Engineering Chemistry · December 2023

Zeolites are hydrated sodium aluminosilicates, Na₂O·Al₂O₃·xSiO₂·yH₂O (x = 2–10, y = 2–6), written simply as Na₂Ze. They may be natural (green sand, natrolite) or synthetic (permutit, made by heating china clay, feldspar and soda ash). They can exchange their Na⁺ ions for Ca²⁺ and Mg²⁺ ions of hard water.

Process

Hard water is passed through a bed of zeolite in a cylinder. Ca²⁺ and Mg²⁺ are retained by the zeolite and an equivalent amount of Na⁺ is released:

Regeneration

When the zeolite is exhausted (all Na⁺ replaced), it is regenerated by passing 10% brine (NaCl) solution; the washings containing CaCl₂/MgCl₂ are drained:

Advantages

• Residual hardness is low (about 10 ppm).
• Plant is compact, occupies little space and can be operated automatically.
• No sludge is formed; the process adjusts itself to water of different hardness.

Limitations

• Soft water contains more sodium salts; NaHCO₃/Na₂CO₃ in boilers form NaOH and cause caustic embrittlement.
• Bicarbonate and carbonate ions are not removed; CO₂ released in boilers causes corrosion.
• Turbid water clogs the pores; acidic water destroys the zeolite; Fe²⁺ and Mn²⁺ form zeolites that cannot be regenerated easily.

Step 1: Standardisation of EDTA

Step 2: Total hardness

Step 3: Permanent hardness

Step 4: Temporary hardness

If the standard CaCO₃ solution is taken as 1.5 g/L (1.5 mg/mL, a more usual strength), 1 mL EDTA ≡ 1.2 mg CaCO₃ and the answers become one-tenth: total 216 ppm, permanent 144 ppm, temporary 72 ppm. The method of working is the same.

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.

Methods of operation

Cold lime-soda process: chemicals are mixed with water at room temperature. The precipitate is finely divided, so a coagulant is needed. Residual hardness is about 50–60 ppm.

Hot lime-soda process: chemicals are mixed with water at 80–150 °C (heated by exhaust steam). The plant has three parts: a reaction tank where raw water, chemicals and steam are mixed; a conical sedimentation vessel where the sludge settles and is removed from the bottom; and a sand filter which removes the remaining fine precipitate.

Advantages of the hot process

• Reactions are much faster, so softening capacity is higher.
• The precipitate and sludge settle quickly and form larger particles, so no coagulant is needed.
• Dissolved gases (CO₂, O₂) are expelled at high temperature, reducing corrosion.
• Viscosity of water is lower, so filtration is easier and faster.
• Residual hardness is lower (about 15–30 ppm) than in the cold process (50–60 ppm).
• Requires smaller quantities of chemicals in practice and a smaller plant for the same output.

General advantages of the lime-soda process

• Economical; removes iron, manganese and some bacteria as well.
• Increases pH, which reduces corrosion of pipes and kills pathogens.
• Removes minerals and reduces total dissolved solids to some extent.

Disadvantages: requires careful control and skilled supervision; produces large amounts of sludge, whose disposal is a problem; water still contains residual hardness, so it is not fit for high-pressure boilers without further treatment.

Role of coagulant (cold process)

The CaCO₃ and Mg(OH)₂ precipitated in the cold process are very fine and settle slowly. A small amount of coagulant such as alum [K₂SO₄·Al₂(SO₄)₃·24H₂O], aluminium sulphate, sodium aluminate or ferrous sulphate is added. It hydrolyses to give a gelatinous, flocculent precipitate of Al(OH)₃ which entraps the fine particles and makes them settle quickly:

Thus the coagulant speeds up settling and also removes colloidal impurities and turbidity, giving clear water.

(i) Caustic embrittlement

Definition: Caustic embrittlement is a type of boiler corrosion in which the boiler metal becomes brittle and develops intergranular (inter-crystalline) cracks at highly stressed parts such as bends, joints and rivet holes, due to a high concentration of caustic soda (NaOH) in the boiler water.

Cause and mechanism

Water softened by the lime-soda process contains some residual Na₂CO₃. In high-pressure boilers it hydrolyses to NaOH:

This alkaline water enters the minute hair-cracks in the boiler walls by capillary action. Water evaporates inside the cracks and NaOH concentration builds up there (it can exceed 10%). Concentrated NaOH attacks iron and dissolves it as sodium ferroate:

The NaOH is regenerated, so the attack continues and the cracks spread. The process can also be explained by a concentration cell, in which iron in contact with concentrated NaOH (inside cracks, under stress) acts as anode and iron in contact with dilute NaOH acts as cathode:

Prevention

• Use sodium phosphate instead of sodium carbonate as the softening reagent.
• Add tannin or lignin to boiler water; they block the hair-cracks and pits and prevent infiltration of NaOH.
• Add sodium sulphate, which also blocks the cracks. Recommended Na₂SO₄ : NaOH ratio is 1:1, 2:1 and 3:1 for boilers working up to 10, 20 and above 20 atm respectively.
• Keep the pH of boiler water around 8–9 and avoid excess alkalinity.
• Reduce stresses by proper design and fabrication: use welding instead of riveting and avoid sharp bends.

A lubricant is a substance introduced between two moving or sliding surfaces to reduce friction between them. The process of reducing friction in this way is called lubrication. The lubricant keeps the surfaces apart, so the frictional resistance is replaced by the much smaller internal resistance of the lubricant layer.

Functions of a lubricant

• Reduces friction, wear and tear of the moving parts.
• Reduces heat generation and acts as a coolant by carrying heat away.
• Reduces loss of power, so efficiency of the machine increases.
• Acts as a seal (e.g., between piston and cylinder) and prevents entry of dust and moisture.
• Protects the surfaces against corrosion and reduces noise.

Bakelite (phenol-formaldehyde resin) is a thermosetting polymer made by condensation of phenol with formaldehyde in the presence of an acid or alkaline catalyst.

Preparation

Step 1 — methylolation: phenol reacts with formaldehyde to give ortho- and para-hydroxymethyl phenols (methylol phenols).

Step 2 — condensation: methylol phenols condense with more phenol, eliminating water, to form linear chains joined by methylene (–CH₂–) bridges at ortho and para positions. With acid catalyst and excess phenol, this linear, fusible product is called Novolac. (With alkaline catalyst and excess formaldehyde, a product called Resol is formed.)

Step 3 — curing: Novolac is heated (about 150–200 °C) with a curing agent, hexamethylenetetramine [(CH₂)₆N₄], which supplies more formaldehyde. Further methylene bridges form between chains, and a hard, infusible, three-dimensional cross-linked network called Bakelite is obtained.

Properties

• Hard, rigid, scratch-resistant and strong; cannot be remoulded (thermosetting).
• Infusible and insoluble in most solvents; resistant to acids, salts and many organic solvents (attacked by strong alkali).
• Excellent electrical insulator, with good heat resistance and dimensional stability.
• Good moisture resistance and high compressive strength.

Uses

• Electrical switches, plugs, switch-boards, sockets, heater handles.
• Handles of cookers, kettles and cooking utensils; telephone and radio cabinets.
• Adhesive (binder) for plywood, laminates and grinding wheels; brake linings.
• Varnishes and paints; sulphonated Bakelite is used as an ion-exchange resin.
• Bearings, propeller shafts and gears in some industries.

Synthetic rubbers (elastomers) are man-made polymers that can be stretched to several times their length and return to their original shape when the force is removed. They are made to overcome the drawbacks of natural rubber (poor oil resistance, ageing, temperature sensitivity). Important examples:

• Buna-S (SBR) — butadiene + styrene; tyres, belts, soles (described below).
• Buna-N (NBR, nitrile rubber) — butadiene + acrylonitrile; excellent oil resistance; used for oil seals, fuel hoses, gaskets.
• Neoprene — polychloroprene; resistant to oil, flame and weathering; hoses, cable jackets.
• Butyl rubber — isobutylene + a little isoprene; very low gas permeability; inner tubes, tank linings.

Suitable example: Buna-S

Buna-S (styrene-butadiene rubber, SBR, GR-S) is a copolymer of about 75% butadiene and 25% styrene. "Bu" stands for butadiene, "na" for sodium (once used as catalyst) and "S" for styrene.

Preparation

It is prepared by emulsion copolymerisation of 1,3-butadiene and styrene at about 5 °C ("cold rubber") using a peroxide or persulphate free-radical initiator:

Properties

• Good abrasion resistance and load-bearing capacity; strength similar to natural rubber after compounding with carbon black.
• Can be vulcanised with sulphur since it contains double bonds.
• Attacked by oils, ozone and oxygen; swells in petrol; less resilient and generates more heat than natural rubber.

Uses

Motor tyres (largest use), shoe soles, conveyor belts, gaskets, hoses, foam rubber, cable insulation, floor tiles, adhesives and chewing gum base.

A phase diagram is a graphical representation of the conditions (temperature, pressure, composition) under which the different phases of a system exist in equilibrium. It shows the areas, curves and points corresponding to one, two or three phases.

For a two-component system, F = 2 − P + 2 = 4 − P, so up to 3 variables would be needed. For solid–liquid systems (condensed systems) the effect of pressure is negligible, so pressure is kept constant (1 atm) and the reduced phase rule F′ = C − P + 1 is used. The diagram is drawn between temperature and composition.

Cu–Ag system (simple eutectic system)

Copper and silver are completely miscible in the liquid state but do not form a compound; they form a simple eutectic.

• Point A — melting point of pure silver, 961 °C. Point B — melting point of pure copper, 1083 °C.
• Curve AO — freezing-point curve of silver: addition of Cu lowers the melting point of Ag. Along AO, solid Ag is in equilibrium with the melt. P = 2, F′ = 2 − 2 + 1 = 1.
• Curve BO — freezing-point curve of copper: addition of Ag lowers the melting point of Cu. Solid Cu ⇌ melt; F′ = 1.
• Eutectic point O — the curves meet at 778 °C and composition 28.1% Cu (71.9% Ag). Solid Ag, solid Cu and melt coexist. P = 3, F′ = 0 (non-variant). Below this temperature no liquid exists.
• Area above AOB — homogeneous liquid melt; P = 1, F′ = 2.
• Area between AO and the eutectic line — solid Ag + melt; area between BO and the eutectic line — solid Cu + melt.
• Areas below the eutectic line — solid Ag + eutectic (left) and solid Cu + eutectic (right).

Cooling of a melt: if a melt richer in Ag than the eutectic is cooled, nothing separates until the temperature reaches curve AO; then pure Ag begins to crystallise, the melt becomes richer in Cu and its composition moves along AO towards O. At O (778 °C) the remaining melt solidifies at constant temperature as the eutectic mixture of Ag and Cu.

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.

Lambert's law: when a beam of monochromatic light passes through a homogeneous absorbing medium, the rate of decrease of intensity with thickness of the medium is proportional to the intensity of the incident light.

Beer's law: the rate of decrease of intensity with thickness is also proportional to the concentration of the absorbing substance.

Beer–Lambert law combines the two: the absorbance of a solution is directly proportional to the concentration of the absorbing species and the path length.

Here I₀ = incident intensity, I = transmitted intensity, c = concentration (mol L⁻¹), l = path length (cm), and ε = molar absorptivity (L mol⁻¹ cm⁻¹), a constant for a substance at a given wavelength. A plot of A against c is a straight line through the origin, used as a calibration curve for finding unknown concentrations.

Limitations: the law holds only for dilute solutions (below about 0.01 M), for monochromatic radiation, and when the solute does not associate, dissociate, react with the solvent, fluoresce or scatter light.

• Chromophore: a covalently bonded unsaturated group responsible for absorption of UV or visible radiation, i.e., for electronic transitions. Examples: C=C, C≡C, C=O, C=N, N=N, NO₂, C₆H₅. Originally, groups that impart colour to a compound.
• Auxochrome: a saturated group with lone pairs (–OH, –NH₂, –OR, –Cl) that does not absorb above 200 nm by itself, but when attached to a chromophore shifts λ_max to longer wavelength and increases intensity.
• Bathochromic shift (red shift): shift of λ_max towards longer wavelength. Caused by conjugation, auxochromes, or change of solvent (π→π* bands shift to red in polar solvents). Example: benzene 255 nm → aniline 280 nm; ethylene 171 nm → 1,3-butadiene 217 nm.
• Hypsochromic shift (blue shift): shift of λ_max towards shorter wavelength. Caused by removal of conjugation or of lone-pair participation, or by polar solvents for n→π* bands. Example: aniline 280 nm → anilinium ion (in acid) 254 nm, because the lone pair of N is no longer conjugated with the ring.
• Hyperchromic shift: increase in the intensity of absorption (ε_max), usually by introducing an auxochrome. Example: pyridine (λ 257 nm, ε 2750) → 2-methylpyridine (λ 262 nm, ε 3560).
• Hypochromic shift: decrease in the intensity of absorption (ε_max), e.g., by groups that distort the chromophore. Example: biphenyl (ε 19,000) → 2-methylbiphenyl (ε 10,250), where the methyl group twists the rings out of plane.

The periodic table is a tabular arrangement of elements in order of increasing atomic number, such that elements with similar properties fall in the same vertical column (group). According to the modern periodic law (Moseley), the physical and chemical properties of elements are a periodic function of their atomic numbers. The long form of the periodic table has 18 groups and 7 periods, and is divided into s-, p-, d- and f-blocks according to the subshell receiving the last electron. Periodicity of properties arises from the regular recurrence of similar outer electronic configurations.

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

(i) Alkalinity

Alkalinity of water is its capacity to neutralise acids. It is due to the presence of hydroxide (OH⁻), carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions, and is expressed in ppm as CaCO₃ equivalent. OH⁻ and HCO₃⁻ cannot exist together because they react: OH⁻ + HCO₃⁻ → CO₃²⁻ + H₂O. So the possible combinations are OH⁻ alone, CO₃²⁻ alone, HCO₃⁻ alone, OH⁻ + CO₃²⁻, or CO₃²⁻ + HCO₃⁻.

Determination

A known volume of water is titrated with standard H₂SO₄ first using phenolphthalein, and then, in the same solution, using methyl orange.

Phenolphthalein alkalinity (P) measures all OH⁻ plus half of CO₃²⁻. Methyl-orange (total) alkalinity (M) measures all three. Alkalinity (ppm) = V_acid × N_acid × 50 × 1000 ÷ V_sample.

Significance: high alkalinity in boiler water causes caustic embrittlement, scale formation (with Ca²⁺), priming and foaming; alkalinity data are also used to calculate lime and soda requirement.