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Note for Applied Chemistry - CHEM by Dheeru Sharma

  • Applied Chemistry - CHEM
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  • Dr. A PJ Abdul Kalam Tech University Lucknow - AKTU
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INDRAPRASTHA ENGINEERING COLLEGE, GHAZIABAD ENGINEERING CHEMISTRY:KAS-102/202 Module : 4 Water and Fuel Question –Answer Part –I Water Analysis Q.1 What do you understand by hardness ? Explain different types of hardness. Ans. Hardness is the property of water which prevents the lather formation of soap. Cause of hardness : Hardness of water is due to the presence of dissolved salts of calcium (Ca), magnesium (Mg) and other heavy metals. These react with soluble sodium soap to form insoluble salts of Ca and Mg. 2C17 H35COONa  CaSO 4  C17 H35COO 2 Ca   Na 2SO 4 Sodium stearate Calcium stearate 2C17 H 35 COONa  MgCl 2  C17 H 35 COO2 Mg  2NaCl Sodium stearate Magnesium stearate Thus the precipitate formed is the insoluble soap of calcium and magnesium. Hence a large quantity of soap is required to produce lather with hard water. Types of Hardness : (1) Temporary Hardness or Carbonate Hardness : It is due to the presence of dissolved salts of bicarbonates of Ca, Mg and other heavy metals and the carbonates of iron. Example: Ca HCO 3 2 , Mg HCO 3 2 . Removal : It can be removed by By boiling of the water Δ Ca HCO 3 2   CaCO 3  CO 2  H 2 O Calcium Calcium bicarbonate carbonate (insoluble) Δ MgHCO 3 2   Mg(OH) 2  CO 2  Magnesium bicarbonate Magnesium hydroxide (insoluble) (2)Permanent hardness or Non-Carbonate hardness : It is due to the presence of chlorides and sulphate of Calcium (Ca), Magnesium (Mg) and other heavy metals like iron (Fe). It cannot be removed by boiling. Example: CaCl2 , MgCl 2 , CaSO4 , MgSO4 Q.2 Why does soap not give lather with hard water ?write chemical reactions in support of your answer. Ans. A sample of hard water ,when treated with soap does not produce lather ,but forms insoluble white scum or precipitate .This is due to the formation of insoluble soaps of calcium & magnesium. 2 C17H35COONa + CaCl2 (C17H35COO)2Ca + 2NaCl Soap(Sodium stearate) (Hardness) Calcium stearate(Insoluble ppt.) 2 C17H35COONa + MgCl2 (C17H35COO)2Mg + 2NaCl 1

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Soap(Sodium stearate) (Hardness) Magnesium stearate(Insoluble ppt.)) Q 3.Why hardness is measured in terms of CaCO3 Equivalents? Give the relationship between various units of hardness. Ans. Hardness is expressed in terms of equivalents of CaCO3 (also known as Degree of hardness) because:  The molecular weight of CaCO3 is 100 , which makes calculations easy.  It is the most insoluble salt that can be precipitated in water treatment. Units of Hardness: • Milligrams per liter (mg/litre) 1ppm=1 part of CaCO3 equivalence hardness causing substance present in 106 parts of water • Milligrams per liter (mg/litre) 1mg/L=1mg of CaCO3 equivalence hardness causing substance present in one liter of water • Clare's Degree(oCl) 1o Clarke= 1part of CaCO3 equivalent hardness in 70000 parts of water • Degrees French (oFr) 1o Fr = 1 part of CaCO3 equivalent hardness per 105 parts of water • Milliequivalent per liter (milli eq./litre) 1milli eq./L= 50 mg of CaCO3 equivalence hardness Relationship between various units: 1mg/L = 1ppm = 0.1 oFr = ( milligrams ( parts per (degree per liter) million) French) 0.07oCl = (degree Clarke) 0.02 millieq./litre ( milliequivalent per liter ) Q.4. Explain zeolite method of water softening. Explain advantages and disadvantages of Zeolite process . Ans. Zeolite process or Permutit process : Zeolites are the hydrated sodium alumino silicate . The artificial zeolite is called permutit having the formula: Na2O. Al2O3. X SiO2 n H 2O (X= 2-10, n = 2-6 ) Process:  Hard water is passed through the zeolite bed at a specified rate.  The hardness causing ions such as Ca2+, Mg2+ are retained by the zeolite bed as CaZe and MgZe respectively.  Sodium salts are released during the reaction, as byproduct. 2

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 The following reactions takes place during softening process: Regeneration: Exhausted zeolite is regenerated by treating the with a concentrated Brine solution(10% NaCl). CaZe + 2 NaCl Na2Ze + CaCl2 MgZe + 2 NaCl Na2Ze + MgCl2 Exhausted Active zeolite zeolite Advantages: i. It removes the hardness completely and water of about 10 ppm hardness is produced. ii. The equipment used is compact, occupying a small space. iii. No impurities are precipitated, so there is no danger of sludge formation . iv. It requires less time for softening. v. It requires less skill for maintenance as well as operation. Disadvantages: i. The treated water contains more sodium salts than in lime-soda process. ii. This method causes caustic embrittlement. iii. High turbidity water cannot be treated efficiently by this method. Q.5 Explain ion exchange resin method. Explain advantages and disadvantages of this process. Ans. Ion Exchange Resin Method :  Ion exchange process also known as demineralization or de-ionization process.  In ion exchange process all the ions present in water are eliminated by using ion-exchange resins.  Ion-Exchange resins are insoluble cross linked long chain organic polymers with a micro porous structure, and the “functional Groups” attached to the chains are responsible for the ion-exchanging properties.  Ion exchange resins are classified as:  Cation Exchange Resins- Resins with acidic functional group are capable of exchanging H+ ions with other cations.  Anion Exchange Resins- Resins with basic functional groups are capable of exchanging OH- ions with other anions. Principle:  Hard water is allowed to pass through cation exchange resins, which remove Ca+2 and Mg+2 ions and exchange equivalent amount of H+ ions. Cation Exchange Resins 2RH+ + Ca+2 R2Ca + 2H+ + +2 2RH + Mg R2Mg + 2H+  After cation exchange resin , the water is passed through anion exchange column which removes all the anions like Cl- ,SO4-2 etc. and equivalent amount of OH- ion is released from this column to water. 3

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Anion Exchange Resins R’OH + ClR’Cl + OH2R’OH + SO4-2 R2SO4+ 2OH H+ and OH- ions, thus released in water from respective cation and anion exchange columns, get combined to produce water molecules. H+ + OHH2O Regenration:  The exhausted cation exchanger is regenerated by passing it through dilute sulphuric acid. R2Ca+2 + 2H+ 2RH+ + Ca+2 Exhausted cation resin  The exhausted anion exchanger is regenerated by passing a dilute solution of NaOH. R2SO4-2 + 2OH2R’OH- + SO4-2 Exhausted anion resin Fig: Demineralization of water Merits of Ion-exchange process: i. The process can be used to soften highly acidic or alkaline water. ii. It produces water of very low hardness (2 ppm) Demerits of Ion-exchange process: i. The equipment is costly and more expensive chemicals are needed. ii. If water contains turbidity, the output of the process is reduced. Q. 6 Discuss the lime -soda method. Explain advantages and disadvantages of this methods. Ans. It is a process in which Lime (Ca(OH)2) and soda (Na2CO3) are added to the hard water to convert the soluble calcium and magnesium salts to insoluble compounds by a chemical reaction. The CaCO3 and Mg(OH)2 so precipitated are filtered off and removed easily. Ca HCO 3 2  Ca OH2  2CaCO 3  2H 2 O MgCl 2  Ca OH2  MgOH2  CaCl 2 4

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