Question Bank
Unit 02
Q-1Explain electrode potential.
The electrode potential is defined as the Potential difference developed between the metal ions from metal to the solution or from solution to the metal. At equilibrium the potential difference remains constant. The electrode potential of a metal is defined as the direct measure of its tendency to get reduced is called reduction potential, its value is +x volts. Similarly the tendency of an electrode to lose electrons is a measure of its tendency to get oxidized is called oxidation potential, its value is –x volts.
Q-2Obtain the Nernst equation for electrode potential.
Consider the following redox reaction
Mn+ + ne- ↔ M
For such a redox reversible reaction, the free energy change (∆ G) and its equilibrium constant (K) are related as;
∆ G = -RT ln k + RT ln [product]/[Reactant]
∆ G0 + RT ln [product]/[Reactant]………………..(i)
Where
∆ G 0 = standard free energy change.
The above equation is known as Van’t Hoff Isotherm.
The decrease in free energy in the reversible reaction will produce electrical energy i.e.
-∆ G = nEF and ∆ G 0 = -nE0F…………………………(ii)
Where
E = Electrode potential
E0 = Standard electrode potential
F = Faraday (96,500 coulombs)
Comparing equation 1 & 2
-nEF = -nE0F + RT ln [M]/[Mn+]
= -nE0F + Rt ln 1/ [Mn+]
Where, concentration of the metal is unity or
-nEF = -nE0F - RT ln [Mn+]
Dividing the equation by –nF
E= E0 + RT ln [Mn+]/nF
E= E0 + 2.303RT log [Mn+]/nF
E= E0 + 0.0591 log [Mn+]/n ……………(iii)
This equation-3 is known as “Nernst Equation” for electrode potential
Q-3What is Reference electrode?
Reference electrode are the electrode with reference to those, the electrode potential of any electrode can be measured. It can acts both as an anode or cathode depending upon the nature of other electrode.
The Reference Electrodes can be classified in to two types
i) Primary reference electrodes Ex: Standard hydrogen electrode
Ii) Secondary reference electrodes Ex: Calomel and Ag/Agcl electrodes
Q-4Describe construction and working of Standard Calomel Electrode.
Construction and working of Standard Calomel electrode (SCE)
1. Calomel electrode is a metal-metal salt Ion electrode.
2. It consists of mercury, mercurous Chloride and a solution of KCl. Mercury is placed at the bottom of a glass tube.
3. A paste of mercury and mercurous chloride Is placed above the mercury. The space above the paste is filled with a KCl solution of known concentration.
4. A platinum wire is kept immersed into the mercury to obtain electrical contact.
5. Calomel electrode can be represented as:
Hg|Hg2Cl2|KCl
Q-5Explain Glass electrode.
Most often used pH electrodes are called glass electrodes and belong to the family of ISE. They are sensitive only to H+ ions. Typical glass electrode is made of glass tube engaged with small glass bubble sensitive to protons. Inside of the electrode is usually filled with buffered solution of chlorides in which silver wire covered with silver chloride is immersed.
PH of internal solution varies- E.g.; it can be 1.0(0.1M HCl) or 7.0 Active part of the electrode is the glass bubble. While tube has strong and thick walls, bubble is made to be as thin as possible. Surface of the glass is protonated by both internal and external solution till equilibrium is achieved. Both sides of the glass are charged by the adsorbed protons, this charge is responsible for potential difference. This potential in turn is described by the Nernst equation and is directly proportional to the pH difference between solutions on both sides of the glass. The majority of pH electrodes available now a day are combination electrodes that have both glass H+ ion sensitive electrode and reference electrode compartments, conveniently placed in one housing.
Q-6What are Lithium cells?
It consists of lithium anode with solid electrolyte or liquid electrolyte and solid or liquid cathode. A thin protective insulating film is formed on lithium anode protecting the anode against corrosion as it is conductive to lithium ions but not electrons while water and alcohol never form such film.
Lithium iodide solid cathode cell consists of iodine PVP cathode with 3V voltage. It is highly stable and dependable and hence used in medical source for electronic flash guns of cameras.
Lithium Ion Cells
Anode: Graphite, Carbon compound.
Cathode: Oxide of Lithium
Uses:
Used in Laptops, cellular phones, electronic vehicles.
Q-7What is Lead Acid storage battery ?
Lead storage battery is the most common device used to store energy in the portable form. This is also called as lead acid battery. Although the batteries are reliable, which contain acidic material inside that required a proper disposal method after its complete use. These batteries have moderate power density and good time. The battery consists of lead grids on its electrodes. The anodic grid opening is filled with spongy lead while the cathodic grid consists of lead oxide (PbO2).
Charge Chemistry of the battery:
Charge batteries are those batteries which can be recharged after single use. In this type of battery each plate contains negative as well as the positive end. The negative plate is of lead while the positive plate is made up of lead oxide in an electrolyte of approx 4.0M sulphuric acid.
Negative plate reaction:
PbSO4(s) + H+(aq) + 2e– → Pb(s) + HSO4–(aq)
Positive plate reaction:
PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4–(aq) + 3H+(aq) + 2e–
Combining these two reactions, the overall reaction is the reverse of the discharge reaction:
2PbSO4(s) + 2H2O(l) → Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4–(aq)
Discharge Chemistry of the Battery:
The positive and negative plate of the batteries becomes lead sulphate. Due to the loss of sulfuric acid from electrolytes it becomes the water.
Negative plate reaction:
Pb(s) + HSO4–(aq) → PbSO4(s) + H+(aq) + 2e–
Positive plate reaction:
PbO2(s) + HSO4–(aq) + 3H+(aq) + 2e– → PbSO4(s) + 2H2O(l)
Combining these two reactions, one can determine the overall reaction:
Pb(s) + PbO2(s) + 2H+(aq) + 2HSO4–(aq) → 2PbSO4(s) + 2H2O(l)
Q-8Explain the causes of corrosion.
Corrosion is the electrochemical process that occurs in various forms such as chemical forms and atmospheric forms. On the contact of acid sucstance with iron it pretend to form rust. Rust is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and moisture. When steel is exposed to water, the iron particles are lost to the water’s acidic electrolytes. The iron particles then become oxidized, which results in the formation of Fe⁺⁺. When Fe⁺⁺ is formed, two electrons are released and flow through the steel to another area of the steel known as the cathodic area.
Oxygen causes these electrons to rise up and form hydroxyl ions (OH). The hydroxyl ions react with the Fe2+ to form hydrous iron oxide, better known as rust. Where the affected iron particles were, has now become a corrosion pit, and where they are now, is called the corrosion product (rust).
Q-9Explain technical effects of corrosion.
Technology sector is also very badly affected by the corrosion. A great deal of the development of new technology is held back by corrosion problems because material is required to withstand in many cases simultaneously higher temperature, higher pressure. Corrosion problems that are less difficult to solve affect solar energy systems, which require alloys to withstand hot circulating heat transfer fluids for long periods of time, and geothermal systems, which require materials to withstand highly concentrated solutions of corrosive salts at high temperatures and pressures.
Q-10Explain electrochemical corrosion.
Corrosion occurs due to the electrochemical process of oxidation and reduction process. In the corroding solution electrons are released by the metal and that is gained by teh elements in the corroding solution. The release of electron from metal is called as the oxidation while vice-versa that is gain of electron by elements is reduction. The regular electron flow in the corrosion reaction can be measured and controlled electronically. This is why controlled electrochemical experimental methods are used to characterize the corrosion properties of metal.
Q-11Explain microbial corrosion.
Microbial Corrosion is caused by micro-organisms. They commonly referred to as microbiologically influenced corrosion. It applies to both metallic and non-metallic materials with or without oxygen. In the presence of oxygen there are some bacteria that directly oxidize iron to iron oxides and hydroxides while in the absence of oxygen sulphate reducing bacteria are active and produce hydrogen sulphide causes sulphide stress cracking.
Q-12What are the factors responsible for affecting the rate of corrosion?
(i)Nature of metal
Position of metal in galvanic series.
If position is higher in galvanic series then it carrode faster
While for 2 metal the difference between them shows the corrosion ratio.
(ii)Potential Difference
If the difference at the electrode potential between two metal is high then the rate of corrosion would be also high while vice versa for lesser difference.
(iii)Purity of metal
Corrosion never took place in pure metals. While if metal itself has a impurity then galvanic cell set up easily which intend increases the rate of corrosion.
(iv)Relative areas of cathode and anode parts
Rate of corrosion is directly depends on the area of cathode and inversely depends on the area of anode. If the area of cathode is larger then there is more demand of electrons while in the smaller anode area the corrosion took place very fast.
(v)Nature of corrosion Product
Metal oxide film is formed on the surface of metal by corrosion due to oxygen. The formed film would be stable, unstable, volatile.
(vi)Temperature
At high temperature the rate of corrosion increases as because there is a consistent increase in the ionization and mobility difference rate while in some cases rate of corrosion decreases at high temperature as the solubility of O2 gas increases.
(vii)Presence of moisture
The rate of corrosion decreases in dry while increases in presence of moisture. Moisture act as the solvent for setting up of electrochemical corrosion.
(viii)Effect of pH
Rate of corrosion is high at acidic pH due to the evolution of H2 gas at cathode.
(ix)Concentration of electrolytes
This is also called as the Oxygen concentration cell. The rate of corrosion would be directly depend on the supply of oxygen on air.
(x)Over Voltage
The difference between the actual value and theoretical value of decomposition potential of electrode.
Q-13Explain metal cladding with example.
It is a process of coating of brass metal on dense homogeneous layer of cladding material. Cladding metals can be pure metals or alloys. In this process arranging Tin sheets of coating metal and the base metal sheet in the form of sandwich which is then passed through rollers under the action of heat and pressure.
E.g.: Duralumin is sandwiched between two layers of 99.5% pure aluminum. This sheet is called as Alclad used in Aircraft industry.
