CSIR NET Chemical Sciences Physical Chemistry Flashcards

24 sample cards from the Physical Chemistry deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. In a galvanic cell, what reactions occur at the anode and cathode?

   Oxidation occurs at the anode (electrons released), reduction occurs at the cathode (electrons consumed). Mnemonic: 'An Ox, Red Cat'. Electrons flow from anode to cathode through the external circuit.

2. Write the Nernst equation for an electrode/cell at 298 K.

   E = E° − (RT/nF) ln Q = E° − (0.0591/n) log Q at 298 K, where n = electrons transferred, F = Faraday constant, Q = reaction quotient. It relates electrode/cell potential to concentrations.

3. What is the standard hydrogen electrode (SHE) and its assigned potential?

   The SHE is the reference electrode: Pt | H₂(1 atm) | H⁺(1 M), assigned a standard electrode potential of exactly 0.00 V at all temperatures. All other electrode potentials are measured relative to it.

4. How is the standard cell EMF related to standard electrode potentials, and to ΔG°?

   E°cell = E°cathode − E°anode (both as reduction potentials). ΔG° = −nFE°cell; a positive E°cell corresponds to a spontaneous reaction (negative ΔG°).

5. Give two applications of electrochemical cells / potential measurements.

   Examples: determining pH using a glass/calomel electrode; measuring solubility products (Ksp) and equilibrium constants via E°; potentiometric titrations; corrosion studies; and powering devices (batteries, fuel cells).

6. For the equilibrium of a reaction, how are Kp and Kc related?

   Kp = Kc(RT)^Δnₘ, where Δnₘ = (moles of gaseous products − moles of gaseous reactants). When Δnₘ = 0, Kp = Kc.

7. State Le Chatelier's principle.

   When a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium shifts in the direction that counteracts (partially offsets) the imposed change.

8. According to Le Chatelier's principle, how does increasing pressure affect a gaseous equilibrium?

   Increasing pressure (decreasing volume) shifts equilibrium toward the side with fewer moles of gas. If moles of gas are equal on both sides, pressure has no effect on the position of equilibrium.

9. Define the common ion effect and its consequence on solubility.

   The common ion effect is the suppression of ionization of a weak electrolyte (or decrease in solubility of a sparingly soluble salt) by adding an ion already present in the equilibrium. It shifts equilibrium per Le Chatelier, lowering dissociation/solubility.

10. State the Henderson–Hasselbalch equation for a buffer.

    pH = pKa + log([conjugate base]/[acid]) (for an acidic buffer). For a basic buffer: pOH = pKb + log([conjugate acid]/[base]). It relates buffer pH to the ratio of components.

11. What is the difference between physisorption and chemisorption?

    Physisorption: weak van der Waals forces, low enthalpy (20–40 kJ/mol), reversible, multilayer, decreases with rising temperature, not specific. Chemisorption: chemical bonds, high enthalpy (80–240 kJ/mol), often irreversible, monolayer, specific, may increase then decrease with temperature.

12. State the Freundlich and Langmuir adsorption isotherm key features.

    Freundlich: x/m = k·P^(1/n) (empirical, no saturation, multilayer at high pressure). Langmuir: x/m = (a·P)/(1 + b·P), assumes monolayer adsorption on uniform sites with no interaction; gives saturation at high pressure.

13. What is the difference between a lyophilic and lyophobic colloid?

    Lyophilic (solvent-loving): reversible, stable, self-forming, high solvent affinity (e.g., starch, gelatin). Lyophobic (solvent-hating): irreversible, less stable, need stabilizers, prepared by special methods (e.g., metal/As₂S₃ sols).

14. What is the Tyndall effect and what does it indicate?

    The scattering of a beam of light by colloidal particles, making the light path visible. It distinguishes colloidal solutions (show Tyndall effect) from true solutions (do not), because colloidal particle size is comparable to/larger than light wavelength.

15. Define surface tension and state how it changes with temperature.

    Surface tension is the force per unit length acting along the surface of a liquid (units N/m), arising from unbalanced cohesive forces at the surface (minimizes surface area). It decreases with increasing temperature and becomes zero at the critical temperature.

16. What is the difference between homogeneous and heterogeneous catalysis (catalysis by surfaces)?

    Homogeneous: catalyst and reactants in the same phase. Heterogeneous (surface) catalysis: catalyst in a different phase (usually solid catalyst with gas/liquid reactants); reaction occurs on the surface via adsorption, reaction, and desorption of products.

17. What are the seven crystal systems and how many Bravais lattices exist?

    Seven crystal systems: cubic, tetragonal, orthorhombic, monoclinic, triclinic, hexagonal, and rhombohedral (trigonal). There are 14 Bravais lattices in total.

18. Compare the packing efficiency and coordination number of SCC, BCC, and FCC.

    Simple cubic: CN 6, packing 52.4%, 1 atom/cell. Body-centered cubic: CN 8, packing 68%, 2 atoms/cell. Face-centered cubic (CCP): CN 12, packing 74%, 4 atoms/cell (most efficient).

19. State Bragg's law of X-ray diffraction.

    nλ = 2d sinθ, where n = order of diffraction, λ = X-ray wavelength, d = interplanar spacing, and θ = glancing/Bragg angle. It gives the condition for constructive interference of X-rays from crystal planes.

20. What is the difference between Schottky and Frenkel defects?

    Schottky defect: equal numbers of cation and anion vacancies (decreases density); favored when ions are similar in size (e.g., NaCl). Frenkel defect: a cation displaced to an interstitial site (density unchanged); favored when cation is much smaller than anion (e.g., AgCl, ZnS).

21. How do n-type and p-type semiconductors arise from doping?

    n-type: doping with a higher-valence (electron-rich) impurity (e.g., Si doped with P) adds free electrons as charge carriers. p-type: doping with a lower-valence (electron-deficient) impurity (e.g., Si doped with B) creates positive 'holes' as carriers.

22. Distinguish addition (chain-growth) and condensation (step-growth) polymerization.

    Addition: monomers with C=C add without losing any atoms, via chain mechanism (initiation, propagation, termination); e.g., polyethylene. Condensation: monomers with two functional groups join with elimination of a small molecule (e.g., H₂O); e.g., nylon, polyester.

23. Define number-average (M̄n) and weight-average (M̄w) molecular weights, and the polydispersity index (PDI).

    M̄n = ΣNᵢMᵢ/ΣNᵢ (counts molecules). M̄w = ΣNᵢMᵢ²/ΣNᵢMᵢ (weights by mass; ≥ M̄n). PDI = M̄w/M̄n ≥ 1; PDI = 1 means a monodisperse polymer.

24. What are biodegradable polymers? Give two examples.

    Polymers that can be broken down by microorganisms/enzymes into simpler harmless products (CO₂, H₂O, biomass). Examples: PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate), PLA (polylactic acid), Nylon-2-nylon-6, and PGA (polyglycolic acid).

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