Engineering Chemistry-II: Comprehensive Study Notes (2076-2081 Question Paper Solutions)

Hardness of Water and Its Removal Processes

Hardness of water is primarily caused by the presence of soluble bicarbonates, chlorides, and sulfates of calcium and magnesium. These ions prevent water from producing a lather easily with soap. Specific salts responsible include Calcium sulphate ($CaSO_4$), Magnesium sulphate ($MgSO_4$), Calcium chloride ($CaCl_2$), Magnesium chloride ($MgCl_2$), Calcium bicarbonate ($Ca(HCO_3)_2$), and Magnesium bicarbonate ($Mg(HCO_3)_2$).

Removal of Temporary Hardness

Temporary hardness, caused specifically by bicarbonates, can be removed through two primary methods:

1. By the Application of Heat: Heating the water sample causes soluble bicarbonates to decompose into insoluble carbonates.

• $Mg(HCO_3)_2 \xrightarrow{\Delta} MgCO_3 \downarrow + CO_2 + H_2O$
• $Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3 \downarrow + CO_2 + H_2O$

1. By Reacting with Calcium Hydroxide (Clark's Process): Treating hard water with lime water ($Ca(OH)_2$) converts soluble bicarbonates into insoluble carbonates.

• $Ca(OH)_2 + Ca(HCO_3)_2 \rightarrow 2CaCO_3 \downarrow + 2H_2O$
• $Ca(OH)_2 + Mg(HCO_3)_2 \rightarrow CaCO_3 \downarrow + MgCO_3 \downarrow + 2H_2O$

Removal of Permanent Hardness by Permutit Method

The Permutit process utilizes a complex compound known as hydrated sodium aluminium silicate ($Na_2Al_2Si_2O_8 \times xH_2O$), also called zeolite. In this process, the sodium ions in the permutit ($Na_2Z$) are exchanged for $Ca^{++}$ and $Mg^{++}$ ions present in hard water, causing the precipitation of calcium and magnesium aluminium silicates.

• Reactions for removal: $CaCl_2 + Na_2Z \rightarrow CaZ \downarrow + 2NaCl$$MgSO_4 + Na_2Z \rightarrow MgZ \downarrow + Na_2SO_4$ (Where $Z = Al_2Si_2O_8 \times xH_2O$)

After a period of use, the zeolite becomes exhausted as it is entirely converted to calcium and magnesium zeolites ($CaZ$ and $MgZ$). It is regenerated by treating it with a 10\% sodium chloride ($NaCl$) solution:

• $CaZ + 2NaCl \rightarrow Na_2Z + CaCl_2$
• $MgZ + 2NaCl \rightarrow Na_2Z + MgCl_2$

Industrial Manufacture of Important Chemicals

Manufacture of Nitric Acid ($HNO_3$) by Ostwald's Process

The Ostwald's process involves the catalytic oxidation of ammonia in a series of steps:

1. Catalytic Oxidation of Ammonia: Ammonia is oxidized to nitric oxide ($NO$) in the presence of air at $800^\circ C$ over a platinum gauze or a rhodium-platinum alloy catalyst. $4NH_3 + 5O_2 \xrightarrow{Pt/Rh, 800^\circ C} 4NO + 6H_2O + \text{Heat}$

2. Oxidation to Nitrogen Dioxide: The generated nitric oxide is cooled and allowed to react with air to form nitrogen dioxide ($NO_2$). $2NO + O_2 \rightarrow 2NO_2$

3. Absorption: Nitrogen dioxide is passed through an absorption tower packed with acid-proof stone or quartz, where it reacts with water and air to form nitric acid. $4NO_2 + 2H_2O + O_2 \rightarrow 4HNO_3$

Manufacture of Sulphuric Acid ($H_2SO_4$) by Contact Process

Sulphuric acid, known as the "king of chemicals," is manufactured through the following steps:

1. Production of Sulphur Dioxide: Sulphur is burnt in air, or iron pyrites are roasted. $S_8 + 8O_2 \rightarrow 8SO_2$$4FeS_2 + 11O_2 \rightarrow 2Fe_2O_3 + 8SO_2$

2. Catalytic Oxidation of $SO_2$ to $SO_3$: Pure $SO_2$ is treated with air at $450^\circ C$ and $2-3\,atm$ pressure in the presence of Vanadium Pentoxide ($V_2O_5$) or platinum asbestos. $2SO_2 + O_2 \xrightleftharpoons{V_2O_5} 2SO_3 + 196\,kJ$

3. Formation of Oleum: $SO_3$ is absorbed in concentrated $H_2SO_4$ (94-98\%) to form Pyrosulphuric acid, commonly called Oleum. $SO_3 + H_2SO_4 \rightarrow H_2S_2O_7$

4. Dilution: Oleum is diluted with a calculated amount of water to obtain concentrated sulphuric acid. $H_2S_2O_7 + H_2O \rightarrow 2H_2SO_4$

Key plant components include the Pyrite burner, Dust catcher, Cooling pipes, Washing tower (removes dust), Drying tower (using conc. $H_2SO_4$), Arsenic purifier (using $Fe(OH)_3$), Testing Box (for light scattering), and the Contact tower.

Manufacture of Ammonia ($NH_3$) by Haber's Process

Ammonia is synthesized directly from nitrogen and hydrogen in a $1:3$ volume ratio. $N_2 + 3H_2 \xrightleftharpoons{Fe, Mo} 2NH_3 + 98.6\,kJ\,mol^{-1}$

Conditions for high yield:

• Temperature: approximately $450^\circ C$.
• Pressure: between $200-500\,atm$.
• Catalyst: Finely divided iron ($Fe$).
• Promoter: Molybdenum ($Mo$).

Carbon and Its Allotropes

Allotropes are different forms of the same element that share chemical properties but differ significantly in physical properties.

Diamond Structure and Properties

Diamond is a crystalline allotrope of carbon where each carbon atom is tetrahedrally bonded to four other carbon atoms.

• Bond Angle: $109^\circ 28'$
• Bond Length: $1.54\,\text{\AA}$
• Conductance: Bad conductor of heat and electricity because it contains no free electrons.
• Hardness: Considered the hardest known substance due to strong covalent bonding.
• Applications: Used in jewelry (gemstones), glass cutting, rock drilling borers, and polishing hard materials.

Graphite Structure and Properties

In graphite, each carbon atom is covalently bonded to three other carbon atoms in a hexagonal planar arrangement.

• Bond Angle: $120^\circ$
• Bond Length: $1.42\,\text{\AA}$
• Van der Waals Forces: Layers are held together by weak forces, making it soft and slippery.
• Conductance: Good conductor of electricity due to the presence of one free electron per carbon atom.
• Applications: Used as a dry lubricant for high-temperature machinery, in electrodes, crucibles, and as "pencil lead."

Properties of Methane ($CH_4$)

Methane is a saturated hydrocarbon and the first member of the alkane series.

• Nitration: Replacement of hydrogen with a $-NO_2$ group at $450-500^\circ C$. $CH_4 + HNO_3 \xrightarrow{450-500^\circ C} CH_3NO_2 + H_2O$
• Combustion: Burns in oxygen to release energy, $CO_2$, and water. $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$
• Pyrolysis (Cracking): Breaking down of propane or higher alkanes into alkenes and hydrogen at $500-800^\circ C$.
• Steam Reaction: Mixture of methane and steam passed over Nickel at $800^\circ C$ produces water gas ($CO + H_2$). $CH_4 + H_2O \xrightarrow{Ni, 800^\circ C} CO + 3H_2$

Important Organic Mechanisms and Concepts

Markovnikov's Rule

In the addition of an unsymmetrical reagent (like $HCl$) to an unsymmetrical alkene, the hydrogen atom of the reagent attaches to the carbon atom that already possesses the higher number of hydrogen atoms. Example ($Propene + HCl$): $CH_3-CH=CH_2 + HCl \rightarrow CH_3-CHCl-CH_3 \text{ (2-Chloropropane)}$

Peroxide Effect (Kharasch Effect)

The addition of $HBr$ to an unsymmetrical alkene in the presence of organic peroxides follows an anti-Markovnikov path. The positive part of the reagent (H) adds to the carbon with fewer hydrogen atoms. Example ($Propene + HBr + Peroxide$): $CH_3-CH=CH_2 + HBr \xrightarrow{\text{Peroxide}} CH_3-CH_2-CH_2Br \text{ (1-Bromopropane)}$

Homologous Series and Functional Groups

A homologous series is a group of organic compounds with the same functional group and general formula, where each subsequent member differs by a $-CH_2$ unit (14 a.m.u.).

• Characteristics: Similar chemical properties, gradual change in physical properties (boiling/melting points increase with mass).
• Functional Group: An atom or radical determining the characteristic chemical properties (e.g., $-COOH$ for carboxylic acids, $-CHO$ for aldehydes, $-OH$ for alcohols).

Metal Chemistry and Alloys

Alkali Metals (Group IA)

Elements include Lithium, Sodium, Potassium, Rubidium, Caesium, and Francium. They are highly reactive, have one valence electron ($ns^1$), and exhibit a $+1$ oxidation state.

• Sodium: Silvery white, soft, density $0.97\,g/cm^3$. Reacts violently with water: $2Na + 2H_2O \rightarrow 2NaOH + H_2$

Alkaline Earth Metals (Group IIA)

Elements include Beryllium, Magnesium, Calcium, Strontium, Barium, and Radium. They exhibit a $+2$ oxidation state ($ns^2$).

• Calcium: Silvery white, reacts with water to form hydroxides and with acids to liberate hydrogen. $Ca + 2H_2O \rightarrow Ca(OH)_2 + H_2 \uparrow$$Ca + 2HCl \rightarrow CaCl_2 + H_2 \uparrow$

Coinage Metals (Group IB)

Includes Copper ($Cu$), Silver ($Ag$), and Gold ($Au$). Characterized by high electrical and thermal conductivity and used historically for making coins.

• Copper Properties: Fleish pink color, malleable, ductile. Copper does not displace hydrogen from dilute $H_2SO_4$ because it is less reactive than hydrogen.

Alloys and Amalgams

An alloy is a homogeneous mixture of two or more metals (or a metal and non-metal) created to improve properties like hardness, corrosion resistance, and strength.

• Ferrous Alloys: Contain iron (e.g., Steel, Stainless Steel).
• Non-ferrous Alloys: Do not contain iron (e.g., Brass ($Cu+Zn$), Bronze ($Cu+Sn$)).
• Amalgam: An alloy containing Mercury ($Hg$) as a constituent (e.g., Sodium amalgam).

Environmental Chemistry and Pollutants

Acid Rain

Caused by high levels of Nitrogen and Sulphur oxides ($NO_x, SO_x$) from industries and automobiles dissolving in rainwater to form acids ($H_2SO_4, HNO_3$).

• Effects: Increases soil acidity, leaches heavy metals (Pb, Al), corrodes historical monuments/buildings, and kills aquatic life.

Oxides of Carbon

• Carbon Dioxide ($CO_2$): A primary greenhouse gas responsible for global warming.
• Carbon Monoxide ($CO$): Highly toxic gas that binds to hemoglobin in human blood to form Carboxyhemoglobin ($CO-Hb$), reducing the blood's oxygen-carrying capacity and leading to suffocation.

The Nitrogen Cycle

A continuous process of recycling nitrogen in the atmosphere.

• Fixation: Conversion of atmospheric $N_2$ into usable forms via lightning or symbiotic bacteria (legumes). During lightning: $N_2 + O_2 \rightarrow 2NO \rightarrow 2NO_2 \xrightarrow{H_2O+O_2} 4HNO_3$

Laboratory Analysis and Reagents

Aqua Regia

A mixture of 1 part concentrated $HNO_3$ and 3 parts concentrated $HCl$. It produces nascent chlorine, enabling it to dissolve noble metals like Gold ($Au$) and Platinum ($Pt$). $HNO_3 + 3HCl \rightarrow NOCl + 2H_2O + 2[Cl]$$Au + 3[Cl] \rightarrow AuCl_3$

Hydrogen Sulphide ($H_2S$) as an Analytical Reagent

Used in salt analysis for detecting basic radicals in groups II and IIIB.

• Group II (Acidic Medium): Precipitates radicals like Copper as sulphides ($CuS$, black).
• Group IIIB (Alkaline Medium): Precipitates Zinc ($ZnS$, white) and Manganese ($MnS$, flesh-colored).
• Reducing behavior: Reduces acidified $KMnO_4$ (purple to colorless) and $SO_2$ to Sulphur.