2.5.4
Intuition Ek-line pictureJo reactions hona chahti hain (energy release karti hain) woh bhi instantly nahi hoti — ek energy hill hoti hai jise reactants ko pehle climb karna padta hai, tab jaake products tak pahunch sakte hain. Activation energy usi hill ki height hai.
Definition Activation energy (E a Activation energy woh minimum amount of energy hai jo reactant molecules ke paas honi chahiye (aur jise bond-stretching/breaking mein arrange karna padta hai) taaki woh transition state tak pahunch sakein aur products mein convert ho sakein.
Symbol: E a
Units: kJ mol − 1
Yeh energy barrier hai, na ki reaction dwara release ki gayi energy.
Barrier exist hi kyun karta hai?
Naye bonds banane ke liye, purane bonds pehle partially break hone chahiye. Existing bonds ko stretch/distort karna energy kharchta hai. Hill ke top par molecule ek strained, unstable arrangement mein hota hai jise transition state kehte hain — adhe purane bonds, adhe naye bonds. Sirf woh molecules jo itni energy rakhte hain ki is point tak pahunch sakein, react kar sakte hain.
Intuition "Energy release" ≠ "fast hona" kyunEk boulder ledge par bahut zyada potential energy rakhta hai aur girna chahta hai ... lekin sirf tab jab aap pehle use ledge ki lip se thoda dhakka dein. Glucose + O₂ bahut zyada energy release karta hai, phir bhi sugar aapki shelf par saalon tak pada rehta hai. Reaction favourable hai (overall downhill) lekin slow hai kyunki E a
Diagram padhna (left → right = reaction progress):
Reactants kisi energy level par start karte hain.Energy badhti hai ek peak tak = transition state . Reactants se is peak tak ki height = E a
Energy phir girti hai products tak.
Reactants aur products ke beech ka antar = Δ H E a alag quantity hai.
Intuition Enzymes hill mein tunnel khod dete hainEk catalyst (enzyme) reactants ya products ko nahi badalta, isliye Δ H lower transition state hoti hai ⇒ chhota E a
Molecules ka woh fraction jo energy ≥ E a E a temperature badhao (molecular speed distribution ko rightward shift karo). Conceptually:
rate ∝ e − E a / R T Yeh form kyun? Exponential kehta hai ki sirf molecules ki ek chhoti tail ke paas zaroori energy hoti hai; E a T
Worked example Example 1 — energy diagram seE a 30 kJ mol − 1 80 kJ mol − 1 10 kJ mol − 1 E a , forward Δ H
E a , forward = E T S − E R = 80 − 30 = 50 kJ mol − 1 Kyun? yeh reactants se peak tak ki chadhai hai.Δ H = E P − E R = 10 − 30 = − 20 kJ mol − 1 Kyun? negative matlab products neeche hain ⇒ energy release hoti hai ⇒ exergonic/exothermic.Note karo ki dono ek saath exist karte hain: ek downhill (favourable) reaction jise phir bhi 50 kJ ka push chahiye.
Worked example Example 2 — enzyme ka effectEk enzyme transition state ko 80 55 kJ mol − 1
New E a , forward = 55 − 30 = 25 kJ mol − 1 Kyun? peak neeche hai, chadhai chhoti hai.
Δ H = 10 − 30 = − 20 kJ mol − 1 Kyun unchanged? enzyme sirf pathway ko affect karta hai, start/end energies ko nahi.Conclusion: utni hi energy release hoti hai, lekin zyada molecules chhoti hill cross kar lete hain ⇒ faster.
Worked example Example 3 — reverse reaction barrierExample 1 ke numbers use karo, reverse barrier nikalo aur Δ H
E a , reverse = E T S − E P = 80 − 10 = 70 kJ mol − 1 Kyun? backward jaate waqt aap products (10) se peak (80) tak chadhte ho.Check: E a , f − E a , r = 50 − 70 = − 20 = Δ H Yeh kyun matters: derived identity confirm hoti hai.
Common mistake "Activation energy woh energy hai jo reaction release karti hai."Kyun sahi lagta hai: dono energies usi diagram par hain, aur exam questions mein dono saath dikhte hain.
Fix: E a barrier height hai (reactants → peak); released energy Δ H shuru hone ke baare mein hai, doosra net outcome ke baare mein. Dono bahut alag ho sakte hain.
Δ H Kyun sahi lagta hai: enzymes reactions karate hain, toh lagta hai jaise woh push add karte hain.
Fix: enzymes sirf E a Δ H constant rehta hai.
Common mistake "Exothermic reactions mein koi activation energy nahi hoti."Kyun sahi lagta hai: agar energy nikalna hai, toh koi energy daalni kyun zarurat hai?
Fix: almost sabhi reactions mein E a > 0
Recall Feynman: 12-saal ke bacche ko samjhaoSocho ek ball ek slide ke top par ek chhoti si dip mein phans gayi hai. Slide ka bottom neeche hai, toh ball wahan jaana chahti hai aur ek baar jaane par tezi se neechi jayegi. Lekin pehle tum use dip ke kinare par ek chhote se bump ke upar dhakka deni padogi. Woh chhota dhakka hi activation energy hai — woh thoda sa effort jo shuru karne ke liye chahiye. Baad mein jo badi giraavat hoti hai woh energy hai jo reaction deta hai. Enzyme ek aisi dost ki tarah hai jo bump ko chhota kar deti hai, toh ball bahut aasaani se neeche jaati hai — lekin slide ka bottom wahi rehta hai.
"Activation = Avalanche se pehle ki Ascent."
Pehle chadhna padta hai (Ascent = E a Δ H
Activation energy define hoti hai woh minimum energy jो reactants ke paas honi chahiye taaki woh transition state tak pahunch sakein aur products ban sakein.
E a kJ mol⁻¹ (energy per mole).
Hill ke top par sabse zyada energy wali, unstable arrangement ko kehte hain transition state.
Forward activation energy formula energies ke terms mein E a = E T S − E R
Forward/reverse barriers aur ΔH ka relationship Δ H = E a , forward − E a , reverse
Kya catalyst ΔH badalta hai? Nahi — yeh sirf E a
Sugar stable kyun hai jabki jalane par energy release hoti hai? Reaction favourable hai (negative ΔH) lekin iska E a
Temperature badhane se reaction kaise fast hoti hai E a Zyada molecules energy ≥ E a ∝ e − E a / R T
Kya activation energy aur released energy same hain? Nahi — E a
wajah hai ki favourable reactions hoti hain
Ea forward minus Ea reverse deta hai
distribution shift karke fraction badhata hai
Bond stretching and breaking
rate proportional to e^-Ea/RT
