BE Civil Engineering (IOE, TU) Engineering Chemistry (IOE, SH 403) Question Paper 2080 Nepal

Galvanic cell: A galvanic (voltaic) cell is an electrochemical device that converts chemical energy of a spontaneous redox reaction into electrical energy. It consists of two half-cells, each with an electrode dipped in an electrolyte, connected externally by a wire and internally by a salt bridge.

Daniell cell working: Zinc (anode) is oxidised, releasing electrons that flow through the external circuit to the copper electrode (cathode), where $\text{Cu}^{2+}$ ions are reduced. The salt bridge maintains electrical neutrality.

Cell representation:

$$\text{Zn(s)} \mid \text{Zn}^{2+}(aq) \parallel \text{Cu}^{2+}(aq) \mid \text{Cu(s)}$$

Electrode reactions:

• Anode (oxidation): $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^{-}$
• Cathode (reduction): $\text{Cu}^{2+} + 2e^{-} \rightarrow \text{Cu}$
• Overall: $\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$

(a) Standard EMF:

$$E^{\circ}_{cell} = E^{\circ}_{cathode} - E^{\circ}_{anode} = (+0.34) - (-0.76) = \mathbf{+1.10\ V}$$

(b) Nernst equation ($n = 2$):

$$E_{cell} = E^{\circ}_{cell} - \frac{0.0592}{n}\log\frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]}$$ $$\frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]} = \frac{0.010}{0.50} = 0.020$$ $$\log(0.020) = -1.699$$ $$E_{cell} = 1.10 - \frac{0.0592}{2}(-1.699) = 1.10 - (0.0296)(-1.699)$$ $$E_{cell} = 1.10 + 0.0503 = \mathbf{1.150\ V}$$

(c) Spontaneity and $\Delta G$: Since $E_{cell} > 0$, the reaction is spontaneous.

$$\Delta G = -nFE_{cell} = -(2)(96500)(1.150) = -221950\ \text{J} = \mathbf{-221.95\ kJ}$$

The large negative $\Delta G$ confirms spontaneity.

Corrosion: Corrosion is the gradual destruction or deterioration of a metal by chemical or electrochemical reaction with its environment, e.g. the rusting of iron.

Electrochemical (wet) theory: When iron is exposed to moisture and oxygen, tiny galvanic cells form on its surface. Impurity-rich or stressed regions act as anodes and pure regions as cathodes; the thin moisture film acts as the electrolyte.

Anodic reaction (oxidation, occurs at anode in all cases):

$$\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^{-}$$

(i) Acidic medium (cathodic reaction, hydrogen evolution):

$$2\text{H}^{+} + 2e^{-} \rightarrow \text{H}_2 \uparrow$$

(ii) Neutral / aerated medium (cathodic reaction, oxygen absorption):

$$\tfrac{1}{2}\text{O}_2 + \text{H}_2\text{O} + 2e^{-} \rightarrow 2\text{OH}^{-}$$

The $\text{Fe}^{2+}$ and $\text{OH}^{-}$ combine and are further oxidised by air to hydrated ferric oxide (rust):

$$4\text{Fe(OH)}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3 \rightarrow \text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$$

Three methods of corrosion control:

1. Barrier protection (coatings): Applying paints, varnishes, enamels, or metallic coatings (galvanizing with Zn, tinning with Sn) isolates the metal from the corrosive environment.

2. Cathodic protection — sacrificial anode method: A more electropositive (more easily oxidised) metal such as Mg, Zn, or Al is electrically connected to the structure (e.g. a buried pipeline or ship hull). This active metal becomes the anode and corrodes preferentially, forcing the protected iron to act as the cathode so it does not corrode. The sacrificial anode is periodically replaced.

   Mg block ----(wire)---- Steel pipe (buried)
     (anode, corrodes)        (cathode, protected)
   

3. Use of inhibitors: Adding chemicals such as chromates, phosphates (anodic inhibitors) or amines (cathodic inhibitors) to the medium reduces the corrosion rate by forming protective films or suppressing electrode reactions.

Polymer: A polymer is a large molecule (macromolecule) made up of many small repeating units called monomers joined by covalent bonds.

Polymerization: The chemical process by which monomer molecules combine to form a polymer.

Addition vs condensation polymerization:

Thermoplastic vs thermosetting (any three):

(i) PVC: Monomer = vinyl chloride ($\text{CH}_2{=}\text{CHCl}$). Use: pipes, electrical cable insulation, flooring.

(ii) Bakelite: Monomers = phenol + formaldehyde. Use: electrical switches, handles, insulating boards.

Hardness of water: Hardness is the property of water that prevents it from forming lather (foam) readily with soap, caused mainly by dissolved salts of calcium and magnesium.

• Temporary (carbonate) hardness: Due to bicarbonates of Ca and Mg; removed by boiling.
• Permanent (non-carbonate) hardness: Due to chlorides and sulphates of Ca and Mg; not removed by boiling.

Conversion to CaCO₃ equivalent uses:

$$\text{CaCO}_3\ \text{equiv} = \text{mass of salt} \times \frac{100\ (M_{CaCO_3})}{M_{salt}}$$

Temporary hardness = $10.0 + 10.0 = \mathbf{20\ ppm}$

Permanent hardness = $10.0 + 10.0 = \mathbf{20\ ppm}$

Total hardness = temporary + permanent = $20 + 20 = \mathbf{40\ ppm}$ (as CaCO₃)

Calorific value: The calorific value of a fuel is the total quantity of heat liberated when a unit mass (or volume) of the fuel is completely burnt in air/oxygen.

• Higher (Gross) Calorific Value (HCV): Heat released when combustion products are cooled to room temperature so that the water vapour formed is condensed and its latent heat is recovered.
• Lower (Net) Calorific Value (LCV): Heat released when the water vapour is not condensed and escapes with the flue gases; $\text{LCV} = \text{HCV} - (\text{latent heat of steam formed})$.

Given (no sulphur): $C = 80,\ H = 6,\ O = 5$ (percent by mass), $S = 0$.

HCV by Dulong's formula:

$$\text{HCV} = \frac{1}{100}\left[8080(80) + 34500\left(6 - \frac{5}{8}\right) + 0\right]$$ $$8080 \times 80 = 646400$$ $$6 - \frac{5}{8} = 6 - 0.625 = 5.375$$ $$34500 \times 5.375 = 185437.5$$ $$\text{HCV} = \frac{1}{100}(646400 + 185437.5) = \frac{831837.5}{100} = \mathbf{8318.375\ kcal/kg}$$

Mass of water formed per kg of coal:

$$\text{H}_2\text{O} = 9 \times \frac{H}{100} = 9 \times \frac{6}{100} = 0.54\ \text{kg}$$

LCV:

$$\text{LCV} = \text{HCV} - (0.54 \times 587)$$ $$0.54 \times 587 = 316.98\ \text{kcal/kg}$$ $$\text{LCV} = 8318.375 - 316.98 = \mathbf{8001.40\ kcal/kg}$$

Catalyst: A catalyst is a substance that alters (usually increases) the rate of a chemical reaction without itself being permanently consumed in the reaction.

Homogeneous vs heterogeneous catalysis:

Three characteristics of a catalyst:

1. A small amount is sufficient to catalyse a large amount of reactant; it is recovered chemically unchanged in mass and composition.
2. It does not change the position of equilibrium ($\Delta G$ unchanged); it only speeds up attainment of equilibrium (speeds forward and reverse equally).
3. It is specific in action and most effective at a particular (optimum) temperature; activity can be destroyed by catalyst poisons.

Mechanism / energy diagram: A catalyst provides an alternative reaction pathway with lower activation energy ($E_a$). More molecules possess the required energy, so the reaction rate increases.

Energy
 |        uncatalysed (high Ea)
 |        .-''-.
 |       /      \
 |   ___/        \___ catalysed (low Ea)
 |  /   _.-''-._     \
 | /  _/        \_    \___ products
 |/__/  reactants  \______
 +-------------------------> Reaction progress

The catalysed curve has a lower peak (lower $E_a$); the energy levels of reactants and products are unchanged, so $\Delta H$ is unaffected.

Raw materials of Portland cement: Limestone (provides CaO), clay/shale (provides SiO₂, Al₂O₃, Fe₂O₃), and a small amount of gypsum (CaSO₄·2H₂O) added after clinkering.

Four major compounds (Bogue's compounds):

Setting and hardening: When water is added, the compounds undergo hydration and hydrolysis, forming gels and crystalline products that interlock and bind the mass.

• Setting (initial stiffening) is mainly due to rapid hydration of C₃A and C₃S.
• Hardening (strength gain over weeks) is due to slow hydration of C₂S.

$$2(3\text{CaO}\cdot\text{SiO}_2) + 6\text{H}_2\text{O} \rightarrow 3\text{CaO}\cdot2\text{SiO}_2\cdot3\text{H}_2\text{O} + 3\text{Ca(OH)}_2$$

The hydrated calcium silicate (C-S-H gel) provides the main binding strength; $\text{Ca(OH)}_2$ is liberated.

Role of gypsum: Gypsum is added (about 2–3%) to retard the very fast setting of C₃A, i.e. to prevent flash set and provide a workable setting time, by forming calcium sulphoaluminate (ettringite) on the C₃A surface.

Paint: A paint is a mechanical dispersion (suspension) of one or more finely divided pigments in a liquid medium (vehicle), applied to a surface as a thin coating that dries to a decorative and protective film.

Constituents of paint and their functions:

Paint vs varnish:

Explosive: An explosive is a substance (or mixture) which, on receiving a suitable stimulus such as heat, shock, impact, or friction, undergoes a very rapid self-propagating exothermic reaction producing a large volume of hot gases and a sudden release of pressure (explosion).

Classification by performance:

Three characteristics of a good explosive:

1. It should be stable and safe to store and handle (insensitive to accidental shock/friction) but readily detonated by a suitable detonator.
2. It should produce a large volume of gas with high heat and high brisance for maximum work.
3. It should be chemically stable with a good shelf-life and should leave little toxic/solid residue.

Civil/engineering application: Blasting of rock in tunnelling, mining, quarrying, and excavation for dams, roads, and foundations (e.g. dynamite for controlled rock blasting).

Specific conductance ($\kappa$): The conductance of a solution placed between two electrodes each of $1\ \text{cm}^2$ area and $1\ \text{cm}$ apart (i.e. of $1\ \text{cm}^3$ of solution). Unit: $\text{S cm}^{-1}$ (ohm$^{-1}$ cm$^{-1}$).

Molar conductance ($\Lambda_m$): The conductance of all the ions produced by dissolving one mole of electrolyte placed between electrodes $1\ \text{cm}$ apart. Unit: $\text{S cm}^2\,\text{mol}^{-1}$.

Step 1 — Conductance ($G$):

$$G = \frac{1}{R} = \frac{1}{250} = 0.0040\ \text{S}$$

Step 2 — Specific conductance:

$$\kappa = G \times \text{cell constant} = 0.0040 \times 0.50 = \mathbf{0.0020\ S\,cm^{-1}}$$

Step 3 — Molar conductance:

$$\Lambda_m = \frac{\kappa \times 1000}{C} = \frac{0.0020 \times 1000}{0.020}$$ $$\Lambda_m = \frac{2.0}{0.020} = \mathbf{100\ S\,cm^2\,mol^{-1}}$$

(Here $C$ is in $\text{mol L}^{-1}$; the factor 1000 converts L to cm³.)

Combustion: Combustion is a rapid exothermic chemical reaction of a fuel with oxygen (air) accompanied by the evolution of heat and usually light (flame).

Balanced combustion equations:

$$\text{C} + \text{O}_2 \rightarrow \text{CO}_2$$ $$2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$$ $$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$$

Air required for combustion of carbon:

From $\ \text{C} + \text{O}_2 \rightarrow \text{CO}_2$:

$$12\ \text{kg C} \ \text{requires}\ 32\ \text{kg O}_2$$

Oxygen required for $2\ \text{kg}$ of carbon:

$$\text{O}_2 = \frac{32}{12} \times 2 = \frac{64}{12} = 5.333\ \text{kg}$$

Since air contains $23\%$ O₂ by mass:

$$\text{Air} = \frac{5.333}{0.23} = \mathbf{23.19\ kg}$$

Theoretical air required = 23.19 kg per 2 kg of carbon (≈ 11.59 kg air per kg carbon).