Surface Chemistry

To prevent coagulation of colloidal gold particles by keeping thermal energy of particles low

Step 1: In Bredig's arc method, gold electrodes produce an arc that vaporises gold, which then condenses in water to form colloidal particles. Step 2: The newly formed colloidal particles are extremely small (1–200 nm) and thermodynamically unstable — they tend to aggregate (coagulate) due to their high surface energy. Step 3: At higher temperatures, colloidal particles gain more kinetic energy, collide more frequently, and coagulate more readily. Ice-cold water removes this thermal energy, reducing particle motion and collision frequency. Step 4: Option A is incorrect — conductivity is not th

The pores of the semipermeable membrane allow small ions (like Fe³⁺ and Cl⁻ from FeCl₃ impurity) to pass through but retain larger colloidal Fe(OH)₃ particles

Step 1: Fe(OH)₃ sol is prepared by hydrolysis of FeCl₃: FeCl₃ + 3H₂O → Fe(OH)₃ (sol) + 3HCl. The impurity is excess FeCl₃ or HCl (electrolytes containing small Fe³⁺, Cl⁻ ions). Step 2: The semipermeable membrane has pores of size intermediate between colloidal particles (1–200 nm) and true solution ions (< 1 nm). Step 3: Small electrolyte ions (Fe³⁺, Cl⁻, H⁺) pass freely through the membrane pores, diffusing into the flowing water and being carried away. Colloidal Fe(OH)₃ particles (larger size) cannot pass through and are retained. Step 4: Option A is wrong — charge of colloidal particles is

Initial increase occurs because chemisorption requires activation energy — higher temperature activates more molecules; decrease at high temperature is due to desorption as kinetic energy of adsorbate exceeds adsorption energy

Step 1: Chemisorption involves formation of chemical bonds between adsorbent and adsorbate, which requires activation energy (unlike physisorption). Step 2: At low temperatures, molecules do not have sufficient energy to overcome the activation energy barrier. As temperature increases, more molecules gain enough energy → chemisorption increases. This is why chemisorption is also called 'activated adsorption'. Step 3: At very high temperatures, the kinetic energy of adsorbed molecules becomes very large. These molecules overcome the adsorption forces and escape from the surface — this is called

H₂S molecules are preferentially and strongly adsorbed on the active centres (steps, cracks, corners) of the Fe catalyst surface, blocking N₂ and H₂ from adsorbing

Step 1: According to adsorption theory, heterogeneous catalysis occurs at 'active centres' — coordinatively unsaturated atoms at steps, cracks, and corners of the catalyst surface that have residual bonding forces. Step 2: H₂S molecules have a strong affinity for iron surfaces and get chemisorbed onto these active centres with great strength. Since chemisorption forms monolayers, these sites become permanently occupied. Step 3: With active centres blocked, N₂ and H₂ molecules cannot adsorb on the Fe surface → no activated complex formation → no catalysis. Step 4: Option A is wrong — H₂S does n