BE Civil Engineering (IOE, TU) Engineering Chemistry (IOE, SH 403) Question Paper 2076 Nepal

Galvanic (Daniell) cell — definition and working

A galvanic cell is an electrochemical device that converts chemical energy of a spontaneous redox reaction into electrical energy. The Daniell cell uses a Zn electrode in $\text{ZnSO}_4$ and a Cu electrode in $\text{CuSO}_4$, joined by a salt bridge.

      e- ---->  (external wire)  ---->
   [Zn]                              [Cu]
  anode (-)    +-- salt bridge --+   cathode (+)
  ZnSO4 soln   |  KCl / KNO3     |   CuSO4 soln
               +-----------------+

• Anode (oxidation): $\text{Zn} \rightarrow \text{Zn}^{2+} + 2e^-$
• Cathode (reduction): $\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}$
• The salt bridge maintains electrical neutrality; electrons flow externally from Zn to Cu.

(a) Cell reaction and standard EMF

Overall: $\text{Zn}(s) + \text{Cu}^{2+} \rightarrow \text{Zn}^{2+} + \text{Cu}(s)$

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

(b) Nernst equation (with $n=2$, $T=298\ \text{K}$):

$$E_{cell} = E^{\circ}_{cell} - \frac{0.0592}{n}\log\frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]}$$ $$\frac{[\text{Zn}^{2+}]}{[\text{Cu}^{2+}]} = \frac{0.010}{0.50} = 0.020,\qquad \log(0.020) = -1.699$$ $$E_{cell} = 1.10 - \frac{0.0592}{2}(-1.699) = 1.10 + 0.0296\times1.699 = 1.10 + 0.0503$$ $$\boxed{E_{cell} = \mathbf{1.150\ V}}$$

(c) Equilibrium constant

At equilibrium $E_{cell}=0$, so $E^{\circ}_{cell} = \frac{0.0592}{n}\log K$:

$$\log K = \frac{n\,E^{\circ}_{cell}}{0.0592} = \frac{2\times1.10}{0.0592} = 37.16$$ $$\boxed{K = 10^{37.16} \approx \mathbf{1.45\times10^{37}}}$$

The enormous $K$ confirms the reaction proceeds essentially to completion.

Electrochemical (wet) corrosion of iron

Wet corrosion occurs when a metal is in contact with a conducting electrolyte (moisture + dissolved $\text{O}_2$/salts). Micro galvanic cells form on the surface: anodic and cathodic regions develop due to impurities, stress, or differential aeration.

• Anodic reaction (oxidation): $\text{Fe} \rightarrow \text{Fe}^{2+} + 2e^-$

The fate of the electrons at the cathode depends on the environment:

(i) Hydrogen-evolution mechanism (acidic media, e.g. low pH):

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

Dissolves the protective film; occurs in acidic, deaerated solutions.

(ii) Oxygen-absorption mechanism (neutral/alkaline aerated media — most common atmospheric rusting):

$$\text{O}_2 + 2\text{H}_2\text{O} + 4e^- \rightarrow 4\text{OH}^-$$ $$\text{Fe}^{2+} + 2\text{OH}^- \rightarrow \text{Fe(OH)}_2 \xrightarrow{\ \text{O}_2\ } \text{Fe}_2\text{O}_3\cdot x\text{H}_2\text{O (rust)}$$

Two factors affecting corrosion rate

1. Nature/pH of medium: acidic media accelerate corrosion (hydrogen evolution); alkaline/neutral media slow it. Presence of dissolved salts (e.g. NaCl) increases conductivity and corrosion.
2. Position in galvanic series / relative electrode potential: the more active (anodic) metal corrodes faster; differential aeration (less-oxygenated regions become anodic) localises attack.

Cathodic protection of a buried steel pipeline

The principle is to make the entire structure act as a cathode so it does not oxidise.

• Sacrificial-anode method: a more active metal (Mg, Zn, Al) is connected to the pipeline. It becomes the anode and corrodes preferentially, protecting the steel.

  [Mg anode] --wire-- [Steel pipeline (cathode)]
        \___ buried in moist soil (electrolyte) ___/
  Mg -> Mg2+ + 2e-   (anode dissolves, steel protected)

• Impressed-current method: an external DC source drives current from an inert/auxiliary anode (graphite, Pt-Ti) into the soil and onto the pipeline, forcing the steel to be cathodic. Suited to large/long structures.

Why Mg over Cu as sacrificial anode: magnesium is more electronegative (more active, $E^{\circ}_{\text{Mg}^{2+}/\text{Mg}}=-2.37\ \text{V}$) than iron, so Mg is anodic to steel and corrodes in its place. Copper ($E^{\circ}=+0.34\ \text{V}$) is nobler than iron; coupling Cu to steel would make the steel the anode and accelerate its corrosion. Hence Cu is unsuitable.

Definitions

• Temporary (carbonate) hardness: due to dissolved bicarbonates of Ca and Mg — $\text{Ca(HCO}_3)_2$, $\text{Mg(HCO}_3)_2$. Removed by boiling.
• Permanent (non-carbonate) hardness: due to chlorides and sulphates of Ca and Mg — $\text{CaSO}_4$, $\text{CaCl}_2$, $\text{MgSO}_4$, $\text{MgCl}_2$. Not removed by boiling.

Conversion to CaCO₃ equivalent

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

Results

• Temporary hardness $= 10.0 + 10.0 = \mathbf{20.0\ ppm}$
• Permanent hardness $= 10.0 + 10.0 = \mathbf{20.0\ ppm}$
• Total hardness $= 20.0 + 20.0 = \mathbf{40.0\ ppm\ (as\ CaCO}_3)$

Zeolite (permutit) process

Zeolite is hydrated sodium aluminosilicate ($\text{Na}_2\text{O}\cdot\text{Al}_2\text{O}_3\cdot x\text{SiO}_2\cdot y\text{H}_2\text{O}$, written $\text{Na}_2\text{Ze}$). Hard water is passed through a bed of zeolite, which exchanges its $\text{Na}^+$ for the $\text{Ca}^{2+}$/$\text{Mg}^{2+}$ causing hardness:

$$\text{Na}_2\text{Ze} + \text{Ca}^{2+} \rightarrow \text{CaZe} + 2\text{Na}^+$$ $$\text{Na}_2\text{Ze} + \text{Mg}^{2+} \rightarrow \text{MgZe} + 2\text{Na}^+$$

The exhausted zeolite (CaZe/MgZe) is regenerated by passing a concentrated (10%) brine ($\text{NaCl}$) solution:

$$\text{CaZe} + 2\text{NaCl} \rightarrow \text{Na}_2\text{Ze} + \text{CaCl}_2$$

The washings (CaCl₂/MgCl₂) are run to drain and the regenerated $\text{Na}_2\text{Ze}$ is reused.

Polymerization: the chemical process in which a large number of small molecules (monomers) combine to form a high-molar-mass macromolecule (polymer).

Addition vs Condensation polymerization

Vulcanization of rubber

Natural rubber (cis-polyisoprene) is soft, tacky, has low tensile strength and poor temperature stability. Vulcanization is heating raw rubber with sulphur (3–5%) at ~140 °C. Sulphur forms cross-links (–S–S– bridges) between adjacent polyisoprene chains.

  ...chain A...  
        |  S
        |  S   <-- sulphur cross-link
  ...chain B...

Effects: increases tensile strength, elasticity, hardness and abrasion resistance; reduces tackiness and water absorption; widens the useful temperature range. Hard rubber (ebonite) results from high sulphur content.

Degree of polymerization (DP): the number of repeating (monomer) units present in one polymer molecule.

$$DP = \frac{\text{number-average molar mass of polymer}}{\text{molar mass of monomer}} = \frac{42{,}000}{28}$$ $$\boxed{DP = \mathbf{1500}}$$

PVC (polyvinyl chloride) — monomer vinyl chloride $\text{CH}_2=\text{CHCl}$.

• Properties: chemically resistant (to acids, alkalis, water), good electrical insulator, rigid yet can be plasticized to be flexible, non-flammable (self-extinguishing), durable/weather-resistant.
• Civil application: water-supply and drainage/sewer pipes; electrical conduit and cable insulation; window and door profiles.

Calorific value: the total quantity of heat liberated by the complete combustion of a unit mass (or volume) of a fuel.

• Gross/Higher CV (GCV): heat liberated when combustion products are cooled to room temperature, so the water vapour condenses and its latent heat is recovered.
• Net/Lower CV (NCV): GCV minus the latent heat of the water vapour formed (vapour leaves uncondensed). NCV < GCV.

Dulong's formula (GCV in kcal/kg, masses as fractions):

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

(a) Gross calorific value

With $C=80$, $H=6$, $O=8$, $S=1$ (as percentages):

$$H-\frac{O}{8} = 6 - \frac{8}{8} = 6 - 1 = 5$$ $$GCV = \frac{1}{100}\big[8080(80) + 34500(5) + 2240(1)\big]$$ $$= \frac{1}{100}\big[646400 + 172500 + 2240\big] = \frac{821140}{100}$$ $$\boxed{GCV = \mathbf{8211.4\ kcal/kg}}$$

(b) Net calorific value

Mass of water formed per kg fuel $= 9\times H_{frac} = 9\times0.06 = 0.54\ \text{kg}$.

Latent heat carried away $= 0.54\times587 = 316.98\ \text{kcal/kg}$.

$$NCV = GCV - 316.98 = 8211.4 - 316.98$$ $$\boxed{NCV = \mathbf{7894.4\ kcal/kg}}$$

(c) Four characteristics of a good fuel

1. High calorific value — large heat output per unit mass/volume.
2. Moderate ignition temperature — easy to ignite but safe to store.
3. Low moisture, ash and non-combustible content; products of combustion non-toxic/non-polluting.
4. Cheap, readily available, easy to store, transport and handle, with a controllable, smokeless combustion rate.

Catalyst: a substance that alters (usually increases) the rate of a chemical reaction by providing an alternative path of lower activation energy, while itself remaining chemically unchanged in mass and composition at the end of the reaction.

Homogeneous vs heterogeneous catalysis

Four characteristics of a catalyst

1. It is not consumed; recovered unchanged in mass/chemical nature at the end.
2. A small quantity is usually sufficient.
3. It lowers the activation energy; it does not alter the position of equilibrium ($\Delta G$, $K$ unchanged) — it speeds both forward and reverse reactions equally.
4. It is generally specific in action and its activity can be enhanced by promoters or destroyed by poisons.

(Enzyme catalysis is a highly specific biochemical example, e.g. invertase converting sucrose to glucose + fructose.)

Raw materials of Portland cement

• Calcareous (lime) materials: limestone, chalk — supply CaO.
• Argillaceous (clayey) materials: clay, shale — supply $\text{SiO}_2$, $\text{Al}_2\text{O}_3$, $\text{Fe}_2\text{O}_3$.
• Small amounts of gypsum ($\text{CaSO}_4\cdot2\text{H}_2\text{O}$) added to the clinker.

Major Bogue compounds

Setting and hardening (hydration)

When water is added, the compounds hydrate and hydrolyse, forming gels and crystals that interlock:

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

• Setting: initial stiffening of the paste (loss of plasticity) — early reaction, largely of C₃A and C₃S.
• Hardening: subsequent gain in strength over days/weeks as the C–S–H gel develops; C₃S gives early strength, C₂S gives later strength.

Role of gypsum: C₃A reacts violently with water and would cause flash (instant) setting. Gypsum reacts with C₃A to form ettringite, retarding the reaction and giving a workable setting time.

Paint: a mechanical dispersion (suspension) of a finely divided pigment in a liquid vehicle (drying oil + thinner) which, when applied as a thin coat to a surface and dried, gives an opaque, adherent, protective and decorative film.

Essential constituents and their functions

Varnish vs paint

• A varnish is a homogeneous transparent/translucent solution of a resin (natural or synthetic) in a drying oil and/or volatile solvent — it contains no pigment.
• It gives a glossy, transparent protective film that shows the grain/colour of the surface beneath, whereas a paint gives an opaque, coloured film due to its pigment.

Explosive: a chemical substance (or mixture) which, on the application of a suitable stimulus (heat, shock, friction, or impact), undergoes an extremely rapid exothermic decomposition producing a large volume of hot gases and a sudden release of energy (a violent explosion).

Classification (by performance/rate of reaction)

Two characteristics of a good explosive

1. It should be chemically stable and safe to store/handle, yet detonate reliably on the intended stimulus.
2. It should produce a large volume of gas with high heat (high brisance/shattering power) per unit mass.

Two safety precautions in civil blasting

1. Store explosives and detonators separately in cool, dry, ventilated magazines away from heat, flame, friction and impact.
2. Use proper detonators/fuses, clear and guard the blast zone, and avoid overcharging/handling near sparks or static; trained personnel only.

Lime requirement (lime–soda process)

For each component expressed as CaCO₃ equivalent, the lime needed is:

$$\text{Lime} = \frac{74}{100}\times(\text{CaCO}_3\ \text{equivalent of the hardness causing component})$$

For temporary hardness due to $\text{Ca(HCO}_3)_2$, one mole of lime is required per mole (as CaCO₃ equiv). Here the hardness is already given as CaCO₃ equivalent $= 25\ \text{ppm} = 25\ \text{mg/L}$.

Step 1 — pure lime needed per litre:

$$= \frac{74}{100}\times25 = 18.5\ \text{mg/L}$$

Step 2 — total pure lime for 50,000 L:

$$= 18.5\ \text{mg/L} \times 50{,}000\ \text{L} = 925{,}000\ \text{mg} = 925\ \text{g} = 0.925\ \text{kg}$$

Step 3 — correct for 90% purity:

$$\text{Lime (90\% pure)} = \frac{0.925}{0.90} = 1.0278\ \text{kg}$$ $$\boxed{\text{Lime required} \approx \mathbf{1.028\ kg}\ (\approx 1028\ \text{g})}$$

Thermoplastic vs Thermosetting polymers

Civil / engineering applications

• Thermoplastic — PVC: water-supply and drainage pipes, electrical cable insulation, window/door profiles.
• Thermosetting — epoxy / Bakelite: epoxy as structural adhesive, flooring and concrete repair/crack injection; Bakelite for electrical switchgear and heat-resistant fittings.