GTU BE01000031 Chemistry Summer 2026 Solved Paper & Solutions

A good fuel should have the following characteristics.

1. High calorific value: it gives a large amount of heat per unit mass.
2. Moderate ignition temperature: it catches fire easily, but not so easily that storage is dangerous.
3. Low moisture and low ash: these lower the heat output and give waste.
4. Moderate rate of combustion: the heat is not lost too fast or too slowly.
5. Easy to transport and store, and cheap and easy to get.
6. No harmful products: it should not give poisonous gases or much smoke.

An alloy is a homogeneous mixture of two or more metals, or of a metal with a small amount of a non-metal, which has metallic properties.

Necessity of alloys: pure metals are often not suitable for use. Alloying improves their properties.

• Hardness and strength: pure iron is soft. Adding a small amount of carbon makes steel, which is much harder and stronger.
• Corrosion resistance: adding chromium and nickel to iron gives stainless steel, which does not rust.
• Lower melting point: solder, an alloy of tin and lead, melts at a low temperature, and it is used to join metals.
• Better casting and appearance: a harder metal is easier to cast, and its surface is bright.

Example: brass is an alloy of copper (about 60 to 70 percent) and zinc. Copper alone is soft, but brass is harder, stronger and resists corrosion. It is used for utensils and pipe fittings.

Hardness of water is the property of water that prevents the formation of lather with soap. It is due to the dissolved salts of calcium and magnesium. Hard water forms a scum with soap: $2\,\text{C}_{17}\text{H}_{35}\text{COONa} + \text{Ca}^{2+} \to (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca} \downarrow + 2\text{Na}^+$.

• Temporary hardness is due to bicarbonates of calcium and magnesium. It is removed by boiling.
• Permanent hardness is due to chlorides and sulphates of calcium and magnesium. Boiling does not remove it.

It is measured in ppm or mg/L as $\text{CaCO}_3$.

Softening of water for boilers

Hard water forms scale and sludge in a boiler. It is softened by two kinds of treatment.

Methods of internal treatment

• Phosphate conditioning: sodium phosphate forms soft, non-sticky calcium phosphate, which is removed by blow-down.
• Calgon conditioning: sodium hexametaphosphate forms a soluble complex with calcium ions.
• Carbonate conditioning: sodium carbonate changes calcium sulphate to calcium carbonate in low-pressure boilers.

Corrosion inhibitors are substances which, when added in a small amount to the environment of a metal, reduce the rate of corrosion.

• Anodic inhibitors: chromates, nitrites and phosphates form a protective film on the anode.
• Cathodic inhibitors: they slow the cathodic reaction. In an acidic solution, organic amines are used, and in a neutral solution, sodium sulphite or hydrazine remove the dissolved oxygen.
• Organic inhibitors: thiourea and amines stick to the metal surface and form a layer.

They are used in cooling water systems, radiators, boilers and pipelines.

Both are caused by hard water, so both are prevented by softening the water. Sludge is removed by blow-down, which means opening the boiler outlet from time to time. Scale cannot be removed in this way, so it is more harmful.

Hybridization is the mixing (intermixing) of atomic orbitals of nearly equal energy of the same atom, to form the same number of new equivalent orbitals, called hybrid orbitals. These hybrid orbitals have the same energy and shape, and they explain the shape of molecules.

Types and examples

Explanation of the examples

• sp in $\text{BeCl}_2$: one 2s and one 2p orbital of beryllium mix to give two sp orbitals, which are $180^\circ$ apart. Each overlaps with a chlorine orbital, so the molecule is linear.
• sp$^2$ in $\text{BF}_3$: one s and two p orbitals of boron give three sp$^2$ orbitals in a plane, at $120^\circ$.
• sp$^3$ in $\text{CH}_4$: one 2s and three 2p orbitals of carbon give four equal sp$^3$ orbitals, directed to the corners of a tetrahedron.
• sp$^3$d in $\text{PCl}_5$: one 3d orbital also takes part, which gives five hybrid orbitals.
• sp$^3$d$^2$ in $\text{SF}_6$: two d orbitals take part, which gives six orbitals pointing to the corners of an octahedron.

Annealing is a heat treatment in which steel is heated to a suitable temperature above its critical temperature, kept there for some time (soaking), and then cooled slowly, usually in the furnace.

Purpose:

• To soften the steel and make it easier to machine.
• To remove internal stresses caused by earlier working.
• To refine the grain size and make the structure uniform.
• To improve the ductility.

Heisenberg's uncertainty principle: it is impossible to find both the exact position and the exact momentum of a moving microscopic particle, such as an electron, at the same time.

Here $\Delta x$ is the uncertainty in position, $\Delta p$ is the uncertainty in momentum, and $h$ is the Planck constant.

Explanation:

• To see an electron, we must hit it with a photon. The photon changes the momentum of the electron. A photon of short wavelength fixes the position better, but it changes the momentum more.
• So if the position is known more exactly, the momentum is known less exactly, and the reverse.
• This is not a fault of the instrument. It is due to the dual (wave) nature of matter.

Consequence: we cannot speak of a fixed path or orbit of an electron in an atom. We can only speak of the probability of finding it in a region, which is the idea of an orbital. For large bodies, the uncertainty is too small to notice.

The electrochemical theory says that corrosion takes place because of the formation of tiny galvanic cells on the surface of the metal, in the presence of moisture or an electrolyte.

Main points

1. Different parts of the metal surface become anodic and cathodic. This happens due to impurities, strain or a difference in the oxygen supply.
2. At the anode, the metal is oxidised. It loses electrons and goes into the solution as ions.

3. The electrons flow through the metal from the anode to the cathode.
4. At the cathode, the electrons are used by a reduction reaction. It depends on the environment.
5. The ions meet in the solution and form the corrosion product.

Diagram

Cathodic reactions

The reaction at the cathode depends on the medium. Two cases are important.

(a) Hydrogen evolution (acidic medium, no oxygen):

(b) Oxygen absorption (neutral or basic medium, with oxygen):

Formation of rust

In the presence of oxygen, $\text{Fe(OH)}_2$ is oxidised to $\text{Fe(OH)}_3$, the rust ($\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$). Corrosion takes place at the anode, but the rust is deposited between the anode and the cathode. The more oxygen reaches a part of the metal, the more cathodic it is, and the part with less oxygen becomes the anode. This is why iron corrodes below a drop of water at its centre and not at the edge.

Biodegradable polymers are polymers that can be broken down by microorganisms such as bacteria and fungi, into simple harmless substances like carbon dioxide, water and biomass. They do not remain in the environment for a long time.

Example: polylactic acid (PLA). It is made from lactic acid obtained by the fermentation of corn starch or sugar cane. It is used for packaging, disposable cups, and for surgical stitches, since the body can absorb it. Another example is PHB (poly-3-hydroxybutyrate).

The Hard and Soft Acids and Bases (HSAB) concept was given by Pearson. Acids and bases are grouped as hard or soft according to their size and polarisability.

• Hard acids: small, with a high positive charge, and not easily polarised. Examples: $\text{H}^+$, $\text{Na}^+$, $\text{Mg}^{2+}$, $\text{Al}^{3+}$, $\text{Fe}^{3+}$.
• Soft acids: large, with a low positive charge, and easily polarised. Examples: $\text{Ag}^+$, $\text{Cu}^+$, $\text{Hg}^{2+}$, $\text{Pt}^{2+}$.
• Hard bases: small, with high electronegativity, and not easily polarised. Examples: $\text{F}^-$, $\text{OH}^-$, $\text{H}_2\text{O}$, $\text{NH}_3$, $\text{O}^{2-}$.
• Soft bases: large, with low electronegativity, and easily polarised. Examples: $\text{I}^-$, $\text{S}^{2-}$, $\text{CN}^-$, $\text{CO}$.

Rule: hard acids prefer to bind with hard bases, and soft acids prefer soft bases. The compounds formed are more stable. For example, $\text{Al}^{3+}$ with $\text{F}^-$ (hard-hard), and $\text{Hg}^{2+}$ with $\text{S}^{2-}$ (soft-soft). This is why hard metals such as calcium and aluminium occur as oxides and carbonates, and soft metals such as mercury and copper occur as sulphides.

Ion exchange process (demineralisation) removes all the dissolved ions from hard water, and so gives pure soft water. It uses two types of resins, which are insoluble, cross-linked organic polymers.

• Cation exchange resin ($\text{RH}^+$): a strongly acidic resin, for example sulphonated polystyrene. It can exchange its $\text{H}^+$ for the cations.
• Anion exchange resin ($\text{R'OH}^-$): a strongly basic resin, for example a quaternary ammonium compound. It can exchange its $\text{OH}^-$ for the anions.

Diagram

Process

1. Cation exchanger: hard water is passed through it first. The calcium and magnesium ions (and sodium ions) are taken up, and $\text{H}^+$ ions are released.

2. Anion exchanger: the water, which is now acidic, goes through the second column. The anions such as $\text{Cl}^-$ and $\text{SO}_4^{2-}$ are taken up, and $\text{OH}^-$ ions are released.

3. The $\text{H}^+$ and $\text{OH}^-$ ions combine: $\text{H}^+ + \text{OH}^- \to \text{H}_2\text{O}$. The water has no hardness and no ions.

Regeneration

After some time, the resins have no more $\text{H}^+$ or $\text{OH}^-$ to give, and the water coming out is hard again.

When the resins are exhausted, they are restored. The cation resin is washed with dilute HCl or $\text{H}_2\text{SO}_4$, and the anion resin with dilute NaOH.

Advantages and limits

• The water is very soft and pure, so it is used in high-pressure boilers.
• The equipment is costly, and turbid or oily water spoils the resin.

Fullerenes are allotropes of carbon that have the form of hollow cages, spheres or tubes. The most common one is $\text{C}_{60}$, or buckminsterfullerene, a football-shaped molecule made of 60 carbon atoms (12 pentagons and 20 hexagons).

Properties

• It is a stable solid, which is soluble in organic solvents such as toluene.
• It is strong and can take pressure.
• It can accept electrons, and with a metal added (as in $\text{K}_3\text{C}_{60}$) it becomes a superconductor.

Applications: drug delivery, lubricants, organic solar cells, catalysts, and antioxidants in cosmetics.

Nanomaterials are materials with at least one dimension of 1 to 100 nm. They have special properties because of their small size and large surface area. Their applications are:

• Medicine: drug delivery to a target cell, cancer treatment, and imaging.
• Electronics: smaller and faster transistors, memory devices, and displays.
• Energy: better solar cells and batteries, and fuel cells.
• Environment: water purification and removal of pollutants, and catalysts.
• Cosmetics and textiles: sunscreens with $\text{ZnO}$ or $\text{TiO}_2$, and stain-resistant cloth.
• Coatings and construction: self-cleaning glass, paints and stronger concrete.
• Sensors: very small sensors that detect gases, toxins and disease markers.
• Agriculture: nano-fertilisers and sensors for soil.

There are two main approaches to the synthesis of nanomaterials.

Top-down approach

In this approach, a bulk material is broken down into nano-sized particles by physical methods. It works from a larger size to a smaller size.

• Ball milling: the material is ground with hard balls in a rotating drum, until the particles are nano-sized.
• Lithography and etching: a pattern is made on a surface, and the extra material is etched away. It is used in making chips.
• Laser ablation: a laser removes atoms from a target.

Advantages: it is simple and suits large production. Disadvantages: the particles are not uniform, there may be defects on the surface, and it wastes material.

Bottom-up approach

In this approach, the nanomaterial is built atom by atom or molecule by molecule.

• Sol-gel method: a solution of a precursor changes to a gel, which is dried to give particles.
• Chemical vapour deposition (CVD): gases react on a hot surface and form a thin layer or nanotubes.
• Co-precipitation, hydrothermal and self-assembly methods.

Advantages: the particles are uniform and have fewer defects, and the size can be controlled. Disadvantages: it is more costly and slower.

Comparison