BE Civil Engineering (IOE, TU) Engineering Chemistry (IOE, SH 403) Question Paper 2079 Nepal

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

Working — Daniell cell:

• Anode (oxidation): Zinc rod in $\text{ZnSO}_4$ solution. $\;\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$
• Cathode (reduction): Copper rod in $\text{CuSO}_4$ solution. $\;\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$
• Overall: $\text{Zn} + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}$

Electrons flow externally from Zn (negative terminal) to Cu (positive terminal); the salt bridge (e.g. KCl in agar) maintains electrical neutrality by allowing ion migration.

  e- -->            e- -->
 [Zn]----wire----[Cu]
   |   salt bridge  |
  ZnSO4 ~~~~~~~~~ CuSO4
 (anode -)        (cathode +)

Nernst equation for a single electrode: For the reduction $M^{n+} + ne^- \rightarrow M$,

$$E = E^{\circ} - \frac{RT}{nF}\ln\frac{1}{[M^{n+}]} = E^{\circ} + \frac{0.0591}{n}\log[M^{n+}] \quad (\text{at } 25\,^{\circ}\text{C})$$

where $E^{\circ}$ is the standard electrode potential, $n$ the number of electrons, $F = 96500\ \text{C/mol}$.

Calculation: Cell reaction transfers $n = 2$ electrons.

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

Reaction quotient:

$$Q = \frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]} = \frac{0.10}{0.50} = 0.20$$

Nernst equation for the cell:

$$E_{\text{cell}} = E^{\circ}_{\text{cell}} - \frac{0.0591}{n}\log Q = 1.10 - \frac{0.0591}{2}\log(0.20)$$ $$\log(0.20) = -0.699$$ $$E_{\text{cell}} = 1.10 - (0.02955)(-0.699) = 1.10 + 0.02066 = 1.1207\ \text{V}$$

EMF of the cell $\approx \mathbf{1.121\ V}$.

Corrosion: Corrosion is the slow, spontaneous destruction of a metal by chemical or electrochemical reaction with its environment, converting the metal into more stable compounds such as oxides, hydroxides or carbonates. For iron the visible product is hydrated ferric oxide (rust), $\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$.

Electrochemical (wet) theory: When iron is exposed to moisture, the heterogeneous surface develops tiny anodic and cathodic regions forming countless short-circuited galvanic cells, with the thin water film acting as electrolyte.

Anodic reaction (oxidation, same for both mechanisms):

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

Cathodic reaction depends on conditions:

1. Oxygen-absorption (neutral/aerated medium):

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

2. Hydrogen-evolution (acidic medium, poorly aerated):

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

Rust formation: $\text{Fe}^{2+}$ migrates and meets $\text{OH}^-$:

$$\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2$$

Further oxidation gives $\text{Fe(OH)}_3$, which dehydrates to rust $\text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O}$.

Two factors affecting rate of corrosion:

1. Nature of the metal / position in galvanic series — more active (anodic) metals and electrode-potential differences between contacting metals increase corrosion. A small anode with a large cathode causes intense localized attack.
2. Nature of environment (pH, humidity, dissolved O₂, salts): Lower pH and higher dissolved oxygen, humidity and salt content (e.g. coastal/industrial air) all accelerate corrosion by improving electrolyte conductivity and depolarization at the cathode.

Two protection methods:

1. Cathodic protection (sacrificial anode): A more active metal such as Mg or Zn is connected to the iron; it becomes the anode and corrodes preferentially, keeping the iron cathodic and protected (used for buried pipelines, ship hulls).
2. Protective coatings: Barrier coatings (paints, enamel, electroplating) or galvanizing (a Zn coating that also gives sacrificial protection) isolate the metal from the corrosive environment.

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

Temporary (carbonate) hardness: due to dissolved bicarbonates of Ca and Mg, $\text{Ca(HCO}_3)_2$ and $\text{Mg(HCO}_3)_2$. It is removed by boiling (decomposes to insoluble carbonate/hydroxide).

Permanent (non-carbonate) hardness: due to chlorides and sulphates of Ca and Mg ($\text{CaCl}_2, \text{MgCl}_2, \text{CaSO}_4, \text{MgSO}_4$). It is not removed by boiling; requires chemical methods (lime-soda, ion exchange).

Calculation — convert each salt to CaCO₃ equivalent:

$$\text{CaCO}_3\ \text{equiv (mg/L)} = \text{amount of salt (mg/L)} \times \frac{100}{\text{molar mass of salt}}$$

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

Permanent hardness $= 10.0 + 20.0 = \mathbf{30\ ppm}$

Total hardness $= 20 + 30 = \mathbf{50\ ppm}$ (as CaCO₃).

Polymerization: The chemical process in which a large number of small molecules (monomers) combine through covalent bonds to form a large macromolecule (polymer) of high molecular mass.

Addition vs condensation polymerization:

Thermoplastic vs thermosetting:

PVC repeating unit:

$$-\!\left(\text{CH}_2\!-\!\text{CHCl}\right)\!-_n$$

(formed by addition polymerization of vinyl chloride $\text{CH}_2{=}\text{CHCl}$).

Engineering use of PVC: Manufacture of water-supply and drainage pipes, electrical cable insulation and roofing sheets (rigid, chemically resistant, durable).

Degree of polymerization (DP): It is the number of repeating monomer units ($n$) in a polymer chain.

$$\text{DP} = \frac{\text{molar mass of polymer}}{\text{molar mass of repeating unit}}$$

For example, a PVC sample of molar mass $125000\ \text{g/mol}$ (repeat unit $\text{C}_2\text{H}_3\text{Cl} = 62.5\ \text{g/mol}$) has $\text{DP} = 125000/62.5 = \mathbf{2000}$. A higher DP generally gives greater tensile strength, higher softening point and better chemical resistance, which is why high-molecular-weight polymers are preferred for load-bearing engineering applications.

Calorific value: The calorific value of a fuel is the total quantity of heat liberated by the complete combustion of a unit mass (or unit volume) of the fuel. Common units: kcal/kg (solids/liquids), kcal/m³ (gases).

Gross (higher) calorific value (GCV): Heat liberated when unit mass of fuel is completely burnt and the combustion products are cooled to room temperature, so the water vapour formed is condensed and its latent heat is recovered.

Net (lower) calorific value (NCV): Heat liberated when the combustion products are allowed to escape (water remains as vapour); the latent heat of vaporization of water is not recovered. $\text{NCV} < \text{GCV}$.

Dulong's formula:

$$\text{GCV} = \frac{1}{100}\left[8080\,C + 34500\left(H - \frac{O}{8}\right) + 2240\,S\right]\ \text{kcal/kg}$$

where C, H, O, S are percentages by mass. The term $O/8$ accounts for hydrogen already combined with oxygen as water (unavailable for combustion).

Step 1 — available hydrogen:

$$H - \frac{O}{8} = 5 - \frac{8}{8} = 5 - 1 = 4$$

Step 2 — GCV:

$$\text{GCV} = \frac{1}{100}\big[8080(80) + 34500(4) + 2240(1)\big]$$ $$= \frac{1}{100}\big[646400 + 138000 + 2240\big] = \frac{786640}{100} = 7866.4\ \text{kcal/kg}$$

GCV $= \mathbf{7866.4\ kcal/kg}$

Step 3 — mass of water formed per kg fuel: Total hydrogen = 5% = 0.05 kg; water formed $= 9 \times 0.05 = 0.45$ kg.

Step 4 — NCV:

$$\text{NCV} = \text{GCV} - (\text{mass of water} \times 587)$$ $$= 7866.4 - (0.45 \times 587) = 7866.4 - 264.15 = 7602.25\ \text{kcal/kg}$$

NCV $= \mathbf{7602.25\ kcal/kg}$

Catalyst: A catalyst is a substance that alters (usually increases) the rate of a chemical reaction without itself undergoing any permanent chemical change; it provides an alternative reaction path of lower activation energy. The phenomenon is called catalysis.

Homogeneous vs heterogeneous catalysis:

Two characteristics of a catalyst:

1. A small quantity of catalyst is usually sufficient, and it is recovered chemically unchanged in mass and composition at the end of the reaction.
2. A catalyst cannot start a thermodynamically impossible reaction; it only changes the rate and does not alter the position of equilibrium (it speeds forward and reverse reactions equally). Catalysts are also highly specific in action.

Raw materials of Portland cement: Calcareous materials (limestone, chalk — source of CaO) and argillaceous materials (clay, shale — source of SiO₂, Al₂O₃, Fe₂O₃), plus small amounts of gypsum added after burning.

Four principal (Bogue's) compounds:

Role of gypsum ($\text{CaSO}_4\cdot2\text{H}_2\text{O}$): Added (2–3%) to retard the flash setting caused by C₃A. It reacts with C₃A to form calcium sulphoaluminate (ettringite), giving workable setting time.

Setting and hardening (hydration): When water is added the compounds hydrate exothermically:

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

The C-S-H gel (tobermorite gel) is the main binding phase. Setting is the initial stiffening into a solid mass; hardening is the subsequent gradual gain of strength as hydration continues and the gel crystallizes over days and weeks.

Paint: A paint is a mechanical dispersion (suspension) of one or more finely ground pigments in a liquid medium (vehicle), applied as a thin film to a surface for protection and decoration. On drying it forms an adherent, opaque, coloured coating.

Essential constituents and their functions:

Paint vs varnish:

Explosive: An explosive is a substance (or mixture) which, on application of heat, shock, friction or detonation, undergoes a very rapid self-propagating exothermic chemical reaction, producing a large volume of hot gases and a sudden, enormous increase in pressure (an explosion).

Classification on the basis of performance:

1. Primary (initiating) explosives: Extremely sensitive to heat/shock/friction; detonate readily and are used to set off other explosives (detonators). Example: mercury fulminate $\text{Hg(ONC)}_2$, lead azide $\text{Pb(N}_3)_2$.
2. Low (deflagrating) explosives: Burn rapidly (deflagrate) rather than detonate; produce a propelling/pushing action. Used as propellants. Example: gunpowder (black powder, $\text{KNO}_3$ + charcoal + sulphur), nitrocellulose.
3. High (detonating) explosives: Detonate with a very high velocity shock wave producing a shattering (brisance) effect; relatively insensitive (need a detonator). Example: TNT (trinitrotoluene), RDX, dynamite (nitroglycerine based).

Two characteristics of a good explosive:

1. It should be stable and safe to store/handle under normal conditions but explode reliably when initiated (controlled sensitivity).
2. It should produce a large volume of gas and high heat (high power/brisance) per unit mass rapidly, and ideally be cheap and chemically stable over time.

Cell constant ($G^*$): The ratio of the distance between the electrodes ($l$) to the cross-sectional area of the electrodes ($A$): $G^* = l/A$. Unit: $\text{cm}^{-1}$. It converts measured conductance into specific conductance.

Specific conductance ($\kappa$): The conductance of a solution contained between two electrodes of 1 cm² area held 1 cm apart, i.e. the conductance of 1 cm³ (unit volume) of the solution. Unit: $\text{S cm}^{-1}$ (ohm⁻¹ cm⁻¹).

$$\kappa = \frac{1}{R} \times \text{cell constant}$$

Equivalent conductance ($\Lambda_{eq}$): The conductance of all the ions produced by 1 gram-equivalent of an electrolyte dissolved in a given volume of solution. Unit: $\text{S cm}^2\,\text{eq}^{-1}$.

$$\Lambda_{eq} = \frac{\kappa \times 1000}{N}$$

where $N$ = normality.

Step 1 — specific conductance:

$$\kappa = \frac{1}{R} \times \text{cell constant} = \frac{1}{250} \times 1.25 = 0.005\ \text{S cm}^{-1}$$

Specific conductance $= \mathbf{5.0 \times 10^{-3}\ S\,cm^{-1}}$

Step 2 — equivalent conductance:

$$\Lambda_{eq} = \frac{\kappa \times 1000}{N} = \frac{0.005 \times 1000}{0.01} = \frac{5}{0.01} = 500\ \text{S cm}^2\,\text{eq}^{-1}$$

Equivalent conductance $= \mathbf{500\ S\,cm^2\,eq^{-1}}$

Ion-exchange (demineralization) process: Hard water is passed successively through two columns containing synthetic ion-exchange resins:

• a cation exchanger ($\text{RH}_2$ — resin with mobile $\text{H}^+$),
• an anion exchanger ($\text{R'(OH)}_2$ — resin with mobile $\text{OH}^-$).

Cation exchange (removes Ca²⁺, Mg²⁺):

$$\text{RH}_2 + \text{Ca}^{2+} \rightarrow \text{RCa} + 2\text{H}^+$$ $$\text{RH}_2 + \text{Mg}^{2+} \rightarrow \text{RMg} + 2\text{H}^+$$

Anion exchange (removes Cl⁻, SO₄²⁻, HCO₃⁻):

$$\text{R'(OH)}_2 + \text{SO}_4^{2-} \rightarrow \text{R'SO}_4 + 2\text{OH}^-$$ $$\text{R'(OH)}_2 + 2\text{Cl}^- \rightarrow \text{R'Cl}_2 + 2\text{OH}^-$$

The liberated $\text{H}^+$ and $\text{OH}^-$ combine to give water ($\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$), so the outflow is almost ion-free (demineralized) water.

Regeneration of exhausted resins:

• Exhausted cation resin is regenerated by washing with dilute acid (HCl or H₂SO₄):

$$\text{RCa} + 2\text{H}^+ \rightarrow \text{RH}_2 + \text{Ca}^{2+}$$

• Exhausted anion resin is regenerated by washing with dilute alkali (NaOH):

$$\text{R'SO}_4 + 2\text{OH}^- \rightarrow \text{R'(OH)}_2 + \text{SO}_4^{2-}$$

The columns are rinsed and reused.

Advantages (two):

1. Produces very low residual hardness (almost zero, ~2 ppm) and essentially ion-free (demineralized) water suitable for high-pressure boilers.
2. The resins can be regenerated and reused many times, and the process can treat both acidic and alkaline water.

Limitation (one): The equipment and resins are expensive, and turbid/coloured water or water containing Fe/Mn must be pre-treated, otherwise the resin bed gets fouled and loses efficiency.