Chemistry - First Year Engineering, Lecture notes of Chemistry

Download Chemistry - First Year Engineering and more Lecture notes Chemistry in PDF only on Docsity! 1 4 Introduction Impurities in water i) Suspended Impurities Notes: Engg. Chemistry Unit-1 Water Technology The impurity particles like soil, sand, organic waste in water having size of particles greater than 1000Ao and visible are the suspended impurities. Removal Method-They can be separated from water by simple filtration or the sedimentation or settlement methods. ii) Colloidal Impurities Finely divided organic or inorganic matter of the colloidal particle size 10 to 1000Ao makes water turbid and do not settle down. Removal Method-The colloidal particles in water are negatively charged. They are separated by coagulation followed by sedimentation or filtration. The coagulants are like FeSO4, alum, puls floors, sodium aluminate and aluminum sulphate. iii) Dissolved Impurities Various metal salts are water soluble. Gases like O2, SO2, NH3, H2S, CO2 etc are soluble in water. Organic solids like sugars alcohols carboxylic acids, urea etc are soluble in water. Under ground and surface water due to contact with soil, rocks, contains cations like Na+, K+, Ca+2, Mg+2, Fe+2, Mn+2, Al+3 etc. and anions like Cl-, NO3 -, HCO3 -, SO 2- etc. Removal Method The inorganic dissolved impurities are removed by chemical treatment. Dissolved gases are removed by warming the water or mechanical deaeration method. The organic dissolved impurities are removed by oxidation or biochemical oxidation. iv) Biological Impurities These impurities in water include bacteria, algae, fungi, and other small size aquatic animals. Removal Method: They are removed by first filtration and killed by sterilization. The sterilization can be done by using chemicals like bleaching powder, sodium hypochlorite, chlorine, chloramines, ozone etc. and by physical methods like boiling of water, UV light. Hardness of water: It is lather forming capacity of water on treatment with soap. It is due to presence of dissolved salts of heavy metals like Ca+2, Mg+2, Fe+2, Zn+2, Al+3, Mn+2 in the form of their Carbonates, bicarbonates, sulphates, nitrates and chlorides. 2 The water which does not produce foam/lather with soap readily, is known as hard water. Hard water does not produces foam with soap because it contains impurities of hardness causing salts which form sticky white precipitates. Rain water, distilled water, softened water form foam readily with soap and it is known as soft water. Causes of Hardness: (i) Dissolution of Minerals : When rain water flows or deposits on ground or percolates deep down up to nonporous hard rock, it comes in contact with various salts in the earth crust. Water soluble salts of heavy metals gives the hardness character to water (ii) Action of Dissolved CO2 : CO2 in air is acidic gas, comes in contact with rain drops and gets dissolved in rain water. When this water comes in contact with CaCO3, MgCO3 on rocks, the insoluble carbonates are slowly attacked to convert them into soluble and hardness causing bicarbonates. e.g. (iii) Action of O2: Oxygen gas is slightly soluble in water and it can convert some insoluble minerals to soluble and hardness causing salts , On the basis of these salts, hardness is divided in to two main types (1). Temporary Hardness/Carbonate Hardness and (2). Permanent/Non carbonates Hardness 1. Temporary Hardness/Carbonate Hardness: The hardness character in water due to presence of bicarbonate and some soluble carbonate salts of heavy metals, is the temporary hardness. Such salts are Ca (HCO3)2, Mg (HCO3) 2, MgCO3, FeCO3. • These water soluble salts can be converted to their water insoluble forms by boiling the water and the temporary hardness gets removed. • This hardness can be removed by boiling the water i.e. CO2 is removed. It is also called as alkaline hardness. 5 Part II: Total hardness of water sample: 𝑦 𝑥 𝑍 𝑥 100 𝑥 1000 Hardness = 𝑉 ppm CaCO3 equivalent. Where y = Volume of EDTA Z = Molarity of EDTA V= Volume of Water sample Temporary & permanent hardness by EDTA: 1. First total hardness of a water sample is found out, by using above formula. 2. Then hardness of the boiled & filtered water sample is found, which contains only permanent hardness. 3. Temporary hardness = Total hardness - Permanent hardness. Advantages: 1. Suitable indicators available for accurate reading. 2. Color change at the end point of titration is sharp. 3. The EDTA titration method is convenient, fast, and easy. Q.100 ml of water requires 18.5 ml M/50 disodium EDTA for end point in titration .100 ml of the same water sample after boiling and filtration takes 10.7 ml of the disodium EDTA for end point in titration .Calculate Temporary & permanent hardness of water sample in pap CaCO3 equivalent. Solution:- Volume of water sample titrated (V) = 100 ml Molarity of EDTA (Z) = M/50 =0.02M Volume of EDTA (y1) = 18.5 ml for Total hardness Volume of EDTA (y2) = 10.7 ml for permanent hardness Total Hardness = 𝑦 𝑥 𝑍 𝑥 100 𝑥 1000 𝑉 ppm CaCO3 equivalent. = (18.5 x 0.02 x 100 x1000) / 100 = 370 ppm CaCO3 equivalent Permanent hardness Hardness = 𝑦 𝑥 𝑍 𝑥 100 𝑥 1000 𝑉 ppm CaCO3 equivalent. = (10.7 x 0.02 x 100 x1000) / 100 = 214 ppm CaCO3 equivalent. Temporary hardness = Total hardness - permanent hardness = 370 - 214 = 156 ppm CaCO3 equivalent. 6 3 3 Q. 50 ml of a water sample requires 12.7 ml of 0.02 M EDTA during titration. Calculate total hardness of the water. Soln: V= Volume of water sample titrated= 50 ml Y = volume of EDTA= 12.7 ml Z = molarity of EDTA= 0.02M 𝑦 𝑥 𝑍 𝑥 100 𝑥 1000 Hardness = 𝑉 ppm CaCO3 equivalent. = (12.7 x 0.02 x 105) / 50 = 508 ppm CaCO3 equivalent.  Alkalinity of water sample Water sample is alkaline due to hydrolysis of salts present or due to presence of hydroxides, carbonates & bicarbonates compounds dissolved in water. Composition of alkaline water Hydroxides : NH4OH, Ca(OH) 2 Bicarbonates : Ca(HCO3)2, Mg(HCO3)2, Fe(HCO3)2 Carbonates : MgCO3, FeCO3 Alkalinity of water sample: Theroy:  When an alkaline water is titrated with strong acid, first all OH– get neutralized, then all the CO3 – CO3 –2 ions are half neutralized to HCO –, upto phenolphthalein end pt.  Till this stage pH of mixture decreases to about 8.7 & completion of this stage is indicated by change in colour of phenolphthalein from pink to colourless.  On continued addition of acid during titration, all the HCO – in the titration mixture get neutralized & completion of this stage, is indicated by methyl orange colour change (yellow to orange) at pH 4-5. Reactions: OH– + H+ → H2O P CO –2 + H+ → HCO – M 3 3 HCO3 – + H+ → H2O + CO2 Procedure:  Take V ml of alkaline water sample in a conical flask & add 2 drops of phenolphthalein indicator in it.  Titrate this sample against std. strong acid solution ( Z N ) from burette, till pink color changes to colourless. Let the burette reading be V1 ml.  To the same solution add few drops of methyl orange indicator & continue the titration till the yellow colour of mixture changes to orange. Note the burette reading as V2 ml. 7 Phenolphthalein alkalinity = 𝑃 = 𝑉1 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 Total or Methyl orange alkalinity = 𝑀 = 𝑉2 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 Types of alkalinities: The possible combinations of alkalinities in water are : i) Only OH– ii) Only CO3 -2 ii) Only HCO3 - iv) OH– & CO3 -2 together v) CO3 -2 & HCO3 - together. OH– & HCO3 - cannot exist together as they form CO3 -2 & water. The types & amounts of alkalinities are calculated from the relation between value of P & M Alkalinity Quantity of OH - -2 Quantity of CO3 - Quantity of HCO3 P =0 0 0 M P =1/2M 0 2P 0 P = M P 0 0 P < 1/2M 0 2P M-2P P > 1/2M (2P-M) 2(M-P) 0 Numericals: Q. 50 ml of a water sample requires 9.2 ml of N/50 HCl up to phenolphthalein end point and total 13.1 ml of the acid for complete neutralization. Find the types and amount of alkalinity in the water sample. Soln: V1= 9.2 ml, V2= 13.1 ml, Z= N/50= 0.02 N, V= volume of water sample= 50 ml Phenolphthalein alkalinity = 𝑃 = 𝑉1 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 = 9.2 X 0.02 X 50 X 1000/50 = 184 ppm CaCO3 equivalent Methyl orange alkalinity = 𝑀 = 𝑉2 𝑥 𝑍 𝑥 50 𝑥 1000 𝑝𝑝𝑚 𝑉 = ( 13.1X 0.02 X 50 X 1000)/50 = 262 ppm CaCO3 equivalent As P >1/2 M, Type of alkalinity present are OH– & CO3 -2 ∴ Quantity of [OH]- = (2P-M) = 106 ppm CaCO3 equivalent. And quantity of [CO3] -2 = 2(M-P) = 156 ppm CaCO3 equivalent. Q. 50 ml of water requires 3.7 ml of 0.025 N H2SO4 upto phenolphthalein end point and further 4.8 ml upto Methyl orange end point. Calculate the types and amounts of alkalinities in the water sample. (Ans: P = 92.5, M = 212.5; Carbonate = 185 ppm; Bicarbonate = 27.5 ppm) 10 Foaming: “It is the formation of continuous foam or bubbles on the surface of water.” Causes: 1. High conc. of dissolved salts in boiler feed water. 2. Presence of oil droplets and alkalies. 3. Presence of Finely dispersed and suspended impurities. 4. Violent agitation of boiler feed water Disadvantages of priming and foaming: 1. Because of foaming actual height of water level cannot be judged well. 2. Because of priming, the salts present in the droplets enter in the part of machineries where steam is being used, thereby decreasing life of machineries. 3. The dissolved salts in droplets of wet steam on evaporation get deposited, which reduces life and efficiency of machine parts. 4. Foaming causes wet steam formation. Prevention of Priming: 1. Priming can be prevented by use of well softened and filtered water 2. Maintaining low water level in boiler prevent priming 3. Rapid changes in steam rate should be avoided. 4. Steam purifier can be used. Prevention of foaming: 1. Foaming can be prevented by adding antifoaming agents like castor oil. 2. Addition of chemicals like sodium aluminate to remove oil from boiler feed water. 3. Blow down operations can be carried out time to time. C) Sludge & scale formation Definition: Sludge: The loose & slimy deposit of precipitated salts in boiler tube, depositing at the bends and valves, affecting free flow of water, is known as sludge. Sludge formation / causes: In boiler water evaporate continuously & the concentration of salt left behind, goes on increasing after the saturation point they get precipitated. The ppt. remains in boiler tubes as loose and slimy matter. These are generally formed at cooler portions of boiler and the parts of boiler where flow rate is slow. Disadvantages:- 1. They waste some portion of heat. 2. It disturbs working of boiler and sometimes may choke up the pipe 3. It reduces the flow rate of water in boiler. Prevention:- 1. Use of water containing very low quantity of total dissolved solids. 2. Frequently making blow down operation. 11 Scale:- Definition: The hard and strong coating formed inside boiler tube by chemical reactions, which is bad conductor of heat, is called as scales. Causes- 1) Decomposition of bicarbonates: At high temp, bicarbonates decompose into sticky water insoluble material. Ca(HCO3)2 → CaCO3 ↓ + H2O + CO2 ↑ Mg( HCo3) 2 → Mg (OH )2 + 2CO2↑ 2) Hydrolysis of magnesium salts: at higher temperature, magnesium salt undergoes hydrolysis, to form sticky. MgCl2 + 2 H2O → Mg (OH)2 ↓ + 2 HCl ↑ 3) Presence of silica: Silica in the form of colloidal particles can deposit as calcium silicate or magnesium silicate as strongly adhered material. 4) Decreased solubility of CaSO4: CaSO4 has lesser solubility at higher temperature. Hence at high temp. CaSO4 present in boiler will precipitate as hard scale forming material. Disadvantage of Scales: 1) Wastage of Fuel: Scales are bad conductors of heat & result in the reduction of heat transfer to the boil. 2) Over heating of boiler: Scale reduces transfer of heat from boiler to boiler water, hence overheating is required to keep the required steam pressure. 3) Boiler Safety: Overheating of boiler is done due to scale formation. To maintain constant steam supply with required pressure boilers are overheated. Overheating makes boiler metal soft & weak which causes distortion of boiler tube & becomes dangerous in high pressure boiler. 4) Danger of explosion: When thick scale cracks due to uneven expansion, the water comes in contact with overheated boiler metal which causes large amount of steam formation & develops sudden high pressure. Due to this boiler metal may burst with explosion. Removal of scales: 1) The scale can be dissolved by addition of suitable chemicals like EDTA, sodium phosphate, calgon, etc & removed by blow down operation. 2) Thin scales can be removed by use of scrapper or wire brush. 3) Thick scales can be removed by hammer & chisel. 4) To remove hard & brittle scale thermal shocks techniques is used i.e. heating empty boiler & cooling suddenly with cold water, which causes the contraction of boiler & scales get cracked. Prevention of scales: 1) Use of softened water. 2) Adding sodium phosphate to the water. (Phosphate conditioning). 3) Frequent blow down operations to remove the sludge & precipitate rich water from boiler. 4) Adding sodium aluminates, which can trap the scale forming particles. 5) Adding organic chemicals like tannin which forms coating on the scale forming particles. This matter becomes easily removable by blow down operation. 12 Differentiate between sludge and scale in boiler Sr. No Sludge Scale 1. Sludge is a loose deposit or slimy matter Scale is hard coating 2 Sludge is less adherent on boiler metal and can be removed easily by brushes, detergents. Scale is strongly adhered to boiler metal and difficult to remove. 3 Sludge is form at the cooler parts & where flow rate is slow. Scale is form at the hotter parts. 4 Sludge may lead to chocking Scale may lead to bulging of metal tube 5 Sludge formation is due to an increase in concentration of salts in boiler water Scales are formed due to CaSO4, CaCO3,Mg(OH)2, MgSiO3,CaSiO3…etc. D. Caustic embrittlement: It is the phenomenon during which the boiler material becomes brittle due to the accumulation of caustic substances. Definition: it is the fast corrosion of boiler caused by highly alkaline condition of water, during steam generation, observed in boilers producing high pressure steam. Causes of Caustic embrittlement: 1) Presence of NaOH in boiler feed water. 2) Sodium carbonate (Na2CO3) used for softening of water (soda-lime process), may remain slightly in unreacted state and gets hydrolyzed to form hydroxide. Na2CO3 + H2O 2NaOH + CO2 3) Hydroxide makes the water alkaline and attacks at the bends, valves and edges of boiler tube. 4) The NaOH penetrates in intergranular spaces or minor cracks on the inner surface of boiler by capillary action. 5) Boiler metal becomes weak and brittle due to presence of the caustic alkali. Disadvantages: 1) The hydroxide formed, comes in contact with boiler metal through cracks or grain boundaries’ in boiler metal and formed sodium ferrite. it decreases strength of boiler metal. 2Fe + 2NaOH + O2 2NaFeO2 + H2 ↑ sodium ferrite 2) Sodium ferrite formed is brittle in nature; if attack of NaOH is continuous then whole boiler becomes brittle and cannot be used further being unsafe. 3) In this type of corrosion two types of cell are formed. A) Galvanic cell: stressed part of boiler metal acts as anode and gets corroded while unstressed part of boiler metal acts as cathode and remains protected. B) Concentration cell: Higher conc. of NaOH in cracks of stressed part acts as anode and gets corrode while lower conc. of NaOH in unstressed part of boiler metal acts as cathode and remains protected. Prevention: 1) Use Sodium phosphate for water softening instead of Na2CO3. 2) Add some Tannin or Lignin to boiler water which blocks the micro cracks. 3) Adjust pH 8-9 of boiler water. 15 Limitations: 1. Warm water should not be softened as it may decompose sodium zeolite slowly. 2. Mn++, Fe+2 like ions get permanently captured by Zeolite bed . 3. Turbid water containing colloidal impurities cannot be softened as it will block zeolite holes. 4. The output water has almost zero hardness but contains equivalent quantities of sodium salts. Applications: 1. Used for removing hardness causing ions from water. 2. Used for removing toxic metal ions & dye cations from polluted water. Numericals: Q. A zeolite bed exhausted by softening 4000 liters of a water sample requires 10 liters of 15% NaCl solution for regeneration. Calculate the hardness of water sample. Solution:- 15% NaCl = 15gm NaCl/100ml = 150gm NaCl/lit i) Amount of NaCl in mg = gm/lit of NaCl x lit. of NaCl x 1000 = 150 x 10 x 1000 = 1.5 x 106 mg of NaCl. ii) Amount of NaCl in terms of CaCO3 eq.(y) = (amount in mg x 50)/58.5 = (1.5 x 106 x 50) /58.5 = 1.28 x 106 mg of CaCO3 equivalent. iii) Hardness of water per liter of water x liters of water = ‘y’ Hardness = 1.28 x 106 /4000 = 320.51 ppm CaCO3 equivalent. Q. A zeolite bed exhausted by softening 3500 litres of a water sample, requires 10 liters of 10% NaCl solution for regeneration. Calculate the hardness of water sample. Solution:- 10% NaCl = 10gmNaCl/100ml = 100gm NaCl/lit i) Amount of NaCl in mg = gm/lit of NaCl x lit. of NaCl x 1000 = 100 x 10 x 1000 = 1 x 106 mg of NaCl. ii) Amount of NaCl in terms of CaCO3 (y) = (amount x 50)/58.5 = (1 x 106 x 50)/58.5 = 8.5 x 105 mg of CaCO3 equivalent iii) Hardness of water per liter of water x liters of water = ‘y’=8.5 x 105 Hardness = 8.5 x 105 /3500 = 242.857 ppm CaCO3 equivalent. 16 3 ii) Demineralization process / Ion Exchange Process: Principle : When water containing cations & anions, is passed through the resins, cation exchange resin captures all cation & anion exchanger resin captures all anions, to give pure & all ions free water. There are two synthetic resins used (in separate vessels) i.e. a cation exchanger resin and an anion exchanger resin, a) Cation exchanger resin : i) This is a polymer having carboxylated / sulphonated aromatic rings attached to the chain; ii) The H+ ions are loosely held on – SO − or – COO− groups and are easily exchanged with cations from water. b) Anion exchanger resin : i) This is a polymer having aromatic rings linked to the polymer chain and the rings are with quaternary ammonium group; ii) The − OH ions are loosely held and are easily exchangeable with all the negative ions from water. Process: The flow sheet of the treatment is shown in fig. 17 i) The water to be treated first enters in porous cation exchanger resin, where all cations are captured by the resin & H+ is released in exchange. e.g. H2R+ Na+ → Na2 R + 2H+ H2R + Ca ++ → Ca R + 2 H+ ii) Then the acidic output water from first resin enters in the porous anion exchanger resin where all anions in water are captured & -OH is released in exchange. e.g. . R’(OH)2 + 2 Cl- → R’Cl2 + 2-OH R’(OH)2 + CO3 -2 → R’CO3 + 2 -OH Regeneration: i) The exhausted cation exchanger is regenerated by washing with dil. HCl solution. Na2R + 2 HCl → H2R + 2 Na+ Ca R + 2 HCl → H2 R + CaCl2 ii) Cation exhausted anion exchanger resin is generated by washing with NaOH solution. R’ Cl2 + 2 NaOH → R’(OH) + 2NaCl R’ SO4 + 2 NaOH → R’(OH)2 + 2Na2SO4 Thus the water flowing out from second exchanger resin contains equivalent amount of H+ & OH- ions which combines to form H2O & there is no ionic impurity in the final water . This method is also called as deionization or demineralization process. Advantages: 1. The method gives water of zero hardness & no ionic impurities. 2. If the output water is passed throw de-gassifier, then the gaseous impurities like O2 , CO2 also get expelled, to get water of ‘distilled water’ standard. 3. Equipment occupies small space. 4. Process is easy to operate, with negligible running cost. Limitations : i) Initial investment high. ii) Process can be operated only for small scale purification of water. Desalination of brackish water: The water contains high concentration of dissolved salts is called brackish water. Desalination is the process of removing common salts (mostly NaCl) from water Commonly used Membrane techniques for Desalination are i) Electrodialysis ii) Reverse Osmosis Electrodialysis:-The process of removing ionic pollutants from water using membranes and electric field is known as Electro dialysis. Construction and working:- An Electro dialysis cell consists of a large number of paired sets of plastic membranes. The membranes are ion selective. E.g. Cation selective membrane will allow only cation to pass through it. (as this membrane consists of negatively charged fixed groups which repel anions and do not allow going it.)