Unit II ENGINEERING CHEMISTRY REDOX REACTIONS AND ELECTROCHEMISTRY Unit II UNIT – II Electrochemistry - Single electrode potential, Nernst equation, Electrochemical series and its applications. Electrolysis of water. Electrodeposition, Electroless deposition, Plating on plastics. Energy devices -Primary battery (Alkaline battery), Secondary battery (Lead-Acid, Ni-Cd and Li-ion). SupercapacitorPrinciple. Fuel cells – Principle and advantages, Hydrogen-Oxygen Fuel cells, Proton exchange membrane fuel cell, Alkaline fuel cell, Solid oxide fuel cell. Corrosion and its control - Chemical and electrochemical corrosion, Microbial induced concrete corrosion and biofouling. Corrosion control - Design, Anodic and cathodic protection. Surface Coatings - Inorganic coatings (Galvanization, Tinning, Electrode position, Anodization) and Organic coatings - oil paints. Inhibitors (cathodic and anodic). Unit II Electrochemistry Galvanic cell Electrolytic cell Unit II Galvanic cell Unit II Ecell = ER – EL Ecell = (+0.34) - (-0.76) Ecell = 1.1 V Ecell = X – Y 10 = X – Y 10 = 0 – Y Unit II Nernst equation •R is the gas constant = 8.314 J/K Mole F = 96487 (≈96500) coulomb/mole T = 298 K Unit II E = 1.05 V Unit II Key advantages of Nernst equation • To identify the electrode potential at real world conditions • To identify the unknown ionic concentrations • The pH of solutions if the electrolyte is acidic in nature. Limitations • Activity of an ion in a very dilute solution is equal to concentration. • For solutions having very high concentrations, the ion concentration is not equal to the ion activity. Unit II Electrochemical series and its applications Arrange the ions or molecules based on their ability to accept electron Ability to accept electron = Ability to undergo reduction Elements high +ve SRP easily accept electron under go reduction (Easily under go reduction reaction) Elements with low -ve SRP easily donate electron to under go oxidation (Easily under go oxidation reaction) Unit II OIL RIG Oxidation Is Loss (of electron) Reduction Is Gain (of electron) Unit II Standard reduction potential values (Zinc easily undergoes oxidation) (Copper easily undergoes reduction) Unit II https://old.iupac.org/ Unit II Application of electrochemical series 1. Electrochemical series helps us to identify a good oxidizing agent or reducing agent. 2. To Calculate the Standard EMF of Electrochemical Cell 3. Useful to predict the feasibility of redox reaction 4. Displacement reactions ( A metal higher in series will displace metal from its solution which its lower in series) 5. Predicting Liberation of H2 gas from acids by metals Electrolysis of water Unit II Electrolysis split H2O into hydrogen and oxygen. This reaction takes place in electrolyzer. https://simple.wikipedia.org/ www.sciencephoto.com Unit II chem.libretexts.org Unit II Electrodeposition Deposition of metal ions on the substrate with the help of electricity The base metal should be taken as cathode The coat element should be present as electrolyte To Gold plate an iron spoon Iron spoon should be taken as cathode Gold chloride solution to be taken as electrolyte solution Unit II https://www.wikiwand.com/en/Electroplating Unit II Advantages of electroplating •Corrosion resistance •Decorative purpose •Cheaper ornaments •Improving mechanical characteristics Disadvantages •Non-uniform plating •Cost •Pollution Unit II Electroless deposition Deposition performed in the absence of electricity Example: Chromate coating Chromate coatings carried out in situ by electroless deposition method in an acidified potassium dichromate bath under optimum pH They get deposited as mixed chromium oxides (Cr2O3. CrO3. xH2O) They impart good corrosion resistant to Zn, Al, Mg and Cd. Unit II Plating on plastics It was accomplished by roughening the outer part of the plastic and then electroplating the combined plastic/metal product. Common types of metals used for plating on plastics are copper, chromium, gold, silver and nickel, although other metals can be used. Unit II Key advantages 1. Process Aesthetic advantage 2. Corrosion resistance 3. Strength 4. Chemical resistance Vapor deposition coatings The plastic component is placed in a vacuum chamber and the metal is vaporized by an electric charge, and it settles on the surface of the plastic, creating a metal coating. Unit II Plating on plastic via electroplating •Cleaning – The surface of the substrate cleaned to remove fingerprints, dirt and other debris. •Pre-dipping – Pre-dipping the plastic parts in a solvent prior to etching can improve the surface •Etching – Etchants typically consist of chromium trioxide or sulfuric acid solutions that increase the surface of the substrate, making it easier for the part to absorb liquids. •Conditioning – This can promote a more uniform absorption during the activation stage www.sharrettsplating.com/ Unit II •Neutralizing – After etching, the part should be thoroughly rinsed to remove any excess acid •Preactivating – A preactivator is a product that is designed to facilitate absorption during the subsequent activation step. •Activating – The next step involves the introduction of a low-concentration precious metal liquid activator (include palladium, platinum and gold) •Bath immersion – After rinsing the plastic parts, the next step is to place them in the electroless bath to deposit a thin metal coating. www.sharrettsplating.com/ Unit II Energy devices Primary battery (Alkaline battery) The redox reactions operating in primary batteries are irreversible Primary batteries such as Leclanchébattery, silver oxide battery, alkaline battery, and etc., are mostly used for low drain applications. Chemistry for Engineers book Unit II ✔ An alkaline battery is similar to dry cell but with the electrolyte been replaced with potassium hydroxide which improves its shelf life. ✔ Alkaline battery delivers higher current than drycell and can withstand heavy drains, hence it is called heavy duty battery. ▪ The critical drawback of the alkaline battery is cell expansion. ▪ This occurs due to the release of hydrogen gas Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Secondary battery ✔ These are batteries in which the redox reactions can be reversed by passing current through them in the opposite direction to that of the discharge current. ✔ Secondary batteries including nickel-cadmium battery, lithium-ion battery, lead-acid battery, nickel-metal hydride battery, etc., can withstand high energy drain. Chemistry for Engineers book Unit II Lead-acid battery Unit II Lead-acid battery The battery is made up of spongy lead anode and lead oxide cathode. 28-30% of sulfuric acid is used as an electrolyte. It consists of six 2V galvanic cells connected in series. The electrolyte is consumed during discharging and it is regenerated during charging. Specific gravity of the electrolyte can be used to predict the state of charge of the battery. Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Chemistry of lead-acid battery (during charging): Chemistry for Engineers book Unit II Nickel-cadmium battery Chemistry for Engineers book Unit II Nickel-cadmium battery is made up of nickel oxyhydroxide as the cathode and spongy cadmium as an anode The two electrodes are separated using nonwoven fabric immersed in potassium hydroxide electrolyte. At anode: Chemistry for Engineers book Unit II Contrary to lead-acid battery, the electrolyte in nickel-cadmium battery is not regenerated during charging, and thus the specific gravity of the electrolyte cannot be used to predict the state of charge of the battery. Chemistry for Engineers book Unit II Lithium-ion battery Chemistry for Engineers book Unit II Among various batteries technologies, lithium-ion batteries are considered the most promising due to their high energy density, little or no memory effect, good rate capability and etc. In commercial lithium-ion battery, graphite is used as the anode, and layered compound such as layered oxide (LiCoO2) or spinel oxide (LiM2O4) or polyanions (LiFePO4) is used as the cathode. The electrolyte is made up of a complex compound of lithium such as LiPF 6, LiAsF6.H2O, LiClO4, etc., dissolved in an organic solvent. Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Owing to the minimal memory effect, low toxicity, low self-discharge, longer cycling stability, excellent efficiency, high theoretical cell potential, and high theoretical energy density, lithium-ion batteries have dominated the portable electronic industries. However, there are several limitations associated with lithium-ion battery technology including low earth reserve, instability when exposed to the aqueous environment, high cost, safety, etc. Chemistry for Engineers book Unit II Supercapacitors (i) Electric double layer capacitor (EDLC) and (ii) Pseudocapacitor In EDLCs, electrical energy is stored by reversible adsorption and desorption of electrolyte ions onto/from the surface of the electrode material. Pseudocapacitors store electrical energy by reversible Faradaic reactions in addition to adsorption and desorption of electrolyte ions onto/from the surface of the electroactive material. Chemistry for Engineers book Unit II Supercapacitors are used in electric vehicles, hybrid electric vehicles, digital cameras, digital communication devices, pacemakers, uninterruptible power supplies, laser techniques, tramways, starting of diesel engines, and etc. https://www.explainthatstuff.com/ Chemistry for Engineers book Unit II Ordinary capacitors store static electricity by building up opposite charges on two metal plates (blue and red) separated by an insulating material called a dielectric (grey). https://www.explainthatstuff.com/ Unit II Supercapacitors store more energy than ordinary capacitors by creating a very thin, "double layer" of charge between two plates, which are made from porous, typically carbon-based materials soaked in an electrolyte. The plates effectively have a bigger surface area and less separation, which gives a supercapacitor its ability to store much more charge. https://www.explainthatstuff.com/ Unit II Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Fuel cells – Principle and advantages A fuel cell is an energy conversion device that converts the chemical energy directly into electrical energy by the electrochemical oxidation of fuel (hydrogen, methanol, etc.) by an oxidant (oxygen). Working principle The fuel fed at the anode undergoes oxidation into cations and electrons. The cations flow from anode to cathode through the electrolyte Oxygen supplied at the cathode undergoes reduction, combines with positive ions and form water. Chemistry for Engineers book Unit II Key advantages ✔ Since the fuel supplied at the anode does not undergo combustion, the process is clean, efficient and noise free. ✔ Unlike batteries, fuel cells do not run down as they produce electrical energy as long as fuel and oxidant are supplied at anode and cathode, respectively. ✔ Based on the choice of electrolyte and fuel, various types of fuel cells are there Proton exchange membrane fuel cell Alkaline fuel cell Phosphoric acid fuel cell Direct methanol fuel cell Molten carbonate fuel cell Chemistry for Engineers book Solid oxide fuel cell Unit II Hydrogen-Oxygen Fuel cells Unit II Proton exchange membrane fuel cell Chemistry for Engineers book Unit II Proton exchange membrane fuel cell PEM is suitable for electric and hybrid vehicles owing to its capability to deliver high energy and power, and low operational temperature. Nafion, a perfluorinated cation exchange membrane (electrolyte) is commonly used in PEM fuel cells. Pt/C based catalyst is uses in the anode to aid the oxidation of hydrogen in proton exchange membrane fuel cell. The stream of air (oxygen) at the cathode reacts with the protons to form water molecules Chemistry for Engineers book Unit II PEM fuel cell operates at around 80 oC with practical efficiency of 60%. Also, it has power output in the range of 5-200 kW. (At anode) (At cathode) (overall reaction) Chemistry for Engineers book Unit II Alkaline fuel cell • Alkaline fuel cell uses sodium hydroxide or potassium hydroxide as an electrolyte • Stream of hydrogen is supplied as a fuel at the anode and pure oxygen (free from carbon dioxide) is fed at the cathode. • Operation temperature (60-80 oC) and hence it requires the use of electrocatalyst (metals, such as Ni, Pt, Pd) Chemistry for Engineers book Working principle Unit II Hydrogen supplied at the anode is oxidized into protons and electrons Protons flow from anode to cathode through the electrolyte Electrons are drawn from anode to cathode through an external circuit, resulting in the generation of electricity At the cathode, oxygen is reduced into hydroxyl ions by the electrons and hydroxyl ions combine with protons and form water. The efficiency of alkaline fuel cell is 40-45% and power output is in the range of 10-100 kW. Alkaline fuel cells are widely used in defense and space-related applications. Chemistry for Engineers book Unit II (At anode) (At cathode) (Overall reaction) Chemistry for Engineers book Unit II Solid oxide fuel cell Chemistry for Engineers book Unit II Solid oxide fuel cell ❖ Solid oxide fuel cell offers several advantages, such ashigh efficiency, low level of pollution, high fuel adaptability, etc. ❖ Solid materials used as an electrolyte, for example, yttria-stabilized zirconia (YSZ). ❖ This fuel cell requires high temperature for operation. Working principle ✔ Oxygen at the cathode reacts with the electrons released from the anode to form oxide which then migrate to through the oxide-ion conducting electrolyte to the anode side. Chemistry for Engineers book Unit II ✔ The migrated oxide ions subsequently react with the fuel (hydrogen) to form water. (At anode) (At cathode) (Overall reaction) Chemistry for Engineers book Unit II Applications of solid oxide fuel cell i. Alkaline fuel cell is used as source of electricity and water in the U.S. space programs ii. Phosphoric acid fuel cell is used for power generation in the remote areas Chemistry for Engineers book Unit II Corrosion and its control Chemistry for Engineers book Unit II Types of corrosion Dry corrosion or chemical corrosion Electrochemical corrosion Dry corrosion or chemical corrosion ❖ Direct reactions take place between metal and the corroding agents. ❖ The corroding agents are usually oxygen or acidic gases such as HCl, SO2, Cl2, H2S, and nitrogen oxides Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Factors influencing chemical corrosion i) Reactivity of metal: Lithium reacts vigorously than gold or platinum ii) Temperature: Dry corrosion increases with temperature iii) Nature of corrosion product: If the corrosion product is volatile, then more corrosion takes place Ex: Reaction of silicon under acidic conditions Chemistry for Engineers book Unit II If the corrosion product is highly porous, localized corrosion takes place Ex: Iron, where the rust (Fe3O4.2H2O) is highly porous in nature. If the corrosion product is a solid, non-porous and adherent layer the product acts as a protective layer over the metal surface. Ex: Al, Ni, Cr etc Electrochemical corrosion Most common types of corrosion reactions take place where the metals are in aqueous conditions. Chemistry for Engineers book Unit II Electrochemical corrosion in acidic media Electrochemical corrosion in ii) Alkaline and neutral media: Chemistry for Engineers book Unit II Factors influencing electrochemical corrosion Nature of anode: Metals with high negative reduction potential values corrode easily Ex: Mg, Fe etc. Chemistry for Engineers book Unit II Nature of cathode: In acidic medium, the cathodic reaction would be hydrogen reduction or hydrogen evolution reactions. The rate of hydrogen evolution or oxygen reduction depends on the nature and area of cathode Nature of corrosion product: When the corrosion product formed is an intact layer on the metal surface ( as in the case of Al) the electrolyte may not be able to diffuse through the cell. Chemistry for Engineers book Unit II Nature of the electrolytic medium: (i) If the electrolyte is highly conductive in nature, it increases the corrosion process. (ii) The pH of electrolyte also influences the type and rate of corrosion reactions. If pH is below 5, the cathodic reaction would be hydrogen evolution. At neutral pH values differential aeration corrosion takes place. Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II iii) Presence of certain ions: the rate increases in presence of ammonium ions for iron, while rate decreases with silicate ions. iv) Temperature v) Presence of corrosion inhibitors Chemistry for Engineers book Unit II Microbial induced concrete corrosion ✔ Chemical ingredients present in cement such as calcium carbonate, silicate and aluminate reacts with water and the products released by microorganisms. ✔ Steel present inside the concrete reacts with corroding agents and reduces the strength of the structure. ✔ Products produced due to biocorrosion occupy more volume compared to the bare metal that leads to mechanical stress in metal, enhancing the corrosion reaction. Chemistry for Engineers book Unit II Biofouling ✔ The first step in biocorrosion is the formation of bacterial colony on the surface of metal (may lead to biofilm formation). ✔ In biofilm formation, bacterial colonies are covered by an extra cellular membrane (ECM) made up of polysaccharides, lipids, nucleic acids and/or proteins. Chemistry for Engineers book Unit II ✔ Locations at which microbial adsorption take place, leading to lower exposure to oxygen acts as anode ✔ The micribial species may release organic or inorganic acids that can change the local environment of the metal. ✔ Bacteria such as Shewanella sp.can directly involve in electron transfer reactions to metal surface. ✔ Bacteria such as Desulfuromonas sp. convert sulphur and its oxides to sulphuric acid. Chemistry for Engineers book Unit II Corrosion control - Design Influence of engineering design on the control of corrosion Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Chemistry for Engineers book Unit II Anodic protection or Sacrificial anode method Chemistry for Engineers book Unit II Cathodic protection method: Chemistry for Engineers book Unit II Surface Coatings Inorganic coatings (Galvanization, Tinning, Electrodeposition, Anodization) Organic coatings - oil paints. Inhibitors (cathodic and anodic). Galvanization Galvanization is the process of zinc coating over ferrous materials such as steel or iron. Carried out through hot-dip method Chemistry for Engineers book Unit II Steps 1. Caustic cleaning (followed by rinsing) 2. Pickling (followed by rinsing) 3. Dipping in flux solution 4. Galvanization (by hot-dipping) Unit II Advantages: ❖ Even majority of zinc coating is removed also, substrate is protected ❖ Less maintenance/Lowest long term cost ❖Toughest coating prevents mechanical damage ❖ Relatively easy process for application Unit II Tinning A thin coating of tin is placed over the metal surface. This method is common for ferrous materials. Electrodeposition It is a well-known method to produce in situ metallic coatings by the action of an electric current on a conductive material immersed in a solution containing a salt of the metal to be deposited. Chemistry for Engineers book Unit II Electrodeposition Deposition of metal ions on the substrate with the help of electricity The base metal should be taken as cathode The coat element should be present as electrolyte To Gold plate an iron spoon Iron spoon should be taken as cathode Gold chloride solution to be taken as electrolyte solution Unit II https://www.wikiwand.com/en/Electroplating Unit II Advantages of electroplating •Corrosion resistance •Decorative purpose •Cheaper ornaments •Improving mechanical characteristics Disadvantages •Non-uniform plating •Cost •Pollution Unit II Anodization The base metal itself is made into an oxide film at the surface either by chemical or electrochemical methods. In chemical oxide formation, the metal is oxidised in an alkaline solution The metal is oxidized in an alkaline solution (typically NaOH) at an elevated temperature (~ 90 0C) in the presence of an oxidising agent such as sodium nitrite or sodium chlorate. Chemistry for Engineers book Unit II Organic coatings - oil paints Unit II ✔ Paint normally consists of one or more pigments, vehicle or drying oil, a thinner, driers, fillers. ✔ Pigments may be of natural or mineral pigment such as clay, mica, etc or it may be synthetic pigments such as TiO2, ZnO, BaSO4, etc. ✔ The vehicle or drying oil is the film forming constituent of the paint that provides toughness, adhesion, elasticity, and hyrophobicity. ✔ Thinners are used to reduce the viscosity of the paint, which may be turpentine, petroleum spirit etc. ✔ A drier is needed in paint to enhance the drying of oil film by carrying oxygen, that lead to oxidation, polymerisation or condensation reactions. Chemistry for Engineers book Unit II Inhibitors (cathodic and anodic) Organic compounds containing phosphorus, Arsenic, Antimony, Chromium, Bismuth or Lead are the commonly used corrosion inhibitors (Antimony, arsenic and phosphorus are called n-type impurities). Anodic inhibitors are chemical substances that form a protective layer of oxide film on the surface of metal, causing resistance to corrosion. These inhibitors force the metallic surface into the passivation region. Examples of anodic inhibitors include: •Chromate •Molybdate •Nitrite •Orthophosphate Chemistry for Engineers book Unit II Cathodic inhibitors slow the reaction at the cathode or precipitate cathodic areas in order to increase the impedance on the surface, thus limiting diffusion of reducible species. Cathodic poisons include substances like antimony, arsenic, sulfur, tellurium, selenium and cyanide ions, which hinder the hydrogen atoms from forming hydrogen gas. Oxygen scavengers: These are chemicals that react with the dissolved oxygen for corrosion reduction. Sulfite and bi-sulfite ions are the best examples that form sulphates when reacting with oxygen. Cathodic precipitates: These include zinc, calcium and magnesium. They are precipitated on the surface of the metal to form a protective layer. Chemistry for Engineers book Unit II REDOX REACTIONS AND ELECTROCHEMISTRY Unit II UNIT – II Electrochemistry - Single electrode potential, Nernst equation, Electrochemical series and its applications. Electrolysis of water. Electrodeposition, Electroless deposition, Plating on plastics. Energy devices -Primary battery (Alkaline battery), Secondary battery (Lead-Acid, Ni-Cd and Li-ion). SupercapacitorPrinciple. Fuel cells – Principle and advantages, Hydrogen-Oxygen Fuel cells, Proton exchange membrane fuel cell, Alkaline fuel cell, Solid oxide fuel cell. Corrosion and its control - Chemical and electrochemical corrosion, Microbial induced concrete corrosion and biofouling. Corrosion control - Design, Anodic and cathodic protection. Surface Coatings - Inorganic coatings (Galvanization, Tinning, Electrode position, Anodization) and Organic coatings - oil paints. Inhibitors (cathodic and anodic).
