Transition state theory — activated complex (intro): Question bank

2.8.10 · D5 · Chemistry › Chemical Kinetics › Transition state theory — activated complex (intro)

Yeh ek rapid-fire trap-clearing deck hai Transition state theory — activated complex ke liye. Har line mein ek reveal chhupi hai. Apna jawab out loud bolein, reason ke saath peek karne se pehle — sirf "true" ya "false" bolne se kuch nahi milega. Goal hai har us misconception ko jala dena jo reaction coordinate picture plant kar deta hai.

True or false — justify

TST collision theory ko poori tarah replace kar deta hai aur kehta hai ki collisions ab matter nahi karti.

False — molecules ko react karne ke liye abhi bhi collide karna padta hai; TST add karta hai collision ke baad kya hota hai, yaani transition state tak climb karna. Yeh Collision theory ko refine karta hai, delete nahi.

Activated complex ko ek normal reagent ki tarah bottle mein band karke store kiya ja sakta hai.

False — yeh energy ke maximum par baitha hai aur sirf ek bond vibration (

\sim 1 0^{- 13}

s) tak survive karta hai, isliye yeh passage ka ek point hai, koi isolable substance kabhi nahi.

Ek intermediate aur ek transition state ek hi cheez hai jo do baar draw ki gayi hai.

False — ek intermediate local minimum mein rest karta hai (measurable lifetime), jabki transition state ek local maximum hai jo ek vibration tak rehta hai; ek valley aur ek peak bilkul opposite hain.

Agar ek reaction exothermic hai, toh uski activation energy zaroor chhoti hogi.

False —

Δ H

(reactant-to-product drop) aur

E_{a}

(reactant-to-peak climb) independent hain;

H_{2} + O_{2}

wildly exothermic hai phir bhi spark chahiye kyunki uska peak abhi bhi high hai.

H_{2} \to 2 H

ke liye, activation energy roughly H–H bond energy ke barabar hoti hai.

True — ek pure dissociation mein koi nayi bond form nahi hoti jo peak ko neeche kare, isliye top tak pahunchne ke liye almost poora bond-dissociation cost dena padta hai.

H_{2} + I_{2} \to 2 HI

ke liye,

E_{a}

H–H aur I–I bond energies ke sum ke barabar hai.

False — nayi H–I bonds transition state par form hone lagti hain aur cost ka bada hissa wapas deti hain, isliye

E_{a} \approx 170

kJ/mol hai,

436 + 151 = 587

kJ/mol se kaafi neeche.

Transition state hamesha structure mein reactants aur products ke bilkul beech mein hota hai.

False — Hammond's postulate kehta hai yeh usi side se milta-julta hota hai jo energy mein closer ho: exothermic ke liye early (reactant-like), endothermic reactions ke liye late (product-like).

E_{a}^{forward} - E_{a}^{reverse} = Δ E_{reaction}

kisi bhi single-barrier reaction ke liye.

True — dono activation energies usi peak se measure ki jaati hain, isliye unka difference bas do valleys ke beech ka vertical gap hai, jo

Δ E

hai.

Ek catalyst reaction ka

Δ H

kam kar deta hai.

False — ek catalyst ek naya lower-energy path (ek chhoti hill) kholta hai jo

E_{a}

change karta hai; valleys (reactants aur products) untouched rehti hain, isliye

Δ H

unchanged rehta hai.

Reverse activation energy negative ho sakti hai.

False ek ordinary single barrier ke liye — peak dono valleys se upar hoti hai, isliye

E_{a}^{fwd}

aur

E_{a}^{rev}

dono

\geq 0

hain (sirf complex multi-step apparent

E_{a}

negative appear ho sakti hai, jo ek alag quantity hai).

Spot the error

"Activated complex reaction ke dauran atoms ka sabse stable arrangement hai."

Error: yeh path ka sabse kam stable point hai — potential energy mein ek maximum — yahi reason hai ki isse isolate nahi kiya ja sakta.

"Ek

S_{N} 2

transition state mein carbon

s p^{3}

hai aur uske paas chaar full bonds hain."

Error: carbon transiently pentacoordinate (trigonal bipyramidal) hai, incoming nucleophile ke saath ek partial bond aur leaving group ke saath ek partial bond simultaneously — dekho SN1 vs SN2 mechanisms.

"Kyunki

H_{2} + I_{2}

ke liye

Δ H = + 10

kJ/mol hai, barrier lagbhag 10 kJ/mol hona chahiye."

Error:

Δ H

sirf valley-to-valley change measure karta hai; peak reactants se

\sim 170

kJ/mol upar baitha hai kyunki bonds pehle toot'te hain naye wale fully form hone se.

"Arrhenius

E_{a}

aur transition-state barrier height unrelated numbers hain."

Error: woh dono usi hill ko describe karte hain — Arrhenius equation ka

E_{a}

transition state tak climb ka experimental read-out hai (Eyring equation isse

Δ H^{‡}

mein refine karta hai).

"Ek exothermic reaction ka transition state products ke closer hota hai."

Error: Hammond's postulate iska ulta kehta hai — ek exothermic step ka transition state early, reactant-like hota hai; product-like (late) wala endothermic steps ka hota hai.

"Kyunki

S_{N} 2

transition state mein ek bond form ho rahi hai, uski energy reactants se kam honi chahiye."

Error: ek bond partially offset karta hai C–Br bond stretch karne ki cost aur paanch groups ko saath squeeze karne ki cost, lekin net result phir bhi reactants se upar ek maximum hai — isliye ek real barrier hai (

\approx 75

kJ/mol).

"Reaction coordinate bas time hai."

Error: yeh ek geometric progress variable hai (potential energy surface par minimum-energy path ke saath atoms ki positions), clock nahi — dekho potential energy surface.

Why questions

Reactants aur products ke beech potential-energy maximum kyun zaroor hona chahiye?

Kyunki bonds ko stretch hona padta hai aur partly break hona padta hai pehle naye wale fully form hon, aur stretching energy cost karti hai (bonding electron density ka loss, Pauli repulsion), isliye system ek unavoidable up-front energy toll bharta hai.

Activation energy usually break hone wale bonds ki total bond-dissociation energy se kam kyun hoti hai?

Kyunki nayi bonds transition state par form hone lagti hain, energy release karti hain jo breaking cost ko partly cancel karti hai — ek concerted step ke dauran koi bond kabhi poori tarah akele nahi toota.

TST predict kyun karta hai ki

S_{N} 2

transition state trigonal bipyramidal hai?

Nucleophile leaving group ke opposite attack karta hai (backside), isliye teen remaining substituents ek plane mein flatten ho jaate hain jabki entering aur leaving groups axial baithte hain — ek five-centre, over-coordinated carbon.

Do alag reactions almost same

E_{a}

lekin opposite signs of

Δ H

kyun rakh sakti hain?

Kyunki

E_{a}

(peak tak climb) aur

Δ H

(valley-to-valley) curve ki alag features se set hote hain; peak height aur net drop geometrically independent hain.

Ek catalyst reaction ko consume hue bina speed up kyun karta hai?

Yeh ek alternative pathway provide karta hai lower transition state ke saath, isliye zyada molecules per second barrier clear kar lete hain; catalyst regenerate ho jaata hai kyunki yeh sirf hill ko shape karta hai, reactant valley nahi banta.

Transition state ek simple hilltop ki jagah "saddle point" kyun hai?

Full potential energy surface par yeh reaction path ke saath maximum hai lekin uske perpendicular har direction mein minimum hai — jaise ek mountain pass, trail ke saath high, across it low.

Edge cases

Ek barrierless reaction, jaise do radicals combine ho rahe hon, ke liye

E_{a}

kya hai?

Effectively zero — koi bond-breaking nahi hoti jiske liye dena pade aur nayi bond form hoti hai jaise atoms approach karte hain, isliye "peak" flat ho jaata hai aur rate sirf is baat se limited hai ki woh kitni jaldi milte hain.

Ek intermediate wale two-step mechanism mein kitne transition states hote hain?

Do — intermediate valley ke har taraf ek peak; unme se zyada uuncha peak (rate-determining step) overall observed rate control karta hai.

Barrier bahut thin aur sharp ho jaane par transition-state lifetime ka kya hota hai?

Yeh ek vibration (

\sim 1 0^{- 13}

s) ke order par rehta hai, height ki parwah kiye bina — transition state ek passage hai, koi well nahi, isliye koi bhi barrier shape hone par population kabhi accumulate nahi hoti.

Ek perfectly thermoneutral reaction (

Δ H = 0

) ke liye, kya forward aur reverse activation energies equal hain?

Haan — kyunki dono usi peak se measure ki jaati hain aur dono valleys level hain,

E_{a}^{fwd} = E_{a}^{rev}

, jo

E_{a}^{fwd} - E_{a}^{rev} = Δ E = 0

ke saath consistent hai.

Agar ek reaction extremely endothermic hai, toh uska transition state structurally kaisa dikhta hai?

Bahut product-like (ek late barrier) Hammond's postulate ke according — peak energy aur geometry dono mein products ke close hota hai, isliye transition state already products jaisa dikhta hai.

Kya temperature badhane se

E_{a}

Δ H

change ho sakta hai?

Dono temperature se nahi bante — dono energy landscape ki fixed features hain; temperature sirf un molecules ka fraction change karta hai jiske paas fixed barrier clear karne ke liye enough energy ho (yahi Arrhenius equation ka kaam hai).

Recall Har trap ki one-line summary

Peak = kinetics = $E_{a}$ = ek maximum = pakad mein nahi aata; Valley gap = thermodynamics = $Δ H$ ; woh independently move karte hain, aur catalysts peak ko reshape karte hain lekin valleys ko kabhi nahi.