Surface Chemistry: Tricky Questions Solved (5)

Question

Surface Chemistry: tricky questions solved for NEET

doubts.ai · Accepted Answer

Solution — Step by Step Criterion Physisorption Chemisorption --- --- --- Nature of forces van der Waals (weak) Chemical bond (strong) H {ads} -20 to -40 kJ/mol -80 to -400 kJ/mol Temperature Decreases with rising T Increases with T initially, then decreases Specificity Non-specific Highly specific Reversibility Reversible Often irreversible The Haber process operates at 700 	ext{ K} — high temperature. Physisorption would decrease with rising T, so N₂ wouldn't stick. The fact that adsorption occurs at high T rules out physisorption. Also, N₂ on Fe shows specificity: not every metal catalyses the reaction. Only Fe (and a few others like Ru) work well. Specificity is a chemisorption hallmark. Conclusion: N₂ on Fe in the Haber process is chemisorption . Chemisorption breaks the strong N N triple bond by adsorbing N 2 onto Fe and forming weaker Fe-N bonds. These weaker bonds are easier to break in the next step (reaction with H 2 on the surface). This is the whole point of a heterogeneous catalyst — it lowers activation energy by making chemisorbed intermediates. Why This Works Adsorption is the accumulation of molecules on a surface. The strength of the molecule-surface interaction determines whether it's physisorption (cold-trap-style sticking by weak forces) or chemisorption (a real chemical bond forms). The five criteria are interconnected: stronger forces larger \Delta H 	o more specific binding 	o harder to reverse. For NEET, the key application is heterogeneous catalysis — every industrial catalyst (Haber, Ostwald, contact process) involves chemisorption of reactants onto the catalyst surface. Alternative Method You can identify the type by enthalpy values alone if given. H = -30 	ext{ kJ/mol} is typical of physisorption; H = -200 	ext{ kJ/mol} unmistakably chemisorption. NEET often hands you the enthalpy and expects you to classify in one step. A clean memory aid: physisorption is like a snug hug, chemisorption is like a marriage . Hugs are easy to break (reversible), can stack up (multilayer), and don't depend on who you are (non-specific). Marriages bind tight (high H), are usually one-on-one (monolayer), and are person-specific. Common Mistake Two traps NEET sets repeatedly: 1. Temperature dependence of chemisorption. It's not monotonic. At low T, raising T helps overcome the activation energy for bond formation, so adsorption increases. Beyond an optimum, the Maxwell-Boltzmann tail starts overcoming the binding energy and adsorption falls. Many students simplify to "chemisorption increases with T" — wrong above the optimum. 2. Calling all gas-on-metal adsorption physisorption. It depends on the gas/metal pair and conditions. Noble gases on metals at low T → physisorption. Reactive gases (N 2, H 2, O 2) on transition metals at high T → chemisorption. Final answer: Five criteria distinguish physisorption (weak, non-specific, reversible) from chemisorption (strong, specific, often irreversible). N 2 on Fe in the Haber process is chemisorption — operates at high T, is specific to Fe, and forms surface Fe-N bonds.

Criterion	Physisorption	Chemisorption
Nature of forces	van der Waals (weak)	Chemical bond (strong)
$\Delta H_{ads}$	$-20$ to $-40 \text{ kJ/mol}$	$-80$ to $-400 \text{ kJ/mol}$
Temperature	Decreases with rising $T$	Increases with $T$ initially, then decreases
Specificity	Non-specific	Highly specific
Reversibility	Reversible	Often irreversible

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

Try These Next