ENGINEERING CHEMISTRY
UNIT -1
(Batteries, Photovoltaics, Fuels, Nuclear energy, Lubricants)
2014-2015
SCHOOL OF CHEMICAL SCIENCES
Basic electrochemistry: Its relevance to electrochemical energy conversion devices.
Electrochemistry deals with transfer of charges (electrons and ions) across the interfaces and the relationship between the chemical changes with electrochemical parameters.
Electrode
Electrodes should electrically conductive and allow charge transfer across the interface. They can be inert or reactive. In case of batteries the electrodes are reactive. In Lithium ion batteries the electrode materials allows ion intercalation and deintercalation. Every electrode has its own potential, this depends on the material and the electrolyte to which its in contact.
Anode – Oxidation takes place at this electrode, Cathode - Reduction takes place at this electrode
When a metal is placed in its own salt solution it may undergo oxidation or reduction according to its tendency to loose or gain electrons.
i) Oxidation: Loss of electrons
M (s) → Mn+ (aq) + ne- , Metal behaves like an anode
For example: When Zn electrode is dipped in ZnSO4 /H2SO4 solution, Zn goes into solution as Zn2+ ions
ii) Reduction: Gain of electrons
Mn+(aq) + ne- → M(s), Metal behaves like a cathode
For example: When Cu electrode is dipped in CuSO4 solution, Cu2+ ions from solution deposits on the metal and Cu electrode attains a positive charge due to reduction.
The positive charge electrode attracts the negative ions from the solution and forms a sort of layer of negative ions around the metal. The layer of positive / negative ions formed on the metal is called Helmholtz Electrical Double Layer. A difference of potential is set up between the metal ions and the solution. At equilibrium, the potential difference becomes a constant value and is called as electrode potential of the metal. The tendency of the electrode to lose electrons is called oxidation potential (EOP) and the tendency of the electrode to gain electrons is called reduction potential (ERP).
REFERENCE ELECTRODES
The electrode of standard potential with which we can compare the potentials of other electrodes is called a reference electrode. The potential of the electrode remains constant at all temperatures. It undergoes specific reduction or oxidation but the potential will be same only sign will be different.
Components:
Electrode component: Pt-H2, Electrolyte component: HCl(1M) Electrode representation:
Pt, H2 (1atm) / H+ (1M)
Saturated Calomel Electrode (SCE)
Type/ class: Metal- metal insoluble salt electrode (Secondary Reference Electrode) Components:
Electrode component: Pt – Hg, Electrolyte component: Hg2Cl2(s) / KCl Electrode representation:
Hg, Hg2Cl2(s) - KCl (sat. solution) E is 0.3335 V for 0.1N KCl (DNCE) E is 0.2810 V for 1N KCl (NCE) E is 0.2422V for Sat. KCl (SCE)
Applying Nernst equation for the net reaction
Ecell = Eocell + 0.0591/n . log [Hg+]2[Cl−]2 / [Hg2Cl2]
The [Hg] and [Hg2Cl2] are unity .Therefore the Ecell depends on the concentration of chloride ions , hence the electrode is said to be reversible wrt chloride. (Another electrode which is reversible wrt to Chloride ions is Ag/ AgCl)
Electromotive Force (E.M.F.)
When two electrodes are connected, current starts flowing through the circuit. The driving force which makes the electrons to flow from a region of higher potential to a region of lower potential is called the electromotive force abbreviated as emf. It is measured in Volts (V).
ELECTROCHEMICAL SERIES (e.m.f series)
A series in which elements are arranged in the ascending (increasing) order of their standard reduction potential is called emf series.
Half cell reaction Eo (V)
Li+ + e- → Li - 3.04
Mg2+ + 2e-→ Mg - 2.37
Al3+ + 3e- → Al - 1.66 Zn2+ + 2e-→ Zn - 0.76 Fe2+ + 2e- → Fe - 0.44 2H+ + 2e- → H2 (g) 0.00
Hg 2 2+ + 2e → Hg (l) 0.24
Cu2+ + 2e- →Cu 0.34
Cu+ + e-- → Cu 0.52
Pt,Fe3+ + e→ Fe2+ 0.77
Ag+ + e- → Ag 0.80
Au+ + e- →Au 1.69
F2 + 2e- → 2F 2.87
Application / Significance of electrochemical series
(i) Relative ease of oxidation or reduction
The metals which lie above hydrogen in the series undergo spontaneous oxidation and the metals which lie below SHE undergo reduction spontaneously ( ie. Acts as Anodes and Cathodes respectively)The metals which lie above hydrogen are good reducing agents and which lies below hydrogen will act as good oxidizing agents
(ii) Replacement tendency
The metal lying above in emf series displaces the metal lying below it from an electrolyte of the later.
Example 1: Ni spatula cannot be used to stir copper sulphate solution due to the following reaction
Ni(s) + Cu2+ (aq) Ni2+ (aq) + Cu (s)
Example 2: when zinc is dipped in copper sulphate solution copper gets deposited (displaced) Zn (s) + CuSO4 (aq) ZnSO4 (aq) + Cu (s)
(v) Calculation of Standard emf of the cell E cell = E cathode − E anode
( if both reduction potentials are considered) E cell = E cathode + E anode
( if oxidation potential of anode and the reduction potential of cathode are considered)
(vi) Corrosion
The metals higher in the series are anodic and are more prone to corrosion.
The metals lower in the series are noble metals (cathodic) and they are less prone to corrosion.
(vii) Predicting the spontaneity of cell reaction
Spontaneity of the redox reaction can be predicted from the emf value of complete cell reaction. If the value of Ecell is positive, the reaction is feasible. as Δ G will be negative ( i.e. it is an electrochemical cell)
If the value of Ecell is negative, the reaction is not feasible. as Δ G will be positive ( i.e. it is an electrolytic cell)
Galvanic Series
In galvanic series, metals and alloys are arranged according to their tendency to corrode. This series can be used to determine whether galvanic corrosion is likely to occur and how strong the corrosion reaction will be.
Importance of galvanic series
Cells
Cell is a simple unit comprising of an anode and a cathode dipped in an electrolyte. Battery is an array of cells.
Reversible cells
A cell works reversibly in the thermodynamic conditions. Ex. Secondary batteries, Rechargeable batteries. The cell is reversible if it satisfies all the following conditions:
(i) If applied emf is equal to derived emf then the net reaction is zero
(ii) If applied emf is infinitesimally smaller than the derived emf then the cell should act as electrochemical cell (forward reaction)
(iii) If applied emf is infinitesimally greater than the derived emf then the cell should act as electrolytic cell (reverse reaction)
Irreversible cells
Cells which do not obey the (above) conditions of thermodynamic reversibility are called irreversible cells. If one of the products escapes from the cell then that cell cannot be made reversible by applying an external current.
Ex. Primary cells
Comparison between Electrolytic and Galvanic cells
Electrolytic cell Galvanic Cell
Conversion of electrical energy into chemical energy
Conversion of chemical energy into electrical energy
The anode is positive plate and cathode is negative plate
The anode is negative plate and cathode is positive plate
The electrodes used may be similar or dissimilar
Here electrodes are dissimilar materials
Electrons are supplied to the cell from the external power supply
Electrons are drawn from the cell. Not a spontaneous reaction , ∆ G is positive Spontaneous reaction. ∆ G is negative
eg. Electroplating eg. Corrosion
The extent of chemical reaction occurring at the electrode is governed by Faraday’s law of electrolysis.
The e.m.f of the cell depends on the concentration of the electrolyte and chemical nature of the electrode (Nernst Equation)
The amount of electricity passed during electrolysis is measured by Coulometer. e.g: Electroplating, Electrolysis
Electrolytes
The electrolytes are the medium which should possess the following properties • It should be Ionically conductive and electronically insulative medium • It can solid, semisolid,(gel), liquid
• It should have high ionic conductivities
• The solvent should have high potential window, specially organic electrolytes have high potential window (in Li-ion batteries). This improves the power and energy densities of the batteries
• In this potential window electrolytes are quite inert under the electrochemical conditions and does not undergo decompositon
Nernst Equation ΔG = Δ Go
+ RT ln Q Δ G = -nFE
For a reaction
Where
Ecell = Ecathode – Eanode
Battery electrochemistry
Batteries are classified under galvanic cell. Here the spontaneous electrode reaction at anode and cathode give the electric power. At Anode oxidation takes place and cathode reduction takes place. The electron flows from anode to cathode. The electrolytes are ionically conducting and chosen according to the types of the batteries.
The following diagram (Ragone) shows the importance of electrochemical devices and their comparison with fossil fuel operated IC engines.
In the above figure the power density and energy densities of the all the devices are the crucial factors that should be considered to replace the fossil fuel operated engines. Higher the energy density (Wh/gm) and power density (W/gm) better will be device. Supercapacitors has high power, low energy density. Batteries have medium energy density but less power density. Fuel cell has highest energy density but low power density. The fossil fuel operated engines has higher power and energy densities. Hence the research work in the electrochemical energy conversion devices is continuous to attain that spot. Considerable level is reached with hybrid system, still the goal is yet to be achieved. This is largely associated with the electrode kinetics, charge transfer, the electrochemical stability and life of the electrochemical devices.
Classification of batteries
Batteries and classified as primary and secondary based on the reversibility of electrode reactions. The primary batteries undergo irreversible electrode reactions.
Daniel cell
This is a galvanic cell. It is a device in which a redox reaction is used to derive electrical energy. During the working of the cell the stored chemical energy decreases and this decrease is gained as electrical energy. In the electrochemical cell the electrode at which oxidation occurs is called anode (− ve) and the electrode at which reduction occurs is called cathode (+ ve).
Example: Zn acts as anode and Cu acts as cathode in Daniel cell
The Zn metal and Cu metal is dipped in the At Anode
Zn Zn2+ At →Cathode
Cu2+ + 2e- Cu Overall cell reaction
Zn + cu2+ + SO42- ZnSO4 + Cu EMF – 1.1 V
Salt bridge: It consists of a U tube filled with a saturated solution of KCl or(NH4)2NO3 in agar-agar gel. It connects the two half cells and performs the following functions. It eliminates the liquid junction potential.Completes the circuit. Maintains electrical neutrality in the two compartments by migration of ions through the porous material thus ensures the chemical reactions proceed without hindrance. It prevents the mixing of the electrode solutions.
Zn/MnO2 dry cell
It is called dry batteries since it has no liquid electrolyte.
The anode is the cylindrical Zn vessel which encloses the pasty ammonium chloride and ZnCl2. The carbon is coated with layers of MnO2 + ammonium chloride and acetylene black paste acts as cathode. The entire setup is sealed in polymeric sheet. The cell voltage is 1.5 V
The following electrode reactions occur in the cell Anode
Zn + 2OH- ZnO + H2O + 2e -Cathode
-Lithium primary batteries (Li/MnO2)
This battery has lithium metal as anode and insertion metaloxide (MnO2) cathodes. The high capacity of the Li metal gives the longer life and higher energy density. The electrode should be free from moisture and air. Lithum metal is highly reactive to moisture and the reaction is explosive. Even lithu react with nitrogen to form nitrides. The passive layer formation on the lithium metal anode is another major problem with this battery.
Li/ Ethylene carbonate + Propylene carbonate+ Li+/MnO2
Li/ Ethylene carbonate + Propylene carbonate+ Li+/organic sulphides
During discharging the following reactions takes place Anode
Li Li+ x+ e-
The Lithium metal is oxidized and the electrons move through the external circuit to produce the power output and reach the cathode.
Cathode
MnO2 + Li+ x + e- LixMnO2
Secondary batteries
These are recharge batteries where the electrode reactions are reversible with supply of electricity.
Pb-PbO2 acid batteries consist of lead anode and lead dioxide cathode. This works in the presence of concentrated sulphuric acid electrolyte (20 % H2SO4). The concentration decreases with discharging and regained on charging. This can tested by specific gravity measurement of H2SO4. Cell voltage 1.88 – 2.15 V
Anode reaction
Pb + 2H2SO4 PbSO4 + 2H+ + 2e-
Cathode reaction
PbO2 + 2H2SO4 + 2H+ + 2e- PbSO4 + 2H2O
Overall
PbO2 + Pb + 2H2SO4 2PbSO4 + 2H2O
Charging Discharging
Charging Discharging
The water level in these batteries should be checked for it affects the formation of H2SO4 during charging. These batteries are used in automobiles, invertors and industrial applications.
Lithium ion secondary batteries
The above figure shows comparison of Lithium ion batteries with other secondary batteries. Due to their highest position in both gravimetric and volumetric energy densities they had reached almost in every hands (Battery in mobiles, laptop, ipads)
Unlike Lithium primary batteries, in Lithium ion batteries both the electrodes are insertion electrodes. This allows both intercalation of lithium ions. The lithium ions shuttle between anode and cathode during charging and discharging process. This is shown in the above figure
At anode
LixC Lix-nC + Li+n At Cathode
LimCoO2 + Li+n Lim+nCoO2
During charging the reverse reactions takes place the anode and cathode. The cell voltage depends on the choice of the electrode materials. The transition metal centers are redox active and, they can undergo the change in their oxidation states during charging and discharging processes. The solidstate structures of the electrodes are important for the Lithium ion diffusion kinetics, cycle life and stability.
Reserve batteries
In these batteries the electrode reactions can be activated when required. This can be done adding one of the component like electrolyte, water, supply of air, mechanical force and heat.Reserve batteries operates when it is required, unlike the primary and secondarry batteries. Primary and secondary batteries has definite shelf life, while in reserve batteries the materials are stored with reaction for long time.
Example
Silver chloride cell: activation by sea water of fresh water Mg/Water/AgCl/Ag (Activation by water)
Mg/water or KBr/O2 (air) (Activation by air and water) Zn/KOH/O2 (air)
Anode: Zn + 2OH- Zn(OH)2 + 2e- Cathode: 2H2O + O2 + 4e- 4OH
-The cathode reaction takes place on introduction of air into the cell. Classification of fuels
Fuels
Characteristics of fuel
A good fuel should have following characteristics • High calorific values, high efficiency • Low emission and low waste
• Moderate ignition temperature
• Low moisture content and low volatile matters • Moderate rate of combustion
• No poisonous product formation • Low cost and high availability • Easy to transport
Comparison of fuels
• Characteristic Solid Liquid Gas
• Cost Cheap More costly Most costly
• Combustion rate Slow Greater Very high
Calorific values
The energy released as heat by the burning the fuel is expressed as calorific value. It is the total quantity of heat liberated by burning a unit mass or volume of fuel completely.
Units calories/gm, Kcal/Kg, BTU/lb
1 calorie is defined as the heat required to raise the temperature of 1 gm of water by 1oC. 1 calorie = 4.184 Joules = 4.184 x 107 ergs
The calorific value of a fuel can be determined using calorimeter. Fuels containing hydrogen is converted to steam during combustion is condensed back to obtain the latent heat of water. In high calorific value the latent heat of condensation of steam also gets included. Thus HCV is defined as the amount of heat produced by burning completely one unit mass or volume of the fuel and allowing the combustion products to cool to room temperature.
The lower calorific value is defined as the net heat liberated by burning completely one unit mass or volume of the fuel and the combustion products are allowed to escape.
• C + O2 CO2 + 394 kJ/mol
• 12 gm of C + 32gm of O2 44 gm of CO2 + 394 kJ/mol • 1gm of C produce 394/12 = 32.84 KJ
• Therefore the calorific value of C is 32.84
• 2gm) H2 + (18gm) ½ O2 H2O (18 gm) • 1 part of hydrogen produce 9 part of water
• LCV = HCV – Mass of hydrogen per unit mass of fuel x 9 x latent heat of steam • LCV = HCV – (H/100) x 9 x 587
Bomb calorimeter
Bomb Calorimeter is used to determine the calorific values of solid and non volatile liquids.
The bomb calorimeter consists of crucible with known weight of fuel. This is connected to the electrodes through the fuse wires. In certain case the cotton thread may be used to initiate the combustion process. High oxygen/air pressure is maintained inside the bomb. The bomb is made of stainless steel container. It can withstand high pressure. This is placed inside the copper calorimeter. The water present inside the calorimeter absorbs the heat liberated. The water equivalence of the calorimeter is specific for the calorimeter. The calorimeter is enclosed inside the air and water jacket to prevent any heat losses. The Calorimeter is provided with thermometer which can record the rise in temperature. The stirrer is provided to maintain the uniform temperature. The intial temperature and the temperature is monitored at regular intervals till it reaches the maximum
Calculation of calorific values
• Mass of the substance – m gms
• Mass of water in calorimeter = W gms
• T1– initial temperature , T2 = Final temperature • HCV = ( W + w) ( T2-T1) / m cal / gm or Kcal/Kg
• Correction - Cooling (Tc), Fuse wire(Tf) , Acid(Ta) , Thread(Tt) • HCV = {( W + w) ( T2-T1 + Tc)} – {Ta + Tf + Tt} / m
• LCV = HCV – 0.09H x 587
Boys Calorimeter
The Boys gas calorimeter is used to measure the calorific values of gases and volatile liquids
This experiment is based on heat transfer from burning the gaseous fuel for heating the water that flows and circulates in a coil heat exchanger. This design ensures maximum heat transfer to the cooling liquid and hence accurate enough for measurement and calculations of calorific value of gas. The apparatus consist of gas burner with the controlled gas supply whose volume can be measured. The water circulates in the copper coil s which absorbs the liberated heat by the combustion of the gases. The rise is temperature is noted using the thermometers introduced in the inlet and outlet. The design of the coil provides the complete absorption of the heat. The exhaust gas temperature can also be monitored. The condensed steam is used to find the LCV.
Calculation of calorific values
• Volume of gas burnt in given time – V ml
• Mass of water in circulate in Cu calorimeter = W gms • Mass of the water condensed from steam = M
• HCV = ( V/M) ( T2-T1) cal / ml or Kcal/L • 1cm3 of gas is burnt to give M/V gm of H2O • LCV = HCV – (M/V)587
Petroleum refining
Petro (rock) leum(oil) obtained from the well is crude and it has several fractions of organic hydrocarbons with inorganic impurities. This has to refined by several methods to use in proper devices or in the processes.
Demulsification.
By this process the inorganic impurities are separated from the crude oil.
Fractionation – Distillation
Distillation of fractions is done to separates the mixtures based on their boiling point. This is done in atmospheric or vacuum condition. Vacuum distillation enhances the easy separation of high volatile fractions.
Cracking
It is the process in which the higher molar mass hydrocarbons are broken down into lower molar mass hydrocarbons. This is done thermal, catalytic and hydro-cracking process.
Different temperatures were used to crack the higher hydrocarbons to lower chain lengths. This can be enhanced by using catalyst. Catalyst work specific and reduce the activation energy. Fixed bed cracking and Fluidized cracking
Catalysts are specific and they enhance the particular fractions in the cracking process. Mainly metals, metaloxides and zeolites were used as catalysts in petroleum refineries. They can be immobilized or mobile. In fixed bed process the catalyst are fixed and the heated petroleum fractions are allowed pass over the surface of catalysts. Thermal cracking usually results in mixtures of saturated and unsaturated hydrocarbons. This has to be further purified.
Reforming
This is done with help of Pt catalyst and pressure to increase the specific fraction. In gasoline isooctane fractions is improved by reforming process.
Isomerisation is done to improve the particular fraction in the refined product. For example isooctane fraction can be enhanced, since octane can give severe knocking characteristics.
Synthetic fuels
Polymerisation of alkenes to alkanes is the important process in synthesis of synthetic petrol. It can be thermal of catalytic polymerization. It is done by hydrogenation of coal. Specific fractions/ composition can be controlled by synthetic methods.
Aviation fuel
The jet engine fuels are kerosene free from aromatics. The jet propulsion with the turbines having wheel of blades operated with air of high velocity. Huge volume of compressed air injected into engine with the fuel. Part of the air is used to combust the fuel. The hot circulating air air along with the combusted products will emanate to turn the blades of the turbine.
For more details on cracking process, reactions and schemes please refer to books
Knocking
It is the sharp metallic sound similar to rattling of hammer which is produced in IC engines, due to premature ignition of air-gasoline mixture. In petrol engine the fuel air mixtures should be ignited by the spark, but some fractions pre-ignited gives this knocking sound in the engine.
• Chemical structure of the fuel related with the knocking characteristics
• Knocking decreases with increase in compactness of molecules, double bonds, cyclic structures
• Normal paraffins knocking increases with increase in chain length • The power output of the IC engines depends on the compression ratio. The trends of knocking characteristics of some compounds is shown below
• n-heptane> n-hexane > n-pentane > n-butane
• n-heptane > 2-methyl hexane > 2,2’ dimethyl pentane > isooctane
• Aromatics such as benzene, toluene have very high antiknocking property
Octane number and octane rating
The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of 2,2,4-trimethylpentane (iso-octane) and heptanes which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel.
For example, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90. Some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.
• n-heptane knocks badly – octane number is zero
• Octane number of the fuel is defined as the percentage of iso octane present in a mixture of iso-octane and n-heptane, which has the same knocking characteristics as that of fuel under examination, under same set of conditions.
• Octane number of the cracked gasoline is higher than the straight run gasoline
• Isooctane – 100, isopentane – 90, cyclohexane – 77, 2-methyl pentane- 71, n-pentane-62, n-hexane – 26, alcohol -95
Anti knocking agents:
Addition of certain additives to reduce the knocking character
• Tetra ethyl lead ( TEL) - It can act as inhibitors for free radical and chain reactions • Anti-knocking agents – tertiary butyl acetate, diethyl telluride, benzol. alcohols, bezene
Cetane rating
• A quantity indicating the ignition properties of diesel fuel relative to cetane as a standard. • Measure of ease with which the fuel ignites or time lag of ignition of fuel.
• Cetane, short time lag – cetane number = 100 • Methylnapthalene – cetane number = 0
• The cetane number of diesel is defined as the percentage of cetane in a mixture of cetane and -methyl napthalene which will have the same ignition characterisitics as the fuel under test, under the same set of conditions.
Nuclear energy
Nuclear reactions associated with the release of high energy radioactive radiation and energy. There are two types of nuclear reaction namely fission and fusion.
Chemical Reactions Nuclear Reactions
• Occur when bonds are broken.
• Atoms remain unchanged, although they may be rearranged.
• Involve only valence electrons.
• Energy changes associated with chemical reactions are less compared to nuclear reactions.
• Reaction rate influenced by temperature, particle size, concentration, etc.
• Occur when nuclei emit particles and/or rays.
• Atoms often converted into atoms of another element.
• May involve protons, neutrons, and electrons
• Energy changes are very high
• Reaction rate is not influenced by temperature, particle size, concentration, etc.
Stability of Nuclei: Mass defect, Binding energy, n/p ratio
Although nucleus is positively charged it is stable. The positively charged protons stay together in a tiny place. This is hold together by strong binding forces. The actual mass of the atom is less than the theoretical mass. This mass is spent as energy and released.
The difference between the actual mass of an isotope of an element and the sum of the masses of protons, neutrons and electrons present in it is called mass defect.
The binding energy of a nucleus is the energy needed to break a nucleus into its individual protons and neutrons or the energy released when the nucleus is formed from its constituent nucleons.
The binding energy per nucleon gives the measure of the stability of the nucleus. The binding energies per nucleon of a number of isotopes are plotted against mass number in the above fig. The inspection of the plot shows the following features:
● The nuclei for helium (4), carbon (12) and oxygen (16) have quiet high B.E. than other nucleides. This shows that the nuclei of these elements are exceptionally stable. This forms the basis of nuclear fusion reaction.
● The B.E. per nucleon rises sharply amongst the lighter elements and attains a maximum value of 8.5 MeV at mass number of about 56 (which corresponds to iron). This means that the nucleus of the iron is thermodynamically most stable.
● The B.E. per nucleon in the central part is nearly constant. This means the addition of a single nucleon to any nucleus in this region increases the binding energy regularly by the same amount. ● The B.E. per nucleon of the nuclei of heavier elements is somewhat less than those of intermediate elements. Consequently, these elements can be easily split up into smaller nuclei. This is the basis of nuclear fission reactions.
From above graph, it is clear that there may be two main types of nuclear reactions:
n/p ratio
Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. But a strong nuclear force helps keep the nucleus from flying apart. Therefore, the ratio of neutrons to protons is an important factor. Neutrons play a key role stabilizing the nucleus.
For small atoms (Z 20), the most stable neutron-proton ratio is 1:1
As atomic number increases, this ratio change to 1.5:1
Protons repel each other in the long range through electrostatic repulsion.
As the nucleus gets larger, the repulsive forces increase and more neutrons are needed to stabilize the nucleus.
Even numbers of nucleons provide the most stable nuclei, this indicates that nucleons are most stable when paired, like electrons.
The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.
Nuclei above this belt have too many neutrons.
They tend to decay by emitting beta particles.
Nuclei below the belt have too many protons.
After atomic number 83, the repulsive force is so great that there are no more stable nuclei.
These nuclei tend to decay by alpha emission.
They tend to become more stable by positron emission or electron capture.
Radioactivity is the spontaneous ejection of radiations (alpha, beta and gamma) by a radioactive element. It refers to the particles which are emitted from nuclei as a result of nuclear instability. Marie Curie named the process by which materials give off such rays radioactivity. The penetrating rays and particles emitted by a radioactive source are called radiation. These findings contradicted Dalton’s theory of indivisible atoms.
Nuclear reactions
Bombardment of the radioactive nuclide with a neutron starts the process.
♣ Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons. This process continues in what we call a nuclear chain reaction. (Chain reaction releases several neutrons which split more nuclei).If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
It is Used for nuclear power and atomic bombs.
Nuclear fusion is a nuclear reaction in which a light nuclei combine to give a stable heavy nucleus plus possibly several neutrons, and energy is released. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons.
Representation of some nuclear fusion reactions. Unstable nuclei is not shown
Stars are giant nuclear reactors. In the center of stars, atoms are taken apart by tremendous atomic collisions that alter the atomic structure and release an enormous amount of energy. This makes stars hot and bright. Stars are powered by nuclear fusion in their cores, mostly converting hydrogen into helium. The enormous energy of the stars comes from nuclear fusion processes.
Depending upon the age and mass of a star, the energy may come from proton-proton fusion, helium fusion, or the carbon cycle.
† For stars like the sun which have internal temperatures less than fifteen million Kelvin, the dominant fusion process is proton-proton fusion.
† For more massive stars which can achieve higher temperatures, the carbon cycle fusion becomes the dominant mechanism.
Lubricants
Lubricants are the materials which are interposed between the moving objects to reduce the friction and wear and tear. It helps to reduce the frictional resistance.
The Van der Waals force of attraction between the moving and static object plays an important role in the retardation of motion. During the motion there will be the interlocking of minute projections – asperities. In general the surfaces are not completely flat. The surface has roughness different energies. It results in increase in frictional forces.
• Friction generates heat
• Higher the surface energy the higher will be the friction
• Higher the load and area of contact there will be greater extent of friction
• Friction will result in wear and tear of the material surface
To avoid direct contact of the surfaces the materials which possess suitable properties of lubricants are interposed between them.
Lubricants have the following functions
• It fills the valleys of the surface and give an smooth motion of the objects
• Reduce deformation, wear and tear between moving/sliding surfaces.
• It acts as coolants
• Reduces the energy losses in the operation of machineries
• Other advantages are like corrosion prevention, electric insulators.
• To reduce the leakage of gases under high pressure like a seal.
• To reduce irregular expansion of metals.
• To reduce welding of the two surfaces.
• To reduce or avoid rough relative motions of moving / sliding parts.
• To reduce running and maintenance cost of the machine.
• Act as corrosion prevention materials
Thick film lubrication (fluid or hydrodynamic)
Here a thick layers of lubricants separates the moving surfaces. It is of 1000 Å or above and hence there is no direct surface to surface contact. It helps to avoid welding of junctions. The thick film lubricant covers/fills the irregularities and also gives lubrication. It reduces the wear and tear of the surface. In general solid lubricants are used to form thick film. The film should not extrude out from the position. Hence the viscosity, thermal stability, flow point of the lubricants is more important.
Eg. Graphite, Molybdenum sulphides, soaps, Heavy oil lubricants, paraffin
Hydrocarbon oils are considered to be satisfactory lubricants.
To maintain viscosity throughout lifecycles long chain polymers are used as blenders with normal hydrocarbons oils.
Small amount of unsaturated hydrocarbons present in hydrocarbon oils produced from petroleum fractions, which causes oxidation and produce gummy like products. Hence, anti-oxidant like aminophenol s are used along with the lubricants.
Thin film lubrication (Boundary lubrication)
These are thin film or few molecular layers over the surfaces. The lubricants are adsorbed chemically or physically on to the moving surfaces. The characteristics of thin film lubrication are:
o High viscosity-index, Resistance to heat and oxidation, Good Oiliness , Low pour-point.
The adsorbed layers on the moving surface should be able to carry the applied load, gives lubrication and protect the surfaces.
There are also chances of incomplete coverage of the surface. Hence the choice of the lubricants and their chemical composition is vital in thin film lubrication.
play an important role in lubrication. Hence for the boundary lubrication, the lubricant molecule should have:
(i) Long hydrocarbon chain with polar groups.
(ii) Polar groups promote spreading and orientation over the metallic surfaces at high pressure.
(iii) Lateral attraction between the chains.
(iv) Active groups or atoms, which can form chemical linkages metal or other surfaces.
Vegetable and animal oils (glycerides of higher fatty acids & their soaps).
o These oils either physically adsorbed to metal surfaces or react chemically at the metal surfaces.
o Although these oils posses greater adhesion property, yet they tend to breakdown at high temperatures. Hence, fatty acids are added to improve the oiliness.
Graphite and Molybdenum disulphide alone or oil suspension may be used because:
o They have Low internal friction
o They can bear/withstand compression
o They should be thermally stable
When moving/sliding surfaces are under very high pressure and speed, a high local temperature is attained. In such conditions, liquid lubricants fail to stick and may decompose and even vaporize. To avoid this, special additives are added to mineral oils. These are called “extreme-pressure additives”.
Mechanism
The “extreme-pressure additives” form on metal surfaces more durable films, capable of withstanding very high loads and high temperatures.
Examples:
Organic compounds containing Chlorine, Sulphur and Phosphorus.
These compounds react with metallic surfaces, at prevailing high temperatures, to form metallic chlorides, sulphides or phosphides.
o These metallic compounds layers possess high melting points.
Classification of Lubricants
Based on Physical state, lubricants are classified as: a) Lubricating oils or liquid lubricants
b) Semi solid lubricants or greases c) Solid lubricants
Solid Lubricants
Solid lubricants are used when: Other lubricants cannot be used, contamination undesirable, Too high temperature or load are involved , Combustible lubricants not acceptable
Examples of solid lubricants used are:
Graphite , Molybdenum disulphide, Soaps, polymers, glass
Graphite
o Slippery nature, soapy, uninflammable and oxidized above 375oC
It undergo slippery motion of the layers during the movement of the surface. Thus it offer the lubrication. It can be mixed with oils (oildag), water(aquadag), Emulsifying agent (tannin) , Greases
MoS2
o Laminar layered structure where Mo separates two layers of sulphide ( ~ 6.4 Å ). It is thermally stable in air up to 400 ° C
Soapstone, talc or mica are also used as solid lubricants.
Semi solid Lubricants
o Semi solid consisting of a soap dispersed throughout a liquid lubricating oil. - May be Petroleum oil or synthetic oil with a specific additive.
Preparation:
Addition to hot lubricating oil under agitation
o To increase the heat resistance of grease, inorganic solid thickening agents (like finely divided clay, bentonite, colloidal silica, carbon block etc.,) are added.
o Have higher shear or frictional resistance than oils and hence support much heavier load at lower speeds.
Applications of Greases:
o When oil cannot remain in place due to high load, low speed, intermittent operation, sudden jerks etc.
o Work at high temperature
o When external contamination may create problem, When dripping or spurting of oil is undesirable
Types of greases:
o Calcium based greases or cup-greases, Soda-based greases, Lithium-based greases, Axle- greases → lime with resin and fatty acids, Graphite greases, Soap stone
Liquid Lubricants
Liquid lubricants must have adequate boiling point., adequate viscosity under operating conditions, thermal stability, High oxidation resistance and non-corrosive properties.
• Animal and Vegetable oils: Usable under very high temperature and heavy load.
Disadvantages of its usages are
1. Undergo oxidation easily in contact with air and forms gummy and acidic products, and get thickened.
2. Tendency to hydrolyze in contact with moist-air or aqueous medium. So, they are used as “blending agents” with other mineral oils.
They are obtained by distillation of petroleum.
o Length of hydrocarbon chain varies between 12 to 50 carbon atoms.
o Shorter- chain oils have lower viscosity than the longer- chain hydrocarbons.
Liquid lubricants are most widely used lubricants because they are
Cheap, available in abundance, Quite stable under service conditions.
o But they have poor oiliness character compared to animal and vegetable oils.
o So, high molecular weight compounds like oleic acid, stearic acid are used to over come this problem.
c) Blended oils:
No single oil serves as the most satisfactory lubricant for many of the modern machines. Hence, additives are used to improve the properties. These blended oils give desired lubricating property required for machinery.
Additives used are:
a) Oiliness- carriers: Coconut oil, caster oil, and palmitic, stearic and oleic acids.
b) Extreme-Pressure additives such as:
o Fatty esters or acids which form oxide film with metal surface. o Organic materials containing sulphur, chlorine, phosphorus.
o Some times lead (Pb) compounds could be used as high pressure lubricants.
c) Pour-point depressing additives:
o Phenol, condensation product of chlorinated wax with naphthalene.
d) Viscosity index improvers : hexanol
e) Thickeners : Polystyrene or polystyers
f) Antioxidants or inhibitors : Aromatic phenolic or amino compounds
g)Corrosion preventers: Phosphorous, Antimony organic compounds
i) Antifoaming agents : glycols and glycerol
j) Emulsifiers : sodium salts of sulphonic acid
k) Deposit inhibitors : detergents such as salts of phenol and carboxylic acids
Properties of Lubricants
1.Viscosity:
The property of a liquid or fluid by virtue of which it offers resistance to its own flow . - Viscosity should not be too low or too high (optimum).
- (viscosity is inversely proportional to temperature)
- Viscosity index (VI) – rate of change of visocity with change in temperature. 2. Flash - Points and Fire - Points :
Flash Point: The lowest temperature at which the oil lubricant gives off enough vapour that ignites for a moment, when a tiny flame is brought near it.
Fire Point: The lowest temperature at which the vapour of the oil burn continuously for at least five seconds, when a tiny flame is brought near it.
3. Oiliness: A measure of its capacity to stick on to the surfaces of machine parts under conditions of heavy pressure or load.
o For high pressure - high oiliness oil should be used. o Important for extreme - Pressure lubrication
4. Cloud and Pour points:
When an oil is cooled slowly, the temperature at which it becomes cloudy or hazy in appearance, is called its CLOUD POINT.
The temperature at which the oil ceases to flow or pour, is called its POUR POINT.
5. Volatility: Good lubricant should have low volatility.
o Emulsions have a tendency to collect dirt, girt, foreign material etc., causing abrasion and wearing out of the lubricating parts of the machinery.
o A good lubricating oil should form an emulsion with water which breaks off quickly. 7. Carbon residue:
Normally lubricants consist of high % of carbon containing compounds.
o Lubricants decompose due to raise in temp. and deposit carbon creating problems to a) IC engines and b) Air compressors.
o A good lubricant should deposit least amount of the carbon .
8. Corrosion stability: A polished copper strip is placed inside a lubricating oil for a specified time and temperature and then checked for any tarnishing .
o To prevent or retard corrosion effect of lubricating oils, additives such as Phosphorous, Arsenic, Antimony, Chromium, Bismuth or Lead are added.
9. Decomposition stability:
o Lubricating oils must be stable to decomposition at the operating temperatures by :
a. Oxidation: To prevent it anti oxidant or inhibitor are used. b. Hydrolysis: Moisture in oils causes hydrolysis of esters c. Pyrolysis : At high temperature
10. Aniline point: (A.P.)
o The minimum equilibrium solution temp. for equal volumes of aniline and oil sample. o A good lubricating oil should have higher aniline points (A.P)
o Higher A.P means higher % of paraffinic hydrocarbons and
hence lower % of aromatic HC. (Aromatic HC dissolves natural rubbers and few synthetic ones)
11. Precipitation Number:
o Precipitation Number is used to differentiate the different classes of lubricants.
12.Specific Gravity:
A.P.I. ( American petroleum Institute) degree A.P.I degree = 141.5/sp. gr. at temp(60°F) -131.5 where 141.5 → modulus of the A.P.I scale.
13. Ash Point:
o For used oil it is important to get an idea as to how much abrasion and wear it may cause
14. Saponification number:
o Number of milligrams of KOH required to saponify 1g of oil. 15. Mechanical stability:
o At very high pressures of operation, the stability of a lubricant is judged by four balls extreme pressure lubricant test.
16. Neutralization Number:
o Is a scale to determine the amount of acidic or basic constituents of an oil.
o Acid Number: Amount of KOH required in milligrams to neutralize the fatty acids in 1g of oil.
o Good lubricating oil → acid number value < 0.1
Photovoltaics
The solar spectrum reaching the earth is mostly visible and IR radiation. This visible part of the spectrum is useful in electronic excitation. Hence the materials which can trap this part of spectrum is major interest. The efficient rapping and conversion to other form of energy depends on the materials which are used. Usually semiconductors whose bandgap matches with the solar spectrum is a major interest. The infrared part of the light is only useful for vibrational excitation of bonds. Hence the electron energy levels and their excitation energy requirements in semiconductors are important in their application.
o About 49% in near IR
o About 3% in UV region and rest in far IR region
Devices of solar energy conversion
There are different types of devices used to convert solar energy to electrical, chemical and thermal energies.
1. Photovoltaic cells
2. Photoelectrochemical cells 3. Photogalvanic cells
4. Solar thermal (eg. water heater) 5. Dye sensitized solar cells
Photovoltaic devices convert the solar energy to electrical energy. The photovoltaic effect involves creation of a voltage (or a corresponding electric current) in a material upon exposure to electro-magnetic radiation. Though the photovoltaic effect is directly related to the photoelectric effect, the two processes are different. In the photoelectric effect electrons are ejected from a material's surface upon exposure to radiation of sufficient energy.
1. These devices are quantum converters, in which a photon is absorbed resulting in an electron-hole pair or breaking of the chemical bond
2. These can use only the relatively high energy photons and considerable portion of the IR radiation cannot be used.
3. The photovoltaic technology has very high efficiencies of the order of 26% (on a laboratory scale) but it is not completely realised.
Photovoltaic effect was observed by French physicist Edmund Becquerel with solid selenium in 1870’s. But the efficieny was 1% and costly. Later in 1950’s highly pure crystalline silicon (Czochralski method) was prepared. With Silicon photovoltaic cells 4% efficiency was achieved in .1954. Bell labs improved to 11 % efficiency. These devices require highly crystalline and pure materials
Silicon is a very common element abundant in nature (it is the main element in sand and quartz). It is considered as the most suitable material for solar energy conversion because of its abundance and optimum band gap of 1.23 eV at 300K. But it has to sepearated, purified and crystallized and doped for use in solar cells.
Effects of light on Si
As Light falls on the Si crystal Electron – hole pair (exciton) generation takes place. The photogenerated holes and electrons have to be separated before it recombines. Hence the p-n junction is formed with doped Silicon. By potential barrier the charges are separated and they are less likely to recombine
Creation of potential barrier
• As n and p type semiconductors are brought together with junction. It forms potential barrier. At the junction there is rapid exchange of electron and hole. Electron moves from n type semiconductor toward p type crossing the junction and vice versa. This form a potential barrier at the junction. The barrier width depends on the area of contact and the
amount of dopants.
• This can be also created by altering the crystal structure
• The line dividing n and p type Si establishes the potential barrier
separate before it recombines. The design of the cell, potential barrier and it width plays an important role in this process.
A photogenerated electron on the p-type side usually has enough time to bounce randomly around the crystal and encounter the junction before it can combine with a hole. As it is a minority carrier, because of the charge imbalance it see near the potential barrier it is accelerated towards the n type side. But the holes at the p type side should have energy to cross the barrier so it is does not happen. This process is same with the holes in the n- type side. Thus a voltage difference is created by this process and the external circuit connected will give the power output.
Photoelectrochemical cell
PEC cells are classified into two types according to their application.
1. Liquid Junction Solar Cell (LJSC) – This cell is used to convert solar energy into electrical energy
2. Photoelectrosynthesis (PES) cells – In this class of cells, solar energy is converted into chemical energy in the form of fuels.
Photoelectrochemical (PEC) cell is a device in which a photoactive semiconductor material as photoelectrode is in contact with an electrolyte. It enjoys the advantage of easy junction formation (mere dipping of the SC electrode in the electrolyte). Even particulate systems can be used.
Irradiation of the SC/electrolyte junction with light of energy > Eg, the band gap of the semiconductor, produces electron - hole pairs.
Heterogenous charge transfer and the kinetics are limited by the materials and their properties
Semiconductor/Electrolyte interface
a) On contact the Fermi level of the n-type semiconductor equilibrates with that of the metal and with the redox couple of the electrolyte.
b) After charge (electron) transfer, a band bending is established as in the case of the previous solid-state junctions, with establishment of the depletion zone.
c) Under light, photoelectrons enter the conduction band; the band bending is reduced and a photovoltage is generated between the semiconductor Fermi level and the redox potential of the electrolyte - equivalent to the potential of the metal counter-electrode.
d) Minority carriers - holes - are then available for an oxidation reaction with the electrolyte at the Semiconductor photoanode. A reduction reaction takes place at the cathodic counter electrode.
Mechanism of Liquid Junction Solar Cells (LJSC)
The simplest LJSC consists of two electrodes (one of them a SC and the other a metal) dipped in an electrolyte containing a redox couple.
Both the electrodes must be inert, i.e., the electrode material itself should not take part in the electro- chemical reactions.
One of the important requirements for the operation of an LJSC is the presence of depletion layer at the surface of the SC electrode.
For this, the initial Fermi level of the SC should be above (in the case of n-type semiconductors) the Eredox.
Working
The non-equilibrium electrons in the valence band are produced by illumination of light with energy hν ≥ Eg.
The electrons are transferred to the bulk, then via the external circuit to the counter electrode, where they are used up for the reverse reaction (reaction).
LJSC
Cell : n-CdSe / Na2S + S + NaOH / Pt At the anode:
Sx2- + S2- + 2h+ ---> Sx+1 At the cathode:
