Engineering Chemistry · June 2025

(i) Sludge and scale

In a boiler, water evaporates continuously and the concentration of dissolved salts keeps rising. When the solution becomes saturated, the salts separate out as a precipitate on the inner walls. If the precipitate is loose and slimy it is called sludge; if it forms a hard, adherent coating it is called scale.

Sludge

Sludge is a soft, loose, slimy precipitate formed in the comparatively colder parts of the boiler and where the flow is slow (bends, bottom of the boiler). It is formed by salts whose solubility is greater in hot water than in cold, such as MgCO₃, MgCl₂, MgSO₄ and CaCl₂.

Disadvantages: sludge is a poor conductor of heat, so fuel is wasted; excess sludge settles in pipes and chokes them, disturbing circulation; when formed along with scale it gets entrapped in the scale and both are deposited together.

Prevention: use well-softened water and carry out frequent blow-down.

Scale

Scale is a hard deposit firmly sticking to the inner surface of the boiler, very difficult to remove even with a hammer and chisel. Causes of scale formation:

1. Decomposition of calcium bicarbonate (low-pressure boilers): Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. CaCO₃ scale is soft and is the main scale in low-pressure boilers.
2. Deposition of calcium sulphate (high-pressure boilers): solubility of CaSO₄ decreases with rise in temperature, so it separates out as a hard, adherent scale on the hot portions.
3. Hydrolysis of magnesium salts: MgCl₂ + 2H₂O → Mg(OH)₂↓ + 2HCl. Mg(OH)₂ forms a soft scale.
4. Presence of silica: SiO₂ forms calcium and magnesium silicates (CaSiO₃, MgSiO₃), which give a very hard, glassy scale that is very difficult to remove.

Bad effects of scale

• Wastage of fuel: scale is a poor conductor of heat, so the metal must be heated more to transfer the same heat. Approximate fuel wastage: 0.325 mm scale → 10%, 0.625 mm → 15%, 1.25 mm → 50%, 2.5 mm → 80%, 12 mm → 150%.
• Lowering of boiler safety: the metal is overheated, becomes soft and weak, and may fail under high steam pressure.
• Decrease in efficiency: scale deposits in valves and condensers and partially chokes them.
• Danger of explosion: scale expands differently from the metal; when it cracks, water suddenly comes in contact with the overheated metal, a large amount of steam forms at once and high pressure may burst the boiler.

Difference between sludge and scale

Prevention and removal of scale

• External treatment: soften feed water by lime-soda, zeolite or ion-exchange process before use.
• Internal treatment (conditioning):
  • Colloidal conditioning (low-pressure boilers): kerosene, tannin or agar-agar coat the particles so they stay as loose sludge.
  • Phosphate conditioning (high-pressure boilers): 3CaCl₂ + 2Na₃PO₄ → Ca₃(PO₄)₂↓ + 6NaCl (soft sludge).
  • Carbonate conditioning (low-pressure boilers): CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄.
  • Calgon conditioning: sodium hexametaphosphate forms a soluble complex: Na₂[Na₄(PO₃)₆] + 2CaSO₄ → Na₂[Ca₂(PO₃)₆] + 2Na₂SO₄.
  • Sodium aluminate: gives gelatinous Al(OH)₃ and Mg(OH)₂ that entrap fine particles as sludge.
• Removal: scraping with wire brush (soft scale), thermal shocks (heating and sudden cooling), chemical washing (5–10% HCl for CaCO₃; EDTA for CaSO₄), and frequent blow-down.

Alkalinity (ppm as CaCO₃) = V_acid × N_acid × 50 × 1000 / V_sample (50 = equivalent weight of CaCO₃).

Phenolphthalein alkalinity (P)

Methyl orange (total) alkalinity (M)

Type of alkalinity

Here P = 200 ppm and M/2 = 112.5 ppm, so P > M/2. Hence the alkalinity is due to OH⁻ and CO₃²⁻ ions (HCO₃⁻ is absent).

The three viscosities at 100 °F are given in the order: test oil, Gulf oil, Pennsylvanian oil. All three have the same viscosity at 210 °F.

A VI above 100 means the test oil's viscosity changes with temperature even less than that of the Pennsylvanian standard (its viscosity at 100 °F, 61 s, is lower than H = 420 s), as is typical of synthetic oils or oils containing VI improvers.

The ion-exchange (demineralisation) process removes all cations and anions from water using ion-exchange resins. These are insoluble, cross-linked long-chain organic polymers (for example styrene–divinylbenzene copolymer) with a microporous structure, carrying functional groups that can exchange ions.

• Cation exchange resins (RH₂): contain acidic groups –SO₃H or –COOH; they exchange H⁺ for cations.
• Anion exchange resins (R′(OH)₂): contain basic groups such as quaternary ammonium –NR₃⁺OH⁻ or amino groups; they exchange OH⁻ for anions.

Process

Hard water is passed first through the cation-exchange column, which removes Ca²⁺, Mg²⁺, Na⁺ etc. and releases an equivalent amount of H⁺:

The acidic water then passes through the anion-exchange column, which removes Cl⁻, SO₄²⁻, HCO₃⁻ and releases OH⁻:

The water coming out is free from all ions and is called deionised or demineralised water. Dissolved CO₂ may be removed in a degasifier.

Regeneration

When the resins are exhausted, the cation exchanger is regenerated with dilute HCl (or H₂SO₄) and the anion exchanger with dilute NaOH. The washings are drained off.

Advantages

• Produces water of very low hardness (about 0–2 ppm) — suitable for high-pressure boilers.
• Removes all ions, including silica (with strongly basic resin), so total dissolved solids become very low.
• Works for highly acidic or alkaline water.

Disadvantages

• Equipment and resins are costly; regenerating chemicals (acid, alkali) are expensive.
• Turbid water reduces efficiency, since suspended matter clogs the resin; water must be filtered first.

Numerical

The NaCl used for regeneration is exactly equivalent to the hardness removed by the exhausted (90%) part of the zeolite bed, which removed all the hardness of 10,000 L of water.

All three are answered below; write any two in the exam.

(i) Aniline point

Aniline point is the minimum equilibrium solution temperature at which equal volumes of aniline and the oil are completely miscible.

Determination: equal volumes of aniline and oil are taken in a test tube and heated with stirring until they mix to form a homogeneous solution. The mixture is then cooled slowly; the temperature at which the two phases separate (turbidity appears) is the aniline point.

Significance: aniline dissolves aromatic hydrocarbons easily, so an oil with high aromatic content has a low aniline point. Aromatic hydrocarbons dissolve and swell natural and many synthetic rubbers. Therefore a lubricant that comes in contact with rubber seals, gaskets, packings and hoses should have a high aniline point. A high aniline point indicates a mainly paraffinic oil with low aromatic content.

Thermoplastic versus thermosetting resins

Chain growth versus step growth

Corrosion is the gradual destruction or deterioration of a metal by chemical or electrochemical reaction with its environment. Examples: rusting of iron (formation of reddish-brown hydrated ferric oxide, Fe₂O₃·xH₂O), green coating on copper (basic copper carbonate, CuCO₃·Cu(OH)₂) and tarnishing of silver (Ag₂S). Most metals occur in nature as oxides, carbonates or sulphides; corrosion is the reverse of extraction (metallurgy), in which the metal returns to a more stable, combined state.

Types of corrosion

1. Dry (chemical) corrosion: oxidation corrosion, corrosion by other gases, liquid-metal corrosion.
2. Wet (electrochemical) corrosion: galvanic (bimetallic) corrosion; concentration-cell corrosion (differential aeration, water-line, pitting, crevice corrosion); stress corrosion and caustic embrittlement; intergranular corrosion; soil and microbiological corrosion; erosion corrosion.

Prevention of corrosion

• Proper design: avoid contact of dissimilar metals, crevices, sharp corners and stagnant areas; allow free drainage; keep anode area large compared with cathode area.
• Proper choice of material: use pure metals, noble metals or corrosion-resistant alloys (stainless steel).
• Cathodic protection: sacrificial anode method and impressed current method.
• Modifying the environment: de-aeration, dehumidification, removal of acidic gases, and addition of inhibitors (anodic inhibitors such as chromates, phosphates; cathodic inhibitors such as amines, thiourea, salts of As and Sb).
• Protective coatings: metallic coatings (galvanising with Zn, tinning, electroplating, metal cladding), inorganic coatings (anodising, phosphating, chromate coating, enamels) and organic coatings (paints, varnishes, lacquers).

Theory of mechanism of corrosion

(a) Dry corrosion occurs by direct reaction of the metal with gases, forming a surface film; its progress depends on the nature of the film (Pilling–Bedworth rule): M → Mⁿ⁺ + ne⁻ and ½O₂ + 2e⁻ → O²⁻, giving the oxide.

(b) Wet (electrochemical) corrosion occurs through anodic and cathodic areas:

Electrochemical (wet) corrosion occurs when (a) two dissimilar metals are in contact with each other in a conducting liquid (electrolyte), or (b) a metal is exposed to different concentrations of oxygen or electrolyte. Separate anodic and cathodic areas are formed and a current flows between them, as in a galvanic cell. Corrosion always takes place at the anode.

Mechanism

At anode (oxidation): Fe → Fe²⁺ + 2e⁻

At cathode (reduction) — two possible reactions:

1. Hydrogen evolution (acidic medium, no oxygen; e.g., iron in an industrial acid waste): 2H⁺ + 2e⁻ → H₂↑. Overall: Fe + 2H⁺ → Fe²⁺ + H₂. Usually the anode area is large and cathode small.
2. Oxygen absorption (neutral or basic aqueous medium with dissolved oxygen, the most common case): ½O₂ + H₂O + 2e⁻ → 2OH⁻.

The Fe²⁺ ions (from anode) and OH⁻ ions (from cathode) diffuse towards each other and form ferrous hydroxide, which is further oxidised to rust:

With limited oxygen, black magnetite Fe₃O₄·6H₂O is formed. Note that the metal is lost at the anode, while rust is deposited near the cathode or between anode and cathode.

Numerical: rust formed from 100 kg iron

Phase rule (J. W. Gibbs): for a heterogeneous system in equilibrium, influenced only by temperature, pressure and concentration,

where F = degrees of freedom, C = number of components and P = number of phases.

Phase (P)

A phase is any homogeneous, physically distinct and mechanically separable part of a system, separated from other parts by definite boundary surfaces.

• A gas or a mixture of gases is always one phase (gases mix completely).
• Two completely miscible liquids form one phase; two immiscible liquids (benzene–water) form two phases.
• Each solid is a separate phase: CaCO₃(s) ⇌ CaO(s) + CO₂(g) has 3 phases.
• Ice ⇌ water ⇌ vapour has 3 phases; a sugar solution in water is 1 phase.

Component (C)

The number of components is the smallest number of independently variable chemical constituents by means of which the composition of every phase can be expressed, directly or in the form of a chemical equation.

• Water system (ice, water, vapour): every phase is H₂O, so C = 1.
• CaCO₃ ⇌ CaO + CO₂: any two species fix the third (CaO = CaCO₃ − CO₂), so C = 2.
• NH₄Cl(s) ⇌ NH₃(g) + HCl(g): C = 1 if NH₃ and HCl are in equal amounts; C = 2 if one is in excess.
• Sulphur system (rhombic, monoclinic, liquid, vapour): C = 1. Salt solution: C = 2.

Degree of freedom (F)

The degree of freedom (variance) is the minimum number of independent variable factors — temperature, pressure and concentration — that must be specified to define the system completely.

• Ice ⇌ water ⇌ vapour at the triple point: F = 1 − 3 + 2 = 0 (non-variant).
• Water ⇌ vapour: F = 1 − 2 + 2 = 1 (univariant); fixing either T or P fixes the other.
• Water vapour alone: F = 1 − 1 + 2 = 2 (bivariant); both T and P must be stated.
• A mixture of N₂ and H₂ gases: C = 2, P = 1, F = 3 (T, P and composition).

Significance of triple point and metastable curve

• Triple point: it is the only condition (0.0098 °C, 4.58 mm) where ice, water and vapour exist together; it is non-variant (F = 0), exactly reproducible and is therefore used as the fixed point for defining the Kelvin scale (273.16 K). It is also the lowest pressure at which liquid water can exist; below it, ice sublimes directly (basis of freeze-drying).
• Metastable curve OA′: it shows that water can be supercooled below its freezing point and still exist as liquid. Because its vapour pressure is higher than that of ice at the same temperature, supercooled water is unstable and changes to ice on slight disturbance or seeding. It explains phenomena such as freezing rain and the need for nucleation in ice formation.

An efficient lubricant should have the following properties:

• Suitable viscosity for the load and speed, and a high viscosity index, so viscosity changes little with temperature.
• High flash and fire point, above the working temperature, for safety against fire.
• Low cloud and pour point, so it flows in cold conditions.
• High aniline point, so it does not attack rubber seals.
• Low neutralisation (acid) value, so it does not corrode metal parts; a good oil has acid value below about 0.1.
• High oiliness, i.e., good ability to adhere to the metal surface under high load.
• Low volatility and good oxidation stability; low carbon residue.
• Good demulsifying property (low steam emulsion number), so it separates quickly from water.
• Chemically stable, non-corrosive and free from moisture and suspended impurities.

Numerical: neutralisation value

Neutralisation (acid) value = mg of KOH required to neutralise the free acids in 1 g of oil.

A high neutralisation value shows that the oil contains excess free acids (from oxidation or poor refining). Such acids corrode machine parts, and the oil also tends to form sludge and gum.

No single technique gives the complete structure of a compound. UV and IR give complementary information:

• UV spectroscopy tells whether the compound contains chromophores and conjugation, and the extent of conjugation (from λ_max and ε_max). Strong absorption above 200 nm indicates conjugated double bonds or an aromatic ring; a weak band at 270–300 nm indicates n→π* of a carbonyl group; no absorption above 200 nm indicates a saturated compound.
• IR spectroscopy identifies the functional groups present (O–H, N–H, C=O, C≡N, C=C, aromatic ring) and, through the fingerprint region, confirms the identity of the compound by comparison with a reference.

Combining these with the molecular formula (and degree of unsaturation) narrows the possibilities to one structure.

Example

An unknown compound has molecular formula C₈H₈O.

UV data: strong band at λ_max ≈ 246 nm (ε ≈ 13,000) and a weak band near 280 nm. The strong band is a π→π* transition of a carbonyl group conjugated with a benzene ring (benzoyl chromophore); the weak band is n→π* of C=O. So C=O is conjugated with the ring.

IR data:

Conclusion: a monosubstituted benzene ring bearing a conjugated ketone group with a methyl group: C₆H₅–CO–CH₃, acetophenone. UV established conjugation of C=O with the ring; IR established that the carbonyl is a ketone and that there is no –OH or –CHO group.

Electronic spectroscopy (UV-visible spectroscopy) deals with the absorption of ultraviolet and visible radiation (about 200–800 nm), which causes transitions of electrons between electronic energy levels (σ→σ*, n→σ*, π→π*, n→π*). Each electronic transition is accompanied by vibrational and rotational changes, so broad absorption bands are observed. Quantitative work is based on the Beer–Lambert law, A = log(I₀/I) = εcl.

Numerical

Principle: In the lime-soda process, calculated amounts of lime [Ca(OH)₂] and soda [Na₂CO₃] are added to hard water. They convert the soluble hardness-causing salts into insoluble CaCO₃ and Mg(OH)₂, which settle out and are removed by filtration.

Reactions of lime

Reactions of soda

Note that permanent Mg hardness (MgCl₂, MgSO₄) needs both lime and soda: lime converts it to an equivalent Ca salt, which is then removed by soda.

Formulae for calculation

74 and 106 are the molecular weights of Ca(OH)₂ and Na₂CO₃; 100 is that of CaCO₃. NaCl, KCl, Na₂SO₄, SiO₂ do not consume lime or soda.

Numerical

All four are answered below; write any two in the exam.

(i) Electron affinity

Electron affinity is the amount of energy released when an electron is added to a neutral, isolated gaseous atom to form a gaseous anion: X(g) + e⁻ → X⁻(g) + energy. (In terms of electron gain enthalpy, ΔH_eg is negative when energy is released.) It is expressed in kJ mol⁻¹ or eV.

Across a period electron affinity generally increases from left to right, because atomic size decreases and effective nuclear charge increases, so the incoming electron is attracted more strongly. Halogens have the highest values. Exceptions: Be and Mg (stable ns² configuration), N (stable half-filled 2p³) have very low or nearly zero values; noble gases (complete octet) have zero or positive values.

Down a group electron affinity generally decreases, because atomic size increases. But F (−328 kJ/mol) has a lower value than Cl (−349 kJ/mol), and O less than S, because the very small 2p subshell of F and O is compact and the added electron feels strong repulsion from the electrons already present. Order for halogens: Cl > F > Br > I.

Second electron affinity is always endothermic (energy must be supplied) because the incoming electron is repelled by the negative ion: O⁻(g) + e⁻ → O²⁻(g), ΔH = +744 kJ/mol.

Factors: atomic size, effective nuclear charge and the stability of the electronic configuration.

All three are answered below; write any two in the exam.

(i) Electronegativity

Electronegativity is the tendency of an atom in a molecule to attract the shared pair of electrons towards itself. It is a property of a bonded atom and has no units. Common scales: Pauling scale (based on bond energies; F = 4.0, highest; Cs = 0.7, lowest), Mulliken scale (EN = (IE + EA)/2) and Allred–Rochow scale (EN = 0.359 Z_eff/r² + 0.744).

Variation across a period: electronegativity increases from left to right — Li 1.0, Be 1.5, B 2.0, C 2.5, N 3.0, O 3.5, F 4.0. Reason: atomic size decreases and effective nuclear charge increases, so the nucleus attracts the bonding electrons more strongly.

Variation down a group: electronegativity decreases — F 4.0, Cl 3.0, Br 2.8, I 2.5; Li 1.0, Na 0.9, K 0.8, Rb 0.8, Cs 0.7. Reason: atomic size increases and the bonding electrons are farther from the nucleus and more shielded by inner electrons.

Other factors: higher oxidation state gives higher EN (Fe³⁺ > Fe²⁺); more s-character in hybrid orbitals gives higher EN (sp > sp² > sp³).

Applications: predicting bond polarity and ionic character (ΔEN > 1.7 → mainly ionic), metallic and non-metallic character, acidic/basic nature of oxides, and bond lengths.