equation Clausius-Clapeyron equation 51 54 Clapeyron equation Exercises 58 62 PHASE RULE 165-229 165 Introduction Phase 148 165 Component 66 Degree of freedom or variance 168 Criterion of phase equilibrium 69 Statement of phase rule 169 Thermodynamic derivation of phase rule 170 72 One component system 72 Water system Carbon dioxide system Sulphur system 75 77 Two component systems Lead-silver system 81 82 Potassium iodide-water system Bismuth-cadmium system 84 186 Binary systems with formation of compounds with congruent melting point Binary systems with formation of compounds with incongruent melting point Solid-gas systems 197 Determination of solid-liquid equilibria Henry's law 202 Systems of liquid in liquid 204 99 87 194 (vii) Solubility of partially miscible liquid pairs Exercises 224 21 DISTRIBUTION LAW Distribution in liquid-liquid systems 230 Thermodynamic derivation of distribution law Different cases of distribution law 232 Applications of distribution law 234 Numerical problems 239 Exercises 230-247 231 ELECTROCHEMISTRY-I Electrical transport-conduction in metals 248 Conduction in electrolytic solutions 250 Arrhenius theory of electrolytic dissociation 260 Migration of ions 263 Transference number or transport number 266 Kohlrausch's law or law of independent migration of ions Ostwald's dilution law 277 Applications of conductivity measurements 279 Conductometric titrations 282 Anomaly of strong electrolytes 283 Exercises 86 ELECTROCHEMISTRY-II Electrochemical cells 291 Reversible and irreversible cells 294 Notations used in cell diagrams 295 Electromotive force 296 EMF of a cell and cell reaction 298 Weston standard cell 299 Reversible electrodes 300 Electrode potential 301 Electromotive series or potential series 308 Calculation of thermodynamic constants 31 Polarisation Oxidation-reduction potential 31 Overvoltage or overpotential 323 Liquid junction potential 328 248-290 273 291-364 (viii) Electrode concentration cells 01 Electrolyte concentration cells amalgam cells 331 Applications of concentration cells Fuel cells 337 350 Potentiometric titrations Exercises 330 353 359 HYDROGEN ION CONCENTRATION, BUFFERS AND HYDROLYSIS pH values 365 Buffer solutions Ionic product 371 of water 375 Salt hydrolysis 376 Degree of hydrolysis Exercises of a sdlt 380 388 10 CORROSION Corrosion 394-418 394 Theories of corrosion 394 Factors influencing corrosion Corrosion inhibitors Passivity 400 402 403 Types of corrosion 403 Protection from corrosion or corrosion control Exercises o o 365-393 408 12 SUBJECT INDEX LOG AND ANTILOG TABLES 419-420 (i)-(iv) Chapter ~ ""I THERMOIj~rAMICS-I BASIC DEFINITIONS [I] Thermodynamics, Objectives and Limitations (a) ,!,bermodynamics means the study of flow of heat It deals with energy changes accompanying all types of physical and chemical processes It is based on three generalisations, known as first, second and third laws of thermodynamics (b) Objectives : Thermodynamics is of great importance in physical chemistry Most of the important generalisations of physical chemistry such as van't Hoff law of dilute solutions, law of chemical equilibrium, phase rule etc can be deduced from the laws of thermodynamics It also lays down the criteria for predicting spontaneity' of a process, i.e., whether a given process is possible or not under given conditions of pressure, temperature and concentration It also helps us to determine the extent to which a process can proceed before obtaining the state of equilibrium (c) Limitations : The laws of thermody-namics apply to the behaviour of assemblages of a large number of molecules and not to individual atoms or molecules It does not tell us about the rate at which a given process may occur, i.e., it does not tell whether the reaction will be slow or fast It concerns only with the initial and final states of a system [II] Thermodynamic System A thermodynamic system is defined as the specified portion (>f matter which is separated from the rest of the universe with a bounding surface It may consist of one or more substances Boiling water in a beaker is an example of a thermodynamic system [III] Surroundings The rest of the universe which might be in a position to exchange matter and energy with the system is termed as surroundings Consider a reaction between :linc and dilute H 2S0 in a test tube Here the test tube forms a system Everything also around this system is called surroundings [IV] Types of Thermodynamic Systems (i) Closed system : In a closed system, exchange of energy with the surroundings is possible, while matter can neither enter into nor leE.ve the system (ii) Isolated system: In this system, there is no exchange of matter or energy b~tween the system and the surroundings PHYSICAL CHEMISTRY-II (iii) Open system: In an open system, both matter and energy can enter into or leave the system and thus there can be an exchange of matter and energy between the system and the surroundings [V] Types of Thermodynamic Systems If a system is kept at constant temperature, it is called an isothermal system If the system is so insulated from its surroundings that no heat flows in or out of the system, it is called an adiabatic system A system is said to be homogeneous when it consists of only one phase, i.e., when it is completely uniform throughout For example, a solution of salt is a homogeneous system A system is said to be heterogeneous when it consists of two or more phases, i.e., when it is not uniform throughout For example, a mixture of two immiscible liquids is a heterogeneous system Besides the above, thermodynamic system can be open, closed or isolated as discussed above [VI] Nature of Work and Heat Whenever a system changes from one state to another, there is always a change in energy, which may appear in the form of heat, work etc The unit of energy is erg It is defined as the work done when a resistance of dyne is displaced through a distance of cm As erg is a small quantity, a bigger unit, called joule (1 joule =: 107 ergs) is used Joule (1850) observed a definite relationship between mechanical work done (W) and heat produced (II), i.e., WocH or W=J.H where J is a constant, known as mechanical equivalent of heat Its value is 4.185 x 107 ergs or 4.185 joules Thus, for the use of 4.185 x 107 erg of mechanical energy, calorie of heat is produced Joule = 4.;85 calorie = 0.2389 calorie Work can be defined as the product of an intensity factor (force, pressure etc.) and a capacity factor (distance, electrical energy etc) Work is done in various ways (i) Gravitational work : The work done is said to be gravitational work if a body is moved through a certain height against the gravitational field If m gm be the mass of a body and h cm be the height of the gravitational field of acceleration gem sec-2 , then the force used to overcome gravity is mg, i.e., the intensity factor is mg dynes The capacity factor is the height h cm The wor:;: done is, therefore, mgh ergs (U) Electrical work: The work done is said to be electrical, if a current flows in an electrical circuit If a potential difference causing the flow is E volts (intensity factor) and the quantity of electricity that flows in a given time is Q coulombs (capacity factor), then electrical work done is EQ volt coulombs or EQ joule (iii) Mechanical work : The work done is said to be mechanical whenever there is a change in the volume of the system As seen in latter articles, the work done is given by fv VI PdV THERMODYNAMICS-I (iv) Maximum work: The magnitude of work done by a system on expansion depends upon the magnitude of the external pressure Maximum work is obtained when the gaseous pressure and the external pressure differ only by an infinitesimally small amount from one another It is obtained in an ideal reversible process [VII] Thermodynamic Variables or State Variables The quantities whose values determine the state of a system are called its thermodynamic variables or state variables The most important state variables are mass, composition, temperature, pressure and volume It is, however, not necessary that we should always specifY all the variables, because some of them are inter-dependent For a single pure gas, composition may not be one of the variables, as it remains only 100% For one mole of an ideal gas, the gas equation PV = RT is obeyed Evidently, if only two out of the three variables (P, V, T) are known, the third can be easily calculated The two variables generally specified are pressure and temperature These two variables are known as independent variables The third variable, viz., volume is known as dependent variable, as its value depends upon the values of pressure and temperature [VIII] Extensive Variable The variable of a system which depends upon the amount of the substance or substances present in the system is known as an extensive variable In other words, those variables whose values in any part of the divided system are different from the values of the entire system are called extensive variables Examples of extensive variables are volume, energy, heat capacity, entropy, enthalpy, free energy, length and mass [IX] Intensive Variable The variable of a system which is independent of the amount of the substance present in the system is known as intensive variable In other words, those variables whose values on division remains the same in any part of the system are called intensive variables Examples of intensive variables are temperature, pressure, concentration, dipole moment, density, refactive index, surface tension, viscosity, molar volume, gas constant, specific heat capacity, specific gravity, vapour pressure, emf of a dry cell, dielectric constant etc [X] State Functions and Path Functions State variables which are determined by the initial and final states ofthe system only and not by the path followed are called state functions These depend upon how the change from initial to the final state is carried out State variables, on the contrary which are determined or depend on the path followed are called path functions Consider the expansion of a gas from PI, VI, Tl toP2, V2 and T2 : (i) in steps and (ii) adiabatically In adiabatic expansion, let the work done by the system be W and heat absorbed is zero In stepwise expansion, heat absorbed is Q and work done is W Here W *" Q and the heat absorbed in the two cases are also different even though the system has undergone the same net change Thus, W and Q are 410 PHYSICAL CHEMISTRY-I! (2) Deactivation: It involves the addition of chemicals, capable of combining rapidly with the oxygen in aqueous solution, e.g., sodium sulphite 2Na2S03 + 02 ~ 2Na2S04 (3) Dehumidification: It reduces the moisture content of air to such an extent that the amount of water condensed on metal is too small to cause corrosion Alumina or silica gel which absorbs moisture preferentially on their surfaces, are used only in closed areas like air-conditioning shop (4) Alkaline neutralization : It consists of preventing the corrosion by neutralizing the acidic character of corrosive environment (due to the presence of HCI, H 2S, S02, CO2 etc.) Such alkaline neutralizers (like NH 3, NaOH, lime, etc.) are generally injected either in vapour or liquid form to the corroding system or to its parts This method has been widely used in controlling the corrosion of refinery equipments [IV] Metallic Coatings Metallic coatings are mostly applied On iron and steel because they are the cheap and most commonly used construction materials and are also the most sucsceptible ones for corrosion The metallic coatings often ,used are of Zn, Sn, Ni, Cu, Cr, Al and Pb Generally, the following methods are used' for metallic coatings (1) Electroplating: Noble metals such as Au, Ag, W, Pb etc and base metal, it protects the base metal by virtue of its noble character Tin plating and nickel plating are generally used The electroplating of zinc is called galvanising In electroplating, the object to be plated is made the cathode ofthe cell The electrolyte is a salt of the metal to be deposited The anode may be ofthe metal to be deposited or it may be an inert electrode such as graphite When the anode is that of the metal to be deposited, the anode dissolves to replenish the metal in the solution Using this method metals like Au, Ag, Cr, Ni, Cu, Zn, Sn etc may be electroplated (2) Hot dipping: Hot dipping is used for coating metals with films of metals having low melting point such as Zn, Su, Pb etc In this process, the metal to be coated is dipped in the molten bath of the coating metal for sufficient time and then removed out along with the adhering film The process of providing a zinc coating is called galvanising and the one providing a tin coating is called tinning (3) Vapourising : Some metals can also be deposited as surface layers by allowing their vapours to strike metallic surfaces with which they undergo alloying Zn and Al can be plated by vapourisation The deposition of zinc by vapourisation is known as sherardising or dry galvanising and deposition of Al is called calorising (4) Metal spraying: In this method, the molten metal is sprayed on the cleaned base metal with the help of a spraying gun or pistol which can be held in hand to direct the molten metal stream as required This process is only used when hot dipping is not possible Metal spraying is utilized for huge structures such as bridges (5) Cementation : In this method, the base metal articles are packed in the powdered coating metal, or a mixture of the powdered metal and a filler and are heated to a temperature just below the melting point of the more fusible metal Generally, an inert or reducing atmosphere is usually maintained during the process This method is used for producing alloy layer on iron and steel surfaces with Zn, Al, Cr, Si etc Steel may be case-hardened by cementation with carbonaceous materials in the pack carburizing process CORROSION 411 (6) Metal cladding: Many processes for cladding a base metal with another metal or alloy have been developed recently to impart corrosion and wear resistance In one of the methods, a duplex ingot is cast with the coating material on the outside and subsequently the ingot is rolled into a plate, sheet or bar or drawn into a wire form Steel sheets clad with stainless stels, copper covered steel articles and tin-cladded lead foils are prepaed by this method Other methods of cladding include : (a) Rolling the clean sheets or plates of the two materials together (b) Applying the coating sheet by spot welding or resistance welding (c) Fusing the cladding material over the surface of the base metal [V] Inorganic Non-Metallic Coatings The inorganic non-metallic protective coating include surface conversion or chemical dip coating, anodized oxide coating and vitreous enamel coating (1) Chemical dip coating or surface conversion: These coatings are produced by covering the surface of a metal or alloy by chemical or electrochemi~al methods The metal is immersed in a solution of a suitable chemical which reacts with the metal surface producing an adherent coating These coating afford good protection of the base metal from corrosion in some environments and sometimes are of decorative value The most commonly used surface conversion coatings are chromate coatings, phosphate coatings and oxide coatings (2) Anodized oxide coatings: Protective oxide films are produced on Al and its alloys in air spontaneously A more protective, thicker and stronger oxide film can be produced by making AI as the anode in an electrolytic bath containing chromic acid or oxalic acid or sulphuric acid After anodizing, the oxide coating is sealed by immersing in boiling water This treatment decreases the porosity and increases corrosion resistance of the film (3) Vitreous enamel coatings : Vitreous or porcelain enamels are modified glass-like materials having different compositions which are usually applied on steel and cast iron equipment The metal part to be enamelled is first cleaned carefully to remove grease and oxide scale The vitreous material for the enamel, called frit is prepared by fusing together refractory acidic substances, e.g., quartz and felspar, with basic fluxes e.g., borax, cryolite, fluorspar, soda ash, litharge, sodium nitrate and this frit is applied to the metal These coatings are widely applied for ferrous materials used for equipments in the pharmaceutical, chemical, dairy food and beverage industries [VI] Organic Coatings Protection of a metal surface from corrosion by using organic protective coatings is an established practice Important organic protective coatings include paints, varnishes, enamels and lacquers When applied on cleaned metal surfaces, they act aD effective inert barriers which not only protect the metal from corrosion but also afford decorative and aesthetic appeal 412 PHYSICAL CHEMISTRY-I! EXERCISES [I] Essay Type or Long Answr Type Questions (a) 10 11 12 13 14 15 How the following factors influer.ce the rate of corrosion: (i) Polarization, (ii) Electrode potential? (b) Explain the following methods of corrosion control: (i) Cathodic protection, (ii) Galvanization (a) Define metallic corrosion Explain electrochemical theory of corrosion (b) Explain how the corrosion can be cOIitrolled by sacrificial anode and impressed emf methods (a) What is corrosion of metals? Describe the mechanism of electrochemical corrosion by: (i) Hydrogen evolution, (ii) Oxygen absorption (b) Describe briefly cathodic protection (a) Describe briefly the following: (i) Pitting corrosion, (ii) Stress corrosion (b) What is cathodic protection? How is it done by using impresed current and sacrificial anode? Explain with suitable examples Differentiate chemical and electrochemical corrosion with suitable examples (a) State the two conditions for wet corrosion to take place (b) Explain the two important factors that affect the corrosion of metals How will you protect an underground pipeline from corrosion by sacrificial anodic and impressed current cathodic protection methods? (a) Discuss the electrochemical principles of corrosion Illustrate with example (b) State two conditions for wet corrosion to take place (c) Explain briefly the various factors affecting corrosion (a) What is "pitting corrosion"? (b) Describe the mechanism of electrochemical corrosion (a) How is corrosion prevented by cathodic protection? (b) How does Fe corrode in neutral or alkaline medium? (a) What is meant by electrochemical corrosion? Explain its mechanism (b) Explain the term cathodic protection Indicate how metal coatings can effectively prevent corrosion (a) What are the causes of corrosion of metals? Explain the mechanism of chemical corrosion (b) Write short note in pitting corrosion (a) Write notes on the following: (i) Experimental determination of rate of corrosion, (in Cathodic protection (b) Write a short note on concentration cell corrosion Discuss the types of corrosion Explain the mechanism of hydrogen evolution and oxygen absorption in electrochemical corrosion Give figures (a) Discuss the various factors which influence corrosion (b) What are the advantages and limitations of anodic protection? (c) Write short notes on the following: (i) Galvanic series, (ii) Waterline corrosion CORROSION 16 (a) 17 18 19 20 21 22 23 24 25 26 27 28 29 413 What are cathodic and anodic protectIOn for controlling corrosions? Discuss their merits and demerits (b) Explain: (i) Boiler corrosion, (ii) Intergranular corrosion (c) How are metals protected against corrosion by modifying the environment? (d) Silver and copper metals not undergo much corrosion like iron in moist atmosphere Explain Explain the mechanism of corrosion and methods of its prevention (a) Write a short note on pitting corrosion and its control (b) Write notes on the following: (i) Galvanic corrosion and its control, (ii) Pitting control (a) Explain: (i) Galvanic corrosion, (ii) Crevice corrosion (b) Write a short note on cathodic inhibitors (c) Write a short note on anodic inhibitors (a) Explain the following factors influencing the corrosion rate : (i) Nature of corrosion product, (ii) The ratio of anodic to cathodic area (b) What is stress corrosion? Give two examples How can it be controlled? (c) Distinguish between anodic and cathodic inhibitors (a) Explain electrochemical theory of corrosion (b) Discuss the effect of : (i) Temperature, and (ii Nature of corrosion product, on the rate of corrosion of metals (a) What is meant by corrosion? Explain the factors which influence corrosion (b) Give an account of inhibitors (a) Write ~ note on cathodic protection by impressed current and sacrificial anode method (b) Give an account of cathodic protection by impressed current and sacrificial anode Give examples (a) What are corrosion inhibitors? Explain with examples how anodic and cathodic inhibitors provide protection against corrosioll (b) Discuss any two factors which influence of corrosion rate (a) Describe the various factors affecting the rate of corrosion (b) Define corrosion Name the different theories of corrosion (a) Define corrosion Explain losses due to corrosion Give the classification of corrosion according to environment or surroundings (b) Distinguish between anodic and cathodic protection What are corrosion inhibitors? Classify different types of inhibitors with examples (a) Write a note on cathodic protection (b) Write brief notes on the following: (ll) Soil corrosion (i) Waterline corrosion (c) Discuss the effect of the following factors on the rate of corrosion: (i) Position of metal in the electrochemical series, (ii) Physical state of metal, (iii) Nature of corrosion products (d) Mention the major corrosion causing substances and how are they eliminated? (e) Write a short note on cathodic protection (a) What you mean by corrosion? How does it differ from erosion? 414 PHYSICAL CHEMISTRY-II (b) (c) :'0 (a) (b) (c) (d) 31 (a) (b) 32 (a) 33 (a) (b) (c) (d) 34 (a) (b) 35 (a) 36 (a) (b) (c) What are the conditions for dry anJ wet corrosion? Name two metals which are noble with respect to corrosion Describe the mechanism of wet corrosion Describe the oxidation corrosion Distinguish between galvanic series and electrochemical series Explain pitting corrosion Explain in detail the atmospheric corrosion Explain differential aeration corrosion by giving suitable examples What happens and why? (i) Iron sheets rivetted with copper rivets (ii) An iron pole is partly buried under earth (ii) A zinc article is under strain (iv) Zinc plate fixed below the ship Give a brief account of cathodic protection method of preventing corrosion (or corrosion control) How are metals protected against corrosion by morlifYing the environment '? Give scientific reasons whether the following statements are true or false: (i) Mechanism of electrochemical corrosion is based on transfer of electrons from anode to cathode (T) Why steel pipe connected to the copper plumbing gets corroded? What are cathodic and anodic protection for controlling corrosion? Discuss their merits and demerits How can corrosion be prevented or controlled? Write short notes on (or explain): (i) Waterline corrosion (ii) Stress corrosion (iii) Anodic and cathodic inhibitors (or corrosion inhibitors) What is meant by differential aeration corrosion? Illustrate with sUltable examples Write short note on factors influencing atmospheric corrosion Explain the different factors affecting the rate of corrosion [II] Short Answer and Very Short Answer Type Questions Define corrosion What is meant by rusting of iron? What is rust? Formation of which types of metal oxide film cause rapid and continuous corrosion Formation of which types of metal oxide film prevents corrosion The rate of metallic corrosion increases with increase in temperature Give reason Iron corrodes faster than aluminium, even though iron is placed below aluminium in the electrical series Why? Impure metal corrodes faster than pure metal under identical conditions Give reason Wire mesh corrodes faster at the joints Why? 10 Corrosion of water filled steel tanks occurs below the waterline Give reason 11 What is meant by the term passivity'? 12 Name four metals, the specific volume oftheir oxides is greater than that of the metals 13 Iron corrodes under drops of salt solution Give reason 14 A steel screw in a brass marine hardware corrodes Give reason CORROSION 415 15 Comment on the use of aluminium in place of zinc for cathodic protection of iron from rusting 16 Rusting of iron is quicker in saline water than in ordinary water Why ? 17 What is cathodic protection? 18 Name the volatile oxidation corrosion product of a metal 19 Where the electrochemical corrosion takes place? 20 What is the effect of grain-size of the metal on the corrosion? 21 What is the effect of carbon dioxide on electrochemical corrosion? 22 What are the factors which affect corrosion? 23 CO2 is always present in natural water Explain its effect (increases, stops or no effect) on rusting of iron 24 A piece of impure zinc and pure zinc are placed in a salt solution Which will corrode faster? 25 Why does c0rrosion occur in steel pipe connected to copper vlumbing? 26 Bolt and nut made of the same metal are preferred in practice Why? 27 State the two conditions for wet corrosion to take place 28 Which of the following metals could provide cathodic protection to iron: AI, Zn, Cu, Ni? 29 Why does a steel pipe in a large copper tank corrode causing rapid destruction? 30 How does addition of amines protect against corrosion or iron? 31 How must rust (Fe203.3H20) will be formed, when 100 kg of iron have completely rusted away? 32 Discuss in brief the liquid metal corrosion 33 How does the physical state of the metal affect corrosion rate? 34 Mention the nature of corroding environment 35 What are cathouic and anodic protection for controlling corrosion? Discuss the merits and demerits 36 Distinguish between anodic and cathodic inhibitors [III] Multiple Choice Questions Metal at the top of electromotive series is : (d) Most active (a) Most stable (b) Least active (c) Most noble Electrochemical corrosion can occur only if : (a) Oxygen is present in contact with metal (b) Air is present in contact with metal (c) Liquid medium is in contact with metal (d) None of the above are present During wet corrosion: (a) The anodic part undergoes oxidation (b) The cathodic part undergoes oxidation (c) The anodic part undergoes reduction (d) Neither anodic nor cathodic parts undergo any changes Corrosion in essence is a process of : (d) Extraction of metals (c) Electrolysis (a) Reduction (b) Oxidation \Vnen a buried pipeline is protected from corrosion by connecting to Mg block, it is called: (b) Sacrificial cathodic protection (a) Impressed voltage protection (d) Any of these (c) Sacrificial anodic protection 416 PHYSICAL CHEMISTRY-II For corrosion of iron to take place : (a) Presence of moisture is sufficient (b) Presence of both moisture and oxygen are essential (c) Hydrogen is required (d) A strong acid is necessary The rusting of iron is catalysed by which one of the following: (a) Fe (b) 02 (c) Zn (d) H+ S Rusting of iron is : (b) Prevented on coating with zinc (a) Enhanced by wet air (c) Retarded in the presence of dissolved salts (d) Prevented, if the article is connected with a wire of Mg Which of the above statements is false? Corrosion is as example of: (a) Oxidation (b) Reduction (c) Electrolysis (d) Erosion 10 In waterline corrosion, the maximum amount of corrosion takes place: (a) Along a line just above the level of the water meniscus (b) Along a line at the level of the water meniscus (c) Along a line just below the level of the water meniscus (d) At the bottom of the vessel 11 Addition of hydrazine hydrate to corrosive environment: (a) Retards anodic reaction (b) Prevents diffusion of protons to cathode (c) Retards cathodic reaction by consuming dissolved oxygen (d) Increases hydrogen overvoltage [IV] Fill in the Blanks In galvanic corrosion, the metal having relatively Eo value will undergo corrosion In acidic environment, lower the value of hydrogen overvoltage is the rate of corrosion The chemical composition of the corrosion product of iron (rust) is When the ratio of anodic to cathodic area decreases, the rate of corrosion During electrochemical corrosion, the corrosion occurs at the part During differential oxygen concentration corrosion, the corrosion occurs at oxygenated part Rusting of iron is faster in air than in dry air S When zinc-copper alloy is placed in moist atmosphere, then undergoes corrosion An example for anodic corrosion inhibitor is [V] True or False State whether the following statements are true (T) or false (F) ? S The metal at the bottom of electromative series is least reactive The rusting of iron is catalysed by hydrogen ions Rusting of iron does not take place in moist air The smaller the size of the metal, greater will be its corrosion Corrosion decreases with increasing temperature Nickel is e passive metal Electroplating of a metal protects it from corrosion An impure metal will corrode faster than a pure metal 417 CORROSION ANSWERS [II] Short Answer and Very Short Answer Type Questions The atmospheric corrosion of iron and steel, leading to the formation of a layer of reddish scale and powder of oxide (Fe30 4) on the surface Hydrated ferric oxide (Fe203) with Fe(OH)a-'Brown rust; mixed ferrous and ferric oxides (Fe304 = FeO.Fe203)-Black rust.' Volatile oxide film and porous oxide film Fine-grained tightly adhering, impervious oxide film; and highly unstable oxide film With increase of temperature of the environment, the rate of reaction as well as rate of diffusion increases, thereby corrosion rate increases This is because aluminium forms a non-porous, very thin, tightly adhering protective oxide film (~03) on its surface and this film does not permit corrosion to occur S Impurities in a metal generally cause heterogeneity and form minute (or tiny) electrochemical cells at the exposed parts, whereby the anodic parts get easily corroded The joints of wire mesh are stressed (due to welding), so these becomes anodic w.r.t unjoined wires At these anodic parts, oxidation takes place and the metal is corroded fast; while the cathodic parts remain unaffected 10 This is because the area above the waterline is highly-oxygenated and acts as cathodic; while the fully immersed part is poorly-oxygenated and acts as anodic So the anodic part (below the waterline) gets corroded, due to electrocheIPical corrosion; while the cathodic part (above the waterline) remains completely unaffectd by corrosion 12 Chromium, nickel, aluminium and titanium 13 This is due to differential aeration Areas of iron covered by drops, having p:lOr access to oxygen, become anodic with respect to other a.eas which are freely exposed to air Due to electrochemical corrosion, the areas under drops (anodic) undergo corrosion; YO' hile the freely exposed parts remain unaffected 14 This is due to galvanic corrosion Iron (higher in series than brass) becomes anodic and is attacked and corroded; while brass (lower in series) acts as cathodic and is not attacked at all 17 It is protection of the parent metal from corrosion by connecting it with a more active metal (more anodic metal) like Mg, AI, etc The connected active metal undergoes corrosion, thereby protecting the parent metal from corrosion 23 In presence of dissolved CO 2, the acidity of water, adjacent to thb iron object increases and its electrical conductivity also increases This, consequently, results in an increased corrosion current flowing in the local or miniature electrochemical cells on the exposed metal surface Hence, rate of rusting of iron increases, if CO is present in water 24 Impure zinc, because impurities lead to the formation of local galvanic cells 25 When iron or steel pipe connected to coppe,r plumbing is exposed to an electrolyte, then iron (the metal higher in the electrochemical series) undergoes corrosion, since iron forms the anode and is attacked and corroded; whereas copper (the metalloyver in the electrochemical series) acting as cathode is protected This type of corrosion is called galvanic corrosion 26 Bolt and nut made of the same metal is preferred, because such a combination will not permit galvanic corrosion to take place 2S AI and Zn 29 Iron or steel anodic w.r.t to copper When a steel pipe fitted in a large copper tank is exposed to atmosphere, galvanic corrosion starts and the anodic metal (iron or steel) is 418 PHYSICAL CHEMISTRY-II starts corroding Moreover, small-sized steel pipe (anode) in a large-sized copper tank (cathode) causes rapid and intense corrosion (or destruction) of steel pipe 30 The added amines keeps the rW] in the solutIon low, thereby shifting the corrosion reaction: 02(S) + 4W (aq) + 4e- ~ 2H20(l) to be left 31 214 g of rust (mol wt.) contains = 112 g of iron : 112 g of iron produces rust = 214 g 214 x 100 kg or 100 kg of Iron produces rust = 112 = 191.1 kg [III] Multiple Choice Questions (d), (c), (a), (b), (c), (b), (d), (d), (a), 10 (a), 11 (c) [IV] Fill in the Blanks lower higher Fe203.xH20 zinc chromate/phosphate increases anodic less moist [V] True or False (T), (T), (F), (T), (F), (T), (T), (T) 000 419 SUBJECT INDEX SUBJECT INDEX Anomaly of strong electrolytes 283 Arrhenius theory 260 - failures 278 Bismuth-cadmium system 186 Bond energy 59 Buffer solutions 371 Carbon dioxide system 176 Carnot's cycle 75 - effi~iency 78 Cell constant 252 Chemical e'-luilibrium 135 Clapeyron equation 154 Clausius-Clapeyron equation 158 Clausius inequality 94 Component 167 Concentration cells 334 - applications 337 Conductivities 254 - applications 279 Conductometric titrations 282 Congruent melting point 187 Conjugate solutions 220 Copper sulphate-water system 197 Corrosion 394 - factors affecting 400 - inhibitors 402 - protection 408 - theories 394 - types 403 Criteria of spontaneity 102 Critical solution temnerature 220 - determination " 223 - influence of impurities 223 - influence of pressure 224 Debye-Huckel-Onsager equation 285 Degree offreedom 168 Degree of hydrolysis 380 Dissociation energy 59 Distribution law 230 - applications 234 - thermodynamic derivation 231 Electrochemical cells 291 Electrochemical series 308 - applications 311 Electrode potential 301 Electrolysis 248 -laws 249 Electromotive force measurement 297 Electrophoretic effect 285 Enthalpy 15 - combustion 47 - compound 46 - formation 45 - neutralisation 50 - reaction 39 - solution (dilution) 52 Entropy 85 - changes 89, 94 - change of isothermal mixing 100 - variation with pressure 99 - variation with temperature 96 - variation with volume 98 Exact differentials Extensive variables Extraction process 236 Faraday 249 Ferric chloride-water system 191 First law of thermodynamics 13 Flame temperature 58 Fractional distillation 210 Free energy 123 Functional equation 82 Glass electrode 346 Heat capacity 15 Henderson-Hassel equation 373 Henry's law 202 Hess law 56 Ideal solutions 204 - activity of component 205 - vapour pressures 205 Incongruent melting point 194 Intensive variables Internal energy 11 Intrinsic energy 54 Inversion temperature 30 Ionic product of water 375 Irreversible cells 297 Irreversible reactions 135, 294 Joule Thomson - coefficient 27 - effect 26 Kelvin scale of temperature 51 Kirchoff's equation 42 Kohlrausch's law 273 420 PHYSICAL CHEMISTRY-II Law of mass action 136 - thennodynamic derivation 139 Laws of thennochemistry 55, 138 Lead-silver system Le Chatelier's prir~iple 141 - thennodynamic derivation 145 Limitations, thermodynamics Liquid junction potential 329 Maximum work Metastable equilibrium Migration of ions 263 175, 180 N ernst equation 302 Nernst heat theorem 105 Non-idcal solutions 208 Non-spontaneous process 74 Objective, thennodynamics Ostwald's dilution law 277 Overvoltage 323 Oxidation-reduction potential 322 Passivity 403 pH 365 Phase 165 Phase equilibrium 169 Phase rule 169 - thennodynamic derivation 170 Polarisation 317 Potassium iodide-water system 184 Potentiometric titrations 353 Reference electrodes 306 Relaxation effect 284 Residual entropy 112 Resonance energy 61 Retroflex solubility 190 Reversible cells 296 Reversible process 5, 74 Reversible reactions 135 Salt hydrolysis 376 Second law of thennodynamics 80 Sodium chloride-water system 196 Specific conductance 250 Spontaneous process 74 State functions State variables Sulphur system 177 Surroundings System Tie line 220 Thennal analysis 199 Thennodynamics of solutions 206 Third law of thermodynamics 112 Transport number 266 Triple point 174 Trouton's law 161 Van't Hoff isochore 151 van't Hoffisothenn 148 Walden rule 256 Water sys~'3m 172 Work function 123 Zeroth law of thennodynamics Zinc-magnesium system 188 12 o (i) COMMON LOGARITHMS log10 X 10 0000 0043 0086 0128 0170 11 0414 0453 0492 0531 0569 12 0792 0828 0864 0899 0934 13 1139 1173 1206 1239 1271 1523 1553 1818 1847 2095 2122 2355 2380 2601 2625 2833 2856 3054 3075 3263 3284 3464 3483 3655 3674 3838 3856 4014 4031 4183 4200 4346 4362 4502 4518 4654 4669 4800 4814 4942 4955 5079 5092 5211 5224 5340 5353 5465 5478 5587 5599 5705 5-717 5821 51h2 5933 5944 6042 6053 6149 6160 6253 6203 6355 0365 6454 6464 6551 6561 6646 6656 6739 6749 6830 6839 6920 6928 1584 1875 2148 2405 2648 2878 3096 3304 3502 3692 3874 4048 4216 4378 4533 4683 4829 4969 5105 5237 5366 5490 5611 5729 5843 5955 6064 6170 6274 6375 6474 6571 6665 6758 6848 6937 0212 0212 0607 0607 0969 0969 1303 1303 1614 1903 2175 2430 2672 2900 3118 3324 3522 3711 3892 4065 4232 4393 4548 4698 4843 4983 5119 5250 5378 5502 5623 5740 5855 5966 6075 6180 6284 6385 6484 6580 6675 6767 6857 6946 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 x 1461 1761 2041 2304 2553 2788 3010 3222 3424 3(,17 3802 3979 4150 4314 4472 4624 4771 4914 5051 5185 5315 5441 5563 5682 5798 5911 6021 6128 6232 6335 6435 6532 6628 6721 6812 6902 1492 1790 2008 2330 2577 2810 3032 3243 3444 3636 3820 3997 4166 4330 4487 4639 4786 4928 5065 5198 5328 5453 5573 5694 5809 5922 6031 6138 6243 6345 6444 6542 6637 6730 6821 6911 No n = 3.14159 e = 2.71828 P logeP loge -p log 0.49715 0.43429 0.4343 1.5657 In x log 0.8686 1.1314 0::53 0294 0334 0374 0645 0682 0719 0755 1004 1038 IOn 1106 1335 1644 1931 2201 2455 2695 2923 3139 3345 3541 3729 3909 4082 4249 4409 4564 4713 4857 4997 5132 5253 5391 5514 5635 5752 5866 5977 6085 6191 6294 6395 6493 6590 6684 6776 6866 6955 1367 1673 1959 2227 2480 2718 2945 3160 1399 1703 1430 1732 2014 2279 2529 2765 2989 3201 3404 3598 3784 3962 4133 4298 4456 4609 4757 4900 5038 5172 5302 5428 5551 5670 5786 5899 6010 6117 6222 6325 6425 6522 6618 6712 6803 6893 6981 3365 3560 3747 3927 4099 4265 4425 4579 4728 4871 5011 5145 5276 5403 5527 5647 5763 5877 5988 6096 6201 6304 6405 6503 6599 6693 6785 6875 6964 = loge x = (11M) log10 X = loglO X = M loge X 1.3029 2.6971 1.7372 2.2628 1'i87 2253 2504 2742 2967 3181 3385 3579 3766 3945 4116 4281 4440 4594 4742 4886 5024 5159 5289 5416 5539 5668 5775 5888 5999 6107 6212 6314 6415 6513 6609 6702 6794 6884 6972 (11M) = 2.30259 ll.m 1\23456i7119 + AnD 42 40 39 37 35 34 33 32 30 28 26 25 24 22 21 20 19 18 18 17 16 16 15 15 14 14 813 1721 25 2') 34 38 812 16 2(J 24 28323(, 812 16 1923 273135 4711 15 1922 263033 4711 14 1821 252832 3710 141720 242731 710 13 10 20 232(,30 10 13 16 19 222629 (, ') 12 15 18 21 2427 308 11 14 J7 2022 25 358 10 13 16 [821 23 257 10 12 15 17 2() 22 25 10 12 14 171922 24 91113 151820 24 11 13 15 17 19 10 12 141618 24 81011 13 15 17 245 18 245 8110 18 245 8299 19 24 (i 8492 19 246 8690 20 246 8892 20 246 90')') 21 Ii 9311 21 24(, 9528 22 247 9750 22 247 9977 23 257 5 5 5 5 5 Ii (, Ii (i 7 7 7 7 8 (, (, Ii (, Ii 11 Ii (i Ii (i Ii 7 8 ( (i 8 9 9 9 10 10 10 10 lOll 1011 t> 10 11 10 11 III 12 10 12 Ii 10 12 10 II 13 10 11 13 10 11 13 1012 13 11 12 14 11 12 14 10 II 13 14 10 11 13 14 [() II 13 14 10 1214 15 10 12 14 15 11 13 14 16 11 13 14 16 11 131416 10 II 13 IS 17 81011 13 15 17 10 12 14 Hi 18 10 12 141(, 18 8J1l13 15 17 19 11 13 15 17 19 91113 15 18 20 11 13 15 18211 11 14 1(, 1821 ... or enthalpy of neutralisation Heat of transition or enthalpy of transition 50 50 Heat of solution and heat of dilution or enthalpy of solution and dilution Intrinsic energy Laws of thermochemistry... can neither enter into nor leE.ve the system (ii) Isolated system: In this system, there is no exchange of matter or energy b~tween the system and the surroundings 2 PHYSICAL CHEMISTRY- II (iii)... (iii) Open system: In an open system, both matter and energy can enter into or leave the system and thus there can be an exchange of matter and energy between the system and the surroundings [V]