2.5.3
Intuition Ek sentence mein core ideaHar reaction energy ka ek deal hoti hai: exergonic reactions energy dete hain (universe ko kharcha karne ke liye free energy milti hai), jabki endergonic reactions loan maangti hain (ye tab hi chalti hain jab koi energy pay kare). Woh ek number jo batata hai ki reaction kaunsi type ki hai woh hai Gibbs free energy change , Δ G
Free energy (G : system ki energy ka woh hissa jo constant temperature aur pressure par useful work karne ke liye available hota hai (Josiah Willard Gibbs ke naam par).Δ G G products − G reactants change .Exergonic reaction : woh reaction jo free energy release karti hai, isliye Δ G < 0 spontaneous hoti hai (thermodynamically favorable).Endergonic reaction : woh reaction jo free energy absorb karti hai, isliye Δ G > 0 non-spontaneous hoti hai — ise proceed karne ke liye energy input chahiye.
Common mistake "Spontaneous" ka matlab "fast" NAHI hotaLagta hai spontaneous ka matlab instant hona chahiye — lekin spontaneity ek thermodynamics wala word hai, speed wala nahi.
Galat idea kyun sahi lagta hai: rozana ki zindagi mein "spontaneous" = achanak. Fix yeh hai: Δ G kar sakti hai ya nahi , yeh nahi ki kitni tez . Glucose jalana bahut zyada exergonic hai phir bhi sugar saalon jar mein stable baith ti hai — kyunki rate activation energy (E a
Hum ek aisi quantity chahte hain jo spontaneity predict kare. Second Law kehta hai ki ek process spontaneous hai agar total entropy badhti hai :
Δ S universe = Δ S system + Δ S surroundings > 0
Surroundings system se heat receive karte hain. Constant temperature/pressure par, system jo heat release karta hai woh uski enthalpy change − Δ H sys Δ S surroundings = T − Δ H sys
Yeh step kyun? Surroundings mein temperature T − Δ H T
Substitute karo:
Δ S universe = Δ S sys − T Δ H sys > 0
− T Δ H sys − T Δ S sys < 0
Is combination ko Gibbs free energy change define karo:
− T Δ S minus ek "tax" hai jo universe disorder ke liye leta hai. High temperature par disorder khareedna sasta ho jaata hai, isliye T Δ S
Exergonic : products free-energy axis par reactants se neeche hote hain → energy downhill jaati hai → released energy = ∣Δ G ∣ Endergonic : products upar hote hain → energy ko uphill push karna padta hai → energy absorb hoti hai.Dono cases mein ek hump (activation energy E a
Intuition Life coupling kyun pasand karti haiCells mein hazaaron endergonic reactions chalti hain (proteins banana, ions pump karna). Woh inke liye coupling karke pay karte hain ek strongly exergonic reaction se — usually ATP hydrolysis (Δ G ≈ − 30.5 Δ G exergonic + Δ G endergonic < 0 combined reaction chalti hai. Yahi metabolism ki poori logic hai.
Worked example 1 — Pure number testEk reaction hai jismein Δ H = − 20 Δ S = + 50 T = 298 Δ G = − 20 , 000 − 298 ( 50 ) = − 20 , 000 − 14 , 900 = − 34 , 900 J/mol Yeh step kyun? Δ H T Δ S exergonic / spontaneous . Dono terms ne yahan help ki (heat bhi release ki AUR disorder bhi badha).
Worked example 2 — Temperature answer ko flip kar deta haiΔ H = + 30 Δ S = + 100
T = 200 Δ G = 30 , 000 − 200 ( 100 ) = + 10 , 000 endergonic .T = 400 Δ G = 30 , 000 − 400 ( 100 ) = − 10 , 000 exergonic .
Yeh step kyun? Crossover temperature hai T = Δ H /Δ S = 30 , 000/100 = 300 − 10° 0°
Worked example 3 — Cell mein ATP couplingGlutamate + NH₃ → Glutamine ka Δ G = + 14.2 Δ G = − 30.5 Δ G total = + 14.2 + ( − 30.5 ) = − 16.3 overall exergonic , toh yeh proceed karta hai.
Yeh step kyun? Coupled reactions ki free energies simply add ho jaati hain (G ek state function hai). Negative sum cell ka green light hai.
Recall Feynman: ek 12-saal ke bachche ko samjhaoSocho ek ball ek pahaad par hai. Agar ball upar hai, toh woh khud neeche roll kar sakti hai aur raaste mein ek chhoti si pawan chakki bhi ghuma sakti hai — yeh exergonic reaction hai: yeh energy bahar deti hai, koi madad nahi chahiye. Agar ball neeche hai, toh woh khud upar nahi chadegi — tum use push karo. Woh pushing woh energy hai jo tumhe kharcha karni hoti hai, aur yeh endergonic reaction hai. "Height" ko free energy kehte hain. Neeche jaana = energy release (negative Δ G Δ G ATP hai!
EX ergonic = EX it (energy exit karti hai, Δ G negative hai, downhill).
EN dergonic = EN ter (energy enter karni padti hai, Δ G positive hai, uphill).
"Exergonic = EXcellent aur Easy (spontaneous); Endergonic = Effort chahiye."
Har type ke liye Δ G
Kya Δ G < 0
Dobara derive karo kyun Δ G < 0 ⇔ Δ S universe > 0
Exergonic reaction ka ΔG sign kya hota hai Δ G < 0
Endergonic reaction ka ΔG sign kya hota hai Δ G > 0
Gibbs free energy equation Δ G = Δ H − T Δ S
Δ G Poore universe ki entropy change, sirf system quantities mein rewrite ki gayi (Δ G < 0 ⇔ Δ S u ni v > 0
Kya negative ΔG matlab hai ki reaction fast hogi? Nahi — ΔG sirf direction/spontaneity batata hai; speed activation energy E a
Cell ek endergonic reaction kaise chala sakti hai? Ise ek exergonic reaction se couple karke (jaise ATP hydrolysis) taaki summed ΔG negative ho jaye.
Crossover temperature jahan ΔG = 0 hota hai T = Δ H /Δ S
ATP hydrolysis ka approximate ΔG about − 30.5
High temperature entropy-driven reactions ko kyun favor karta hai? − T Δ S T Δ S Δ G
Exergonic releases energy
Endergonic absorbs energy
Second Law: S universe up
