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SURFACE CHEMISTRY • Introduction -Adsorption is the phenomenon of concentration or assimilation of a gas or liquid on the surface of a solid or liquid with which it is in contact. Adsorbent is the material which provides the surface on which adsorption occurs and the substance adsorbed or attached is called adsorbate. Examples of adsorbents are charcoal, silica gel, clay, fullers earth, alumina gel, etc. Adsorbent molecule (adsorbate) Fig. 8.1 The adsorption of gas on solid surface
Differences between absorption and Adsorption • Adsorption is a surface phenomenon and it is a fast process. In absorption, diffusion into the interior of matter takes place, hence it is a slow process. For example, when a blotting paper is kept in contact with ink, ink is absorbed into the paper. on the other hand, if a dilute solution of litmus is shaken with animal charcoal the surface of charcoal takes away some of the litmus and all the litmus is concentrated on charcoal. In adsorption, equilibrium is easily attained but in absorption it takes some time to reach equilibrium. Absorption is a bulk phenomenon but adsorption is a surface phenomenon. Adsorption depends on the surface area of adsorbent. Consequently, it is more rapid on finely divided and on a rough surface. Such effect is not observed in absorption. Fig. 8.2 Differences between absorption and adsorption
Types of Adsorption • Adsorption is not necessarily a physical phenomenon. It may be a chemical process involving chemical reaction between the surface atoms of adsorbent and the atoms of adsorbate. This type of adsorption is known as chemisorption. It results in surface complex. For example, O2 is chemisorbed by carbon and H2 by nickel under suitable conditions. Chemisorption differs from physical adsorption in the following respects: • 1. Physical adsorption occurs appreciably at very low temperature, i.e, below boiling point of adsorbate. chemisorption occurs at all temperatures. • 2. The magnitude of physical adsorption decreases with rise in temperature. The magnitude of chemisorption increases with temperature. • 3. The heat evolved during physical adsorption is very low, i.e., 4–40 kj mol–1. It is very high (40–400 kj mol–1) in chemisorption.
4. Chemisorption is irreversible as the gas adsorbed cannot be recovered from adsorbent as such on lowering the pressure of the system at the same temperature. Physical adsorption is reversible as the gas adsorbed can be recovered by simply lowering the pressure of the system. • 5. Physisorption may extend beyond a monolayer. Chemisorption operates within short distance only. It does not extend beyond the monolayer of gas molecules. • 6. In physisorption, the adsorbate molecules are held together by weak van der Waals forces. Hence, activation energy for disorption in very low. In chemisorption, adsorbate molecules are held by strong valence forces and its activation energy for disorption is high. • Adsorption isotherms-The adsorption of a gas on a solid adsorbent in a closed vessel is a reversible process. The amount of gas adsorbed depends on equilibrium pressure (p) and temperature. Adsorption isotherm is a graph plotted between a given magnitude of adsorption and pressure at a given temperature. It may be given on a graphical curve which is known as Freundlich adsorption isotherm
Freundlich Adsorption Isotherm • The relation between the magnitude of adsorption and pressure can be expressed mathematically by an empirical equation known as Freundlich adsorption isotherm. • where x/m is amount of gas adsorbed per unit mass of the adsorbent at pressure P. k and n are variables which depend on the nature of the solid, gas and the nature of adsorbent. • The extent of adsorption x/m increases with increase in pressure (P) and becomes maximum at saturation pressure P0. At P0, the rate of adsorption becomes equal to the rate of desorption. Further increase of pressure has no effect on adsorption.
From the adsorption isotherm (Fig. 8.4), the following observations can be made:
Limitations of Freundlich adsorption isotherm • 1. It fails when pressure is high and when the concentration of adsorbent is very high. It is valid only • within a limited range of pressure. • 2. The constants k and n change with temperature. • 3. It has no theoretical foundation. It is only an empirical formula.
Langmuir’s theory of adsorption (MonoLayer adsorption) • Langmuir proposed a better equation to explain adsorption isotherms on the basis of theoretical consideration. Langmuir proposed that the surface of solid possesses fixed number of adsorption sites per unit area and each site could adsorb only one molecule of gas. Therefore, the surface of solid is covered by mono-molecular gaseous layer. Since the solid surface is assumed to be homogeneous, the molecular adsorption at each site is independent of other adjacent sites occupied or vacant. The adsorbed gas molecules remain localised without any interaction with neighbouring molecules. One site adsorbs one molecule. When a uni-molecular layer is formed by the adsorption of gas, no further adsorption occurs, i.e., saturation is obtained. In Langmuir’s adsorption, there is a dynamic equilibrium between adsorbed gas molecules on the surface of solid and evaporation of adsorbate from the surface of adsorbent.
Before adsorption, the surface remains vacant and the rate of condensation stays maximum. As the surface becomes covered, the rate of desorption (evaporation) is less in the beginning and it increases when the surface becomes covered. At equilibrium, the number of molecules evaporating/unit time from the same surface, i.e., rate of adsorption and rate of disorption, is in equilibrium. Based on the above postulates, the rate of adsorption depends on the pressure P and the number of vacant sites on the surface (1 – θ), where q is the fraction of surface occupied by gas molecules. • Now, since the rate of adsorption is proportional to the pressure (P) of the gas as well as uncovered surface (1 – q) of the adsorbent available for adsorption, thus • Rate of adsorption αP (1 – θ) = k1P(1 – θ)
Adsorption of Gases by Solids • Several methods for determining the adsorption of gases on solid adsorbent have been derived. • In one such method, the gas is contained in a vessel of known volume at a given temperature. The pressure of the gas is measured by a monometer attached to the vessel. The adsorbent is slowly introduced into the vessel. Adsorption takes place quickly and the pressure of the gas falls which is noted on the monometer. Knowing the fall of pressure, the quantity of gas adsorbed by solid can be calculated, assuming Boyle’s law to hold good. • As a result of adsorption, the residual forces on the surface of adsorbent decrease. Hence, there is a decreaseof surface energy which appears as heat. Adsorption is invariably accompanied by decrease in the enthalpy of the system.
Factors influencing Adsorption • The magnitude of gaseous adsorption depends on the following factors: (1) temperature, (2) pressure, • (3)nature of the gas and (4) nature of adsorbent. • Effect of temperature and pressure: Since adsorption is accompanied by evolution of heat, according to Le Chatelaine's principle, the magnitude of adsorption should increase with fall in temperature and increase with increase in pressure. • Nature of gas and nature of adsorbent: The more readily soluble and easily liquefiable gases such as ammonia, hydrochloric acid, chlorine, sulfur dioxide are adsorbed more than permanent gases such as hydrogen, nitrogen and oxygen. This is because the van der Waal’s forces involved in adsorption are more predominant in the former category than the latter category of gases. Since adsorption is a surface phenomenon, it is evident that the greater the surface area per unit mass of the adsorbent, the greater is its capacity for adsorption under the given conditions of temperature and pressure.
COLLOIDS • In Greek, kolla means glue and eidos means like. Depending on their ability to diffuse in liquid medium ,substances are classified into (1) crystalloids and (2) colloids. • 1. Crystalloids: Crystalloids diffuse rapidly in solution and can pass very easily through animal and vegetable membranes. • Examples: urea, sugar and other crystalline substances. • 2. Colloids: Colloids diffuse very slowly in solution and cannot pass through vegetable and animal membranes. Examples: Gelatin, starch and proteins. But according to Graham, every substance irrespective of its nature can be a colloid or crystalloid under suitable conditions, e.g., soaps show crystalloid character in alcohol in which they are freely soluble and show colloidal character in water in which they are sparingly soluble. Thus, it is not feasible to classify the substances into crystalloids and colloids. Hence, the term colloidal substance has been replaced by colloidal state.
True Solution, Colloidal Solution and Suspension • A true solution is one in which the particles are invisible and do not settle on standing, e.g., molecules of sugar in water. • When the particles settle down on standing and can be separated very easily from the solvent, such mixtures are called suspensions. They are large enough to be visible to the naked eye or under a microscope. They are hetergeneous in nature. • In between these two limits, there are particles which are bigger than molecules but smaller than suspension particles. They cannot be seen under a microscope. Such particles are said to be in ‘colloidal state’ and when suspended in liquid are referred to as colloidal solutions. • A colloidal system is made of two phases. The substance distributed as colloidal particles is called dispersed phase. The continuous phase in which the colloidal particles are dispersed is called dispersion medium, e.g., in a colloidal solution of copper in water, copper particles constitute dispersed phase and water the dispersion medium.
Classification of Colloids or Sols • Colloids are of two types: (1) lyophilic or reversible and (2) lyophobic or irreversible. If water is the dispersion medium, the terms hydrophilic and hydrophobic are used. • Lyophilic Colloids (Lyo = Liquid, Philic = Love) • A colloidal system obtained by warming or shaking the substance with a suitable solvent is known as lyophilic colloid, e.g., gelatin, starch, protein, gum, rubber. These colloids are reversible in nature. On evaporating the dispersion medium, the residue can be reconverted into colloidal state by the addition of liquid. They are stable and cannot be easily precipitated. The affinity of colloidal particles for the medium is due to hydrogen bonding with water. If a protein (as in egg) is considered as dispersed phase, hydrogen bonding would occur between water molecules and the amino group of protein.
Lyophobic Colloids (Lyo = Liquid, Phobic = Hate) • These colloids are not formed easily. Lyophobic colloids are formed by substances such as As2S3, Fe(OH)3, gold and other metals which are sparingly soluble and thus their molecules do not readily pass through colloidal state. There is no hydrogen bonding when these colloids are dispersed in water. These colloids are known as irreversible colloids since the residue obtained by evaporating dispersion medium cannot be readily reconverted into colloid by ordinary means. These sols or colloids are not stable and can be readily precipitated. • Examples: Dispersion of gold, iron (III) hydroxide and sulphur in water.
Characteristics of Lyophilic and Lyophobic Colloids • The important properties of lyophilic and lyophobic colloids are described below. • Heterogeneous character: Unlike true solutions, colloidal systems are heterogeneous in nature. They consist of two phases, viz., dispersed phase and dispersion medium. • Ease of preparation: Lyophilic colloids are prepared by mixing the material (starch, protein) with a suitable solvent. Lyophobic sols cannot be prepared by simply mixing the solid material with the solvent.
Charge on particles: Lyophilic colloidal solutions have little or no charge at all, whereas lyophobic colloids carry positive and negative charges which give them stability. Viscosity: Lyophilic sols are viscous in nature. The particle size increases due to solvation. Viscosity of lyophobic colloid is almost similar to that of dispersion medium. Colligative properties: The colligative properties such as osmotic pressure, lowering of vapour pressure, elevation of boiling point and depression of freezing point depend on the number of solute particles in a given weight of solvent. Since the colloidal particles are aggregation of molecules, the number of particles will be very small as compared to the number of particles in true solution for a given mass of colloids. Hence, unlike true solution, colloidal system gives very low osmotic pressure and shows very small elevation of boiling point and depression of freezing point.
Optical properties: • (1) Tyndall effect: A colloidal solution exhibits Tyndall effect. When a strong beam of light is passed through a colloidal solution and observed at right angles, the path of light shows up as a hazy beam or cone. This is due to the fact that colloidal particles absorb light energy and emit it in all directions in space. This phenomenon is known as Tyndall effect and the illuminated path is known as Tyndall cone. • Tyndall effect is due to the scattering of light from the colloidal surface. The appearance of dust particles in a semi-darkened room when a sun beam enters or when a light is thrown from a projector in cinema hall is the well-known example of Tyndall effect. The dust particles are large enough to scatter light which makes the path of light visible.
Tyndall effect is used to distinguish between a true solution and a colloidal solution, since the ions of solute molecules are very small to scatter light and the beam of light passing through true solution is not visible when viewed from the side. • Conditions for the system to show tyndall effect are as follows: • (a) The diameter of particles of the dispersed phase must not be much smaller than the wavelength of light used. • (b) The difference between the refractive indices of the dispersed phase and dispersion medium must be appreciably high. Lyophilic colloids have very small difference of refractive indices and hence the Tyndall effect is very small. On the other hand, lyophobic colloids shows Tyndall effect because they satisfy the above condition.
(2) Colour: The colloidal sols are often coloured. The colour of a colloidal solution is not always the same • as that of the dispersed phase in bulk. It depends on the size and shape of the dispersed particles. For example, a gold sol is red when the particles are extremely fine and in spherical shape, but when the dispersed particles are bigger and flat it appears blue in colour. • (3) Visibility: Even under a powerful microscope, colloidal particles connot be seen. It is not possible to see a particle whose diameter is less than half the wavelength of light used. Thus, the particles of diameter less than 200 mμ cannot be seen, since the shortest wavelength of visible light is 400 mμ, e.g., gold sol cannot be seen by a microscope.
Kinetic Property of Colloidal Solutions • Brownian movement • When a colloidal sol is observed under an ultra microscope, we can find a continuous, zigzag random motion of particles (Fig. 8.14). This kinetic activity of particles of colloid is known as Brownian movement. It depends on the size of the dispersed phase and viscosity of the dispersion medium. Brownian movement is due to molecular impacts from the medium on all sides of dispersed particles. The movement of colloidal particles is much slower than that of molecules of the medium, because colloidal particles are heavier than molecules of dispersion medium.
Brownian movement proves the existence of molecules and thermal motion of molecules, since the colloidal particles acquire almost same energy possessed by the molecules of the dispersion medium. Fig. 8.14
Coagulation or precipitation • The stability of lyophobic solution is due to the adsorption of positive or negative ions by dispersed particles. • The repulsive forces between the charged particles do not allow the particles to settle down. If by any means the charge is removed, then the particles get precipitated and settle down under gravity. • The settling down or flocculation of discharged solution particles is called coagulation or precipitation of sol. Coagulation can be brought about by (1) the addition of ectrolytes, (2) electrophoresis, (3) mixing two oppositely charged sols and (4) boiling.
Addition of electrolytes: When excess of electrolyte is added to the sol, the dispersed particles get precipitated. Sol particles adsorb oppositely charged ions and get discharged. Electrically neutral particles then aggregate and settle down as precipitate. It depends on the valency of the effective ion. the higher the valency of the effective ion, greater is its power of precipitation. • Example: For precipitating As2S3 sol (negative), the precipitating power of Al+3, Ba+2, Na+ ions is in the order • Al+3 > Ba2+ > Na+ • For precipitating Fe(OH)3 sol (positive), [Fe(CN)6]3–, SO4 2–, Cl– have been used. Then the order of precipitating power is [Fe(CN)6]3– > SO4 2– > Cl– • The precipitating power of an electrolyte is expressed by its flocculation value. It is the minimum concentration in millimoles per litre required to cause the precipitation of a sol in 2 hours. the smaller the flocculation value, the higher is the precipitating power of the ion.
By electrophoresis: The charged particles of colloids migrate to oppositely charged electrodes. They get discharged and precipitated soon after they come into contact with electrode. • By mixing oppositely changed sol: Positive particles of one sol are attracted by negative particles of second sol. Mutual adsorption and precipitation of both the sols occur. • By boiling: On boiling, the collision between sol particles and water molecules removes adsorbed layer, charges from particles are taken away and precipitation occurs.
Protective action of colloids • Lyophilic colloids are stable and reversible. The stability depends on the degree of hydration. Lyophobic sols are easily precipitated when a small amount of the electrolyte is added. These sols are also stabilised if a small amount of lyophilic sol is added. • The property of lyophilic sols to prevent the precipitation of a lyophobic sol is called protection. The lyophilicsol used to protect a lyophobic sol. from precipitation is known as protective colloid. • Example: When a small amount of gelatin (lyophilic colloid) is added to a gold sol (lyophobic sol), lyophobic sol is protected which no longer gets precipitated. It is because the particles of lyophobic sol adsorb the particles of lyophilic sol, and the lyophilic colloid forms a protective cover around lyophobic sol particles.
The protective power of lyophilic sol is expressed in terms of gold number. Gold number is defined as the number of milligrams of a lyophilic colloid that will just prevent the precipitation of 10 ml of gold sol on addition of 1 ml of 10% solution of NaCl. The start of precipitation of gold sol is indicated by a colour change from red to blue with increase in particle size. The smaller the gold number, the greater is the protective action of the lyophilic colloid.
Micelles • The molecules of colloids at low concentration act as strong electrolytes. At higher concentration, they form thermodynamically stable particles of colloidal dimensions called association colloids or ‘micelles’. Micelles have lyophobic tails which get congregated and lyophilic heads which provide protection. • Example: Colloidal aggregate of soap (sodium oleate, sodium stearate) or detergent molecules formed in the solvent. Sphere represents lyophilic groups. Stalks represent lyophilic groups. The zigzag hydrocarbon tail is shown by a wavy line and the polar head by a hollow circle (Fig. 8.15).
Cleansing Action of Soaps and Detergents • The cleansing action of soap is due to (a) solubilisation of grease into the micelle and (b) emulsification of grease. • Solubilisation: When soap solution is added to a fabric, the tails of the soap anions penetrate into the grease stain. The polar heads protrude from the grease surface and form charged layer around it. The hydrocarbon tails are in the interior of the micelle and the COO– ions on the surface. The grease droplets are suspended by mutual repulsions. The emulsified stains of grease are washed away with soap solution. • Emulsification: When soap or detergent molecules are ionised in water, the anions are made of oil-soluble hydrocarbon tails and water-soluble polar heads. The polar heads protrude from grease surface and form charged layer. By mutual repulsion, the grease droplets are suspended in water. The emulsified grease stains are washed away.
Applications of colloids • 1. Electroplating of rubber: Rubber is a colloidal suspension of negatively charged particles in water. By electrophoresis, it can be made to deposit on various tools. These negative particles migrate towards anode and get deposited on it during electrolysis. • 2. Leather tanning: The coagulating ability of colloids is used in leather tanning. Leather is a positively charged colloid when mixed with wood, mutual coagulation takes place and leather surface gets hardened. • 3. Chrome tanning: Leather can be subjected to chrome tanning when hydrated chromic oxide penetrates into leather under the influence of electric field. • 4. Medicine: The ease of adsorption and assimilation of colloids makes their use in a number of medicinal and pharmaceutical preparations where colloidal gold, iron, calcium, etc., are administered (orally or injected) to raise the vitality of human system. Trivalent Al+3 and Fe+3 colloids are used in the coagulation of blood. Many skin ointments consist of physiologically active components dissolved in oil and made emulsion with water. Penicillin and streptomycin antibiotics are produced in colloidal form suitable for injection. and scatter the light of blue colour. This is an application of the Tyndall effect. • 5. Food: Many food materials are colloidal in nature. For example, butter-milk (emulsion of fat in water), cheese, fruit jelly, eggs, whipped cream, protoplasm, blood, etc. Ice cream is a dispersion of ice in cream. Bread is a dispersion of air in baked dough. • 6. Artificial rain is due to the aggregation of minute colloidal particles. Clouds are charged particles of water dispersed in air. Electrified sand is thrown into clouds to get rain. • 7. Blue colour of the sky: The outer atmosphere contains colloidal dust particles dispersed in air. As the rays of the sun strike the colloidal particles, they absorb sunlight
EXPLOSIVES AND ROCKET PROPELLANTS • Explosives and rocket fuels are closely related materials, since in both the cases exothermic chemical reaction resulting in large amount of energy takes place. The materials which were earlier used as explosives are now used as rocket fuels. • An explosive is a substance which when subjected to thermal or mechanical shock gets oxidized exothermically into the product of increased volume with sudden release of large amount of potential energy. • In explosive reactions, being exothermic, the products get heated up to very high temperatures and exert high pressure on the surroundings which can be exploited for constructive or destructive purposes. When an explosion occurs in a confined space, the high-pressure conditions developed within the system shatter the confining walls or if developed under relatively slower and controlled rate, the energy may be used to propel projectiles. The quantity of power realizable from a given weight (or volume) of an explosive is called powerto- weight (or volume) ratio which is small in case of gases. In order to achieve a better ratio, solids and/or liquids are used as explosives.
Classification of Explosives Explosives are broadly classified into three groups: • (1) primary explosives or detonators, • (2) low explosives or propellants and • (3) high explosives. • Primary Explosives or Initiating High Explosives or Detonators These are highly sensitive explosives which explode under a slight shock or blow by ignition. They have to be very carefully handled. They are used in very small quantities to initiate explosion of less sensitive explosives such as TNT. Hence, they are called initiating high explosives. Primary explosives are used in blasting caps and catridges. Examples of primary explosives include:
Leadeazide (PbN6): Beacause of its low cost, excellent initiating action, stability in storage lead azide reacts with brass, hence caps loaded with lead azide are made of aluminium. It cannot initiate explosion of TNT • Tetracene (C2H7N7O): Tetracene ignites easily and has high heat of explosion and produces large volume of gases. It has low initiating property hence it is not used by military. It is only used as a detonator. • Mercury fulminate [Hg(CNO)2]: It is more sensitive and expensive and more toxic than lead azide. Hence, its usage is limited. • Diazonitrophenol (DDNP): It is quite sensitive and can initiate explosion in less-sensitive high explosives. It is widely used in commercial blasting caps.
Low Explosives or Propellants • They do not explode suddenly but burn. The chemical reactions takes place comparatively slowly and burning proceeds from the surface inwards in layers at approximately 20 cm/s and the evolved gases disperse readily without building up pressure. Examples: Cellulose nitrate, gun powder. Low explosives are used as propellants (to propel missiles) and in fire works (i.e., pyrotechniques). Low explosives are divided into two categories depending on their applications. • Millitary explosives: RDX, HMX, TNT, nitrocellulose, tetryl, picric acid, PETN, DDNP, Lead azide, etc. • Industrial explosives: GTN, dynamite, etc., are used for industrial purposes.
High Explosives • These explosives are insensitive to mechanical shock and to the flame i.e.; they do not explode on ignition. They explode with great violence only when initiated with the help of detonators. They are stable and possess high energy content than primary explosives. The rate of explosion is about 1500 to 10,000 m per second. Under the influence of high temperature large volume of gases are evolved which produces shattering effect. High explosives are sub-divided into 1. Military high explosives, 2. Blasting or industrial purpose explosives. Examples: GTN: Glyceryltrinitrate RDY: Cyclotrimethylenetrinitroamine PETN: Pentaerythetoltetranitrate TNT: Trinitrotoluene
Ammonium nitrate: • It is half as powerful as TNT (2,4,6 trinitrotoluene) and employed in making binary explosives. It is dangerous to store near any inflammable material. It cannot be used in contact with brass, since it produces a detonator – tetrammino cupric nitrate. • TNT: • It is used in shell-firing and under-water explosives. It can be loaded in containers because of its low melting point (81°C). Because of its (1) non-hygroscopic nature and (2) inertness to metals, TNT is used as safe explosive in the manufacture, storage and transportation. • RDX or cyclonite (cyclotrimethylenetrinitroamine): • It is a powerful high explosive. It is more sensitive and less toxic than TNT. It is used both in military and industrial purpose explosive. • Picric acid (or trinitrophenol): • It is replaced largely by TNT since it forms shock-sensitive picrates with metals. Explosives, based on their state of aggregation are classified as solid (e.g.,TNT), liquid (e.g., nitroglycerine) and gaseous (e.g., mixture of oxygen and acetylene).
Requisites of Explosives • The following are the requisites of explosives: • 1. Any compound to serve as an explosive, must have atleast one easily breakable chemical bond. Most explosives contain O-Cl, N-N, N-O, N-Cl bonds. The electronegativity difference in each of these cases is either marginal or nil. In order to facilitate cleavage of bonds by thermal or mechanical shock they should have low energy of dissociation. • 2. It must be stable in thermal conditions of storage • 3. It must be cheap. • 4.It must undergo decomposition rapidly and exothermically and release large volume of gaseous products. With sharp rise in temperature gases expand rapidly. For example, 1 gram of trinitroglycerine yields about 40 ml of gases when cold, but occupies about 750 ml at the temperature of explosion
Precautions During Storage of Explosives • Following precautions should be taken while storing the explosives • 1. All wiring and electrical fittings should be properly insulated and checked regularly. • 2. Jerks or drops of explosive should not be allowed to take place. • 3. Detonators and explosives should be stored separately. • 4. Different explosives should be stored in separate boxes. • 5. Fire or smoking should be strictly prohibited within the radius of 50 m from the explosive store (magazine). • 6. Only authorized persons with magazine shoes should be allowed to the explosive store. • 7. The explosive store should not be within 500m from any working kiln or furnace. • 8. Lighting conductors should be provided to safeguard the magazine.
Rocket Propellants • A rocket engine is a tube-like missile which carries both the fuel and the oxidant, collectively referred to as propellant. The propellant is burnt in a combustion chambers and a hot jet of gases at a pressure of 300 kg cm–1 and a temperature of 3000oC comes out through a small nozzle at a supersonic velocity. This act of pushing of the gases downwards produces an equal and opposite reaction (Newton’s third law of motion) causing the rocket to move upward
Charateristics of a Good Propellant • 1. It should burn at a slow and steady rate. • 2. It should have low ignition lag [it is the time taken by the propellant to catch fire in the presence of an oxidising agent]. • 3. It should possess high density. • 4. It should be stable over a wider range of temperatures. • 5. It should be non-corrosive and non-hygroscopic. • 6. After ignition, a good propellant should not leave any residue. • 7. It should not produce toxic products • 8. It should have high specific impulse [it is the thrust delivered divided by the rate of propellant burnt]. • 9. It should produce high temperatures on combustion. • 10. It should produce low molecular weight products (such as H2, CO, CO2, N2) during combustion. • 11. It should be safe to handle and store under ordinary conditions and it should not detonate under shock, heat or impact.
Classification of Propellants The chemical propellants are classified into solid and liquid categories. • Solid Propellants Depending on their physical structure, solid propellants are divided into two main groups: homogeneous and composite (or heterogeneous). A solid propellant mixture in which the propellant mixture is intimately mixed in a colloidal state is known as homogeneous solid propellant. Examples: Nitroglycerine and nitro cellulose (gun cotton or smokeless powder combination). It is an example of single-base propellant. When a single propellant is employed, it is called a single-base propellant. In a double-based propellant, 50–55% nitrocellulose and 40–45% nitroglycerine are present. In order to make it a homogeneous plastic mass, diethyl phthalate is added as a solvent cum plasticizer. These propellants give a flame temperature of 800–1650oC and the volume of gases is about 1500 times the original volume of charge
Liquid Propellant They have several advantages over solid propellants. They are more versatile and possess high specific impulse. But the engine is more delicate and is less suitable for rough handling than the engine for solid propellant.Liquid propellants may be mono-propellants or bi-propellants. Mono-propellants: They require single storage tank and one fuel injection and control system. The fuel as well as oxidizer are in the same solution. For example hydrogen peroxide, nitromethane, hydrazine, ethylene oxide, 21.4% methanol and 78.6% hydrogen peroxide is highly reactive, metal oxides catalyse the decomposition, hence storage tanks must be made of special materials.
Bi-propellants: Bi-propellants are widely used. Liquid oxygen (LOX), hydrogen peroxide, ozone, fuming nitric acid, liquid fluorine are the common oxidizers used in bi-propellants. In bi-propellants, liquid fuel and oxidizers are kept separately and injected separately into the combustion chamber. Ethyl alcohol and 25% water is a good fuel. Addition of water reduces flame temperature and molecular mass of combustion gases, which compensates for reduction in performance. Liquid oxygen is safe, non-toxic and good oxidant, but it has to be stored under pressure in insulated containers. Ozone though a powerful oxidant but explodes at high concentration. Liquid fluorine is toxic, corrosive, volatile, very reactive, but a good oxidant. It is difficult to store and handle.
Blasting Fuses A fuse is a thin water proof canvas tube containing gun-powder (or TNT) arranged to burn at a given speed for setting off charges of explosives. Fuses are of two types: (1) saftey fuse and (2) detonating fuse. • 1. Safety fuse: It is analogous to the wick of a fire cracker. Safety fuse is employed in initiating caps, where electrical firing is not used. It consists of a small core of black powder enclosed in a convering of wrapper of water-proofed fabric. Its burning speed is approximately 30 to 40 seconds per foot. While blasting a sufficient length of the fuse is used so as to allow sufficient time for the short fires to reach a point of safety. • 2. Detonating fuse: It consists of a charge of high velocity explosives, such as TNT placed in a small bent tube. The line of fuse (cordean) is in contact with the charge through-out its length and this causes instantaneous detonation of the whole charge, irrespective of its velocity. These fuses are used for exploding charges of explosives in deep holes.
Applications of Explosives They can be used for constructive as well as destructive puposes. They do their work faster, cheaper and more effective than by any other means. • 1. Destructive uses: They are mainly used for military purposes during times of war. Explosives ammunitions include grenades, torpedoes, aerial bombs, rockets, etc. • 2. Constructive uses: In industry, they are employed for blasting ores or iron and other metals, breaking down coal, mining salts, quarrying lime stones (for road construction) blasting holes in mountains for the construction of tunnels, excavating earth for dams, seismic prospecting, dislodging rocks, drilling oil wells, etc.
Multiple-Choice Questions • 1. Physical adsorption occurs rapidly at ____________ temperature. (a) low (c) absolute zero (b) high (d) none of these • 2. Physical adsorption generally ____________ with increasing temperature. (a) decreases (b) increases (c) first increases and then decreases (d) first decreases and then increases • 3. Multi-molecule layers are formed in (a) absorption (b) physical adsorption (c) chemisorption (d) reversible absorption • 4. Freundlich isotherm is not applicable at (a) high pressure (b) low pressure (c) 273 K (d) room temperature • 5. The process of adsorption is (a) exothermic (b) endothermic (c) both endothermic and exothermic (d) none of the above • 6. The adsorption of hydrogen on charcoal is (a) physical adsorption (b) chemical adsorption (c) desorption (d) none of these • 7. In gas masks, the poisonous gases are removed by (a) adsorption (b) absorption (c) catalysis (d) none of these • 8. The process of desorption increases with ____________ of pressure (a) decrease (b) increase (c) sometimes increases, sometimes decreases (d) none of these. • 9. Which of the following is incorrect in chemisorption? (a) It is caused by bond formation (b) It is specific in nature (c) It is reversible (d) It increases with increase in temperature • 10. In an adsorption process, unimolecular layer is formed. It is (a) chemical adsorption (b) physical adsorption (c) ion exchange (d) all the above
11. The efficiency of absorption increases with increase in (a) viscosity (b) surface tension (c) surface area (d) number of ions • 12. Absorbate is one (a) where adsorption occurs (b) which concentrates on the surface (c) which evaporates from the surface (d) none of these • 13. The adsorption of gases on metal surface is called (a) catalysis (b) adsorption (c) occlusion (d) absorption • 14. At low pressure, the amount of gas adsorbed is ____________ to pressure (a) directly proportional (b) inversely proportional (c) not related (d) none of the above • 15. A colloidal solution consists of (a) a dispersed phase (b) a dispersion medium (c) a dispersed phase in a dispersion medium (d) a dispersion medium in a dispersed phase • 16. The scattering of light by the dispersed phase is called (a) Brownian movement (b) Tyndall effect (c) adsorption (d) electrophoresis • 17. In lyophobic sols, the dispersed phase has no ____________ for the medium or solvent. (a) repulsion (b) attraction (c) solvation (d) hydration • 18. The sol in which the dispersed phase exhibits a definite affinity for the medium or the solvent is called (a) lyophobic sols (b) lyophilic sols (c) emulsions (d) hydro sols • 19. Lyophilic sols are (a) reversible in nature (b) irreversible in nature (c) sometimes reversible (d) none of the above • 20. ____________ does not show Tyndall effect. (a) True solution (b) Colloidal solution (c) Suspension • 21. The continuous zig-zag movement of colloidal particle in the dispersion medium is (a) Tyndall effect (b) Brownian movement (c) electrophoresis (d) peptisation • 22. The movement of particles under an applied electric field is (a) electrophoresis (b) electro-osmosis (c) electrofiltration (d) none of these
