Engineering Chemistry Exam Notes

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1. Scale and its Causes Definition: Scale refers to hard, adherent deposits formed on the internal surfaces of boilers and pipes. Causes: Decomposition of Bicarbonates: $\text{Ca(HCO}_3)_2 \xrightarrow{\Delta} \text{CaCO}_3 \downarrow + \text{H}_2\text{O} + \text{CO}_2$ Hydrolysis of Magnesium Salts: $\text{MgCl}_2 + 2\text{H}_2\text{O} \to \text{Mg(OH)}_2 \downarrow + 2\text{HCl}$ Decrease in Solubility of $\text{CaSO}_4$: $\text{CaSO}_4$ becomes less soluble at higher temperatures. Presence of Silica: Forms hard silicate scales (e.g., $\text{CaSiO}_3$, $\text{MgSiO}_3$). 2. Demineralization Process (Ion Exchange) Principle: Removal of all dissolved ionic salts from water using ion-exchange resins. Process: Cation Exchanger: All cations are exchanged for $\text{H}^+$ ions. $\text{RH}_2 + \text{CaCl}_2 \to \text{RCa} + 2\text{HCl}$ $\text{RH}_2 + \text{MgSO}_4 \to \text{RMg} + \text{H}_2\text{SO}_4$ Anion Exchanger: Anions from the water and from the cation effluent are exchanged for $\text{OH}^-$ ions. $\text{R'OH}_2 + \text{H}_2\text{SO}_4 \to \text{R'SO}_4 + 2\text{H}_2\text{O}$ $\text{R'OH}_2 + 2\text{HCl} \to \text{R'Cl}_2 + 2\text{H}_2\text{O}$ Regeneration: Cation resin: Washed with dilute acid (e.g., $\text{HCl}$). $\text{RNa} + \text{HCl} \to \text{RH} + \text{NaCl}$ Anion resin: Washed with dilute alkali (e.g., $\text{NaOH}$). $\text{RCl} + \text{NaOH} \to \text{ROH} + \text{NaCl}$ 3. Reverse Osmosis (RO) Principle: Application of external pressure greater than osmotic pressure to force solvent (water) from a region of higher solute concentration through a semi-permeable membrane to a region of lower solute concentration. Mechanism: The membrane allows water molecules to pass but rejects dissolved salts and other impurities. Real-world Scenarios: Desalination: Converting seawater into potable drinking water. Wastewater Treatment: Removing dissolved contaminants from industrial and municipal wastewater. 4. Priming and its Prevention Definition: Priming is the carrying over of water droplets by steam into the steam line, often due to rapid boiling. Causes: Rapid changes in steaming rate. High water level in the boiler. Presence of suspended solids in boiler water. Faulty boiler design or insufficient steam space. Preventive Measures: Maintaining a steady steam output. Controlling the water level in the boiler. Effective blowdown to remove suspended solids. Using steam purifiers or separators. 5. Wet Corrosion (Hydrogen Liberation Type) Mechanism: Occurs in acidic environments ($pH Anode: Iron oxidizes, releasing electrons: $\text{Fe} \to \text{Fe}^{2+} + 2\text{e}^-$ Cathode: $\text{H}^+$ ions accept electrons to form hydrogen gas: $2\text{H}^+ + 2\text{e}^- \to \text{H}_2 \uparrow$ Overall Reaction: $\text{Fe} + 2\text{H}^+ \to \text{Fe}^{2+} + \text{H}_2 \uparrow$ Common in boiler pipes where water is acidic. 6. Factors Affecting Rate of Corrosion Nature of Metal: Purity of metal, position in galvanic series, overvoltage of $\text{H}_2$ on metal, nature of oxide film. Nature of Environment: Temperature: Higher temperature generally increases corrosion rate. Humidity: Presence of moisture is essential for electrochemical corrosion. pH: Acidic environments ($pH Concentration of Oxygen: Higher $\text{O}_2$ concentration accelerates corrosion (e.g., oxygen absorption). Presence of Impurities: Dissolved salts (e.g., $\text{NaCl}$) increase conductivity and corrosion. 7. Cathodic Protection System Design (Underground Steel Pipeline) Principle: Making the metal to be protected (pipeline) the cathode of an electrochemical cell, preventing its dissolution. Methods: Sacrificial Anodic Protection: A more active metal (e.g., Mg, Zn, Al) is connected to the pipeline. The active metal acts as the anode and corrodes sacrificially, protecting the pipeline. Example: $\text{Mg} \to \text{Mg}^{2+} + 2\text{e}^-$ (anode), $\text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^- \to 4\text{OH}^-$ (cathode on pipeline). Requires periodic replacement of sacrificial anodes. Impressed Current Protection: An external DC power source is used to impress a current on the pipeline. An inert anode (e.g., graphite, high silicon cast iron) is buried in the soil and connected to the positive terminal of the DC source. The pipeline is connected to the negative terminal, making it the cathode. Suitable for large structures like long pipelines. 8. Galvanization Definition: Galvanization is the process of applying a protective zinc coating to steel or iron to prevent rusting. Process (Hot-dip galvanizing): Surface Preparation: Steel is cleaned (degreasing, pickling in acid) to remove dirt, oil, and rust. Fluxing: The cleaned steel is immersed in a flux solution (e.g., zinc ammonium chloride) to remove residual oxides and prevent further oxidation. Galvanizing: The steel is dipped into a bath of molten zinc (typically at $450^\circ\text{C}$), forming a metallurgical bond and a series of zinc-iron alloy layers, topped by a layer of pure zinc. Cooling/Finishing: The galvanized steel is withdrawn and cooled. Diagram: (Imagine a simple diagram showing steel immersed in molten zinc bath, with layers forming on the surface). Applications: Corrosion Protection: Widely used for roofing sheets, pipes, guardrails, and automotive parts to protect against rust. Construction: Used for structural steel, fasteners, and rebar in construction for enhanced durability.