GATE Chemical Engineering Chemical Technology Syllabus

Define Single Super Phosphate (SSP), give its key reaction, and its typical available $\ce{P2O5}$ content.

SSP is made by treating phosphate rock with sulfuric acid: $\ce{Ca5(PO4)3F + 5H2SO4 -> ...}$ giving mainly monocalcium phosphate $\ce{Ca(H2PO4)2}$ plus gypsum. It contains roughly $16$–$20\%$ available $\ce{P2O5}$.

Write the simplified reaction for making SSP and identify why gypsum is present in the product.

$\ce{Ca3(PO4)2 + 2H2SO4 -> Ca(H2PO4)2 + 2CaSO4}$. The $\ce{CaSO4}$ (gypsum) forms because sulfuric acid converts excess calcium into calcium sulfate; it remains in SSP, diluting its $\ce{P2O5}$ content.

Define Triple Super Phosphate (TSP), give its manufacturing reaction, and its $\ce{P2O5}$ content.

TSP is made by treating phosphate rock with phosphoric acid (not sulfuric): $\ce{Ca3(PO4)2 + 4H3PO4 -> 3Ca(H2PO4)2}$. It contains about $44$–$48\%$ available $\ce{P2O5}$, roughly triple that of SSP.

Why does TSP have a much higher $\ce{P2O5}$ content than SSP despite both yielding monocalcium phosphate?

SSP is made with $\ce{H2SO4}$, which generates inert gypsum ($\ce{CaSO4}$) that dilutes the product. TSP is made with $\ce{H3PO4}$, which adds extra phosphate instead of sulfate, so no gypsum forms and the phosphate concentration is far higher.

Compare SSP and TSP on acid used, by-product, and approximate $\ce{P2O5}$ content.

SSP: uses $\ce{H2SO4}$, by-product gypsum ($\ce{CaSO4}$), $\sim 16$–$20\%$ $\ce{P2O5}$. TSP: uses $\ce{H3PO4}$, essentially no gypsum, $\sim 44$–$48\%$ $\ce{P2O5}$.

In pulp and paper manufacture, what is the purpose of pulping, and name the dominant chemical pulping process.

Pulping separates cellulose fibers from lignin and other wood components. The dominant chemical process is the Kraft (sulfate) process, which uses an alkaline 'white liquor' of $\ce{NaOH}$ and $\ce{Na2S}$.

What are the active chemicals in Kraft pulping white liquor, and what does the recovery boiler regenerate?

White liquor contains sodium hydroxide ($\ce{NaOH}$) and sodium sulfide ($\ce{Na2S}$). Spent black liquor is concentrated and burned in the recovery boiler; the smelt of $\ce{Na2CO3}$ and $\ce{Na2S}$ is causticized with lime to regenerate $\ce{NaOH}$.

Write the causticizing reaction used to regenerate $\ce{NaOH}$ in the Kraft recovery cycle.

$\ce{Na2CO3 + Ca(OH)2 -> 2NaOH + CaCO3}$. The $\ce{CaCO3}$ (lime mud) is calcined back to $\ce{CaO}$ in a lime kiln: $\ce{CaCO3 -> CaO + CO2}$.

Contrast the Kraft (sulfate) and sulfite pulping processes on liquor chemistry and fiber strength.

Kraft uses alkaline $\ce{NaOH}/\ce{Na2S}$ liquor, tolerates most wood species, and gives strong fibers with efficient chemical recovery. Sulfite uses acidic $\ce{H2SO3}$/bisulfite liquor, gives weaker but easily bleached pulp and is less common today.

What component of wood must be removed or degraded during pulping, and what is its rough mass fraction in wood?

Lignin, the amorphous polymer that binds cellulose fibers, makes up roughly $20$–$30\%$ of wood and must be dissolved/degraded so the cellulose fibers can be freed.

Outline the main unit operations in cane sugar manufacture from juice extraction to crystallization.

Milling/extraction of juice, clarification (liming and heating to remove impurities), evaporation to thick syrup, crystallization (boiling under vacuum to grow sugar crystals), and centrifugation to separate sugar crystals from molasses.

What is the role of the clarification (defecation) step with lime in sugar processing?

Lime ($\ce{Ca(OH)2}$) is added and the juice heated so that $\ce{CaO}$/phosphate forms a precipitate that adsorbs colloidal and suspended impurities, neutralizes acids (preventing sucrose inversion), and clarifies the juice before evaporation.

Define molasses in sugar manufacture and name a major use.

Molasses is the dark, viscous mother liquor remaining after sucrose crystals are removed by centrifugation and no further economic sugar can be crystallized. It is used for ethanol fermentation, cattle feed, and as a raw material for various fermentations.

Why is vacuum evaporation/boiling used in sugar concentration and crystallization?

Operating under vacuum lowers the boiling point so concentration occurs at reduced temperature, preventing thermal degradation and caramelization (inversion) of sucrose and saving energy via multiple-effect evaporators.

What chemical reaction underlies the hydrogenation of oils and fats, and what is its purpose?

Catalytic hydrogenation adds $\ce{H2}$ across the $\ce{C=C}$ double bonds of unsaturated fatty acid chains over a nickel catalyst, converting liquid oils to semi-solid fats (e.g., vanaspati/margarine) and improving oxidative stability.

Define the saponification of fats and write its general reaction.

Saponification is the alkaline hydrolysis of a triglyceride to produce soap and glycerol: $\ce{fat (triglyceride) + 3NaOH -> 3 R-COONa (soap) + glycerol}$.

What does the iodine value of an oil/fat measure, and what does a high value indicate?

The iodine value is the mass of iodine (in g) absorbed by $100\ \mathrm{g}$ of fat, measuring the degree of unsaturation ($\ce{C=C}$ bonds). A high iodine value indicates a highly unsaturated (drying/liquid) oil.

What is the saponification value of a fat and what does a high value imply about chain length?

The saponification value is the mg of $\ce{KOH}$ required to saponify $1\ \mathrm{g}$ of fat. A high saponification value implies low average molecular weight, i.e., shorter fatty-acid chains.

Name the two principal industrial methods of recovering oil from oilseeds.

Mechanical expression (expeller/screw pressing) and solvent extraction (typically with hexane), often used in sequence — pressing for high-oil seeds and solvent extraction to recover residual oil from the cake.

What monomer makes polyethylene, and how do LDPE and HDPE differ in structure and process?

Monomer: ethylene, $\ce{CH2=CH2}$. LDPE is highly branched, made by high-pressure free-radical polymerization, giving low density and crystallinity. HDPE is mostly linear, made at low pressure with Ziegler–Natta catalysts, giving higher density, crystallinity, and strength.

Write the polymerization of ethylene to polyethylene and classify the polymer.

$\ce{n\ CH2=CH2 -> [-CH2-CH2-]_n}$. Polyethylene is an addition (chain-growth) thermoplastic polymer.

What catalyst system enables low-pressure production of linear HDPE and stereoregular polypropylene?

Ziegler–Natta catalysts (e.g., $\ce{TiCl4}$ with an aluminium alkyl such as $\ce{Al(C2H5)3}$ as co-catalyst), which give stereoregular, linear polymers at low pressure.

What is the monomer for polypropylene, and why is tacticity important for it?

Monomer: propylene, $\ce{CH2=CH-CH3}$. Tacticity (spatial arrangement of the methyl groups) governs crystallinity: isotactic polypropylene is highly crystalline and useful, whereas atactic PP is soft and largely amorphous.

Define the three tacticities of polypropylene.

Isotactic: all methyl groups on the same side of the chain (highly crystalline). Syndiotactic: methyl groups alternate regularly side to side. Atactic: methyl groups randomly placed (amorphous, low value).

What is the monomer for PVC, and write the polymerization reaction.

Monomer: vinyl chloride, $\ce{CH2=CHCl}$. Polymerization: $\ce{n\ CH2=CHCl -> [-CH2-CHCl-]_n}$ (poly(vinyl chloride)), an addition thermoplastic.

Why are plasticizers added to PVC, and what is the difference between rigid and flexible PVC?

Plasticizers (e.g., phthalate esters like DOP) reduce intermolecular forces, lowering glass transition temperature to make PVC soft and flexible (used for cables, films). Rigid (unplasticized) PVC has little or no plasticizer and is used for pipes and window frames.

What is the dominant industrial route to the vinyl chloride monomer (VCM) for PVC?

Via ethylene: direct chlorination/oxychlorination of ethylene to 1,2-dichloroethane (EDC), $\ce{CH2=CH2 + Cl2 -> ClCH2CH2Cl}$, then thermal cracking (pyrolysis) of EDC: $\ce{ClCH2CH2Cl -> CH2=CHCl + HCl}$.

By what type of polymerization is polyester (PET) made, and write the monomers reacting.

PET is made by step-growth (condensation) polymerization of a diacid/diester and a diol: terephthalic acid (or dimethyl terephthalate) reacts with ethylene glycol, releasing water (or methanol). $\ce{HOOC-C6H4-COOH + HO-CH2CH2-OH -> [-OC-C6H4-CO-O-CH2CH2-O-]_n + H2O}$.

Classify polyester fiber (PET), polyethylene, and PVC as addition or condensation polymers and as thermoplastic or thermoset.

Polyethylene, polypropylene, and PVC are addition (chain-growth) thermoplastics. Polyester (PET) is a condensation (step-growth) thermoplastic. All four are thermoplastics (not thermosets).