Reversible reactions and dynamic equilibrium

2.6.1 · Chemistry › Equilibrium

Reversible reaction KYA hoti hai?

Ek irreversible reaction (jaise lakdi jalana) essentially completion tak jaati hai — reverse rate negligible hoti hai. Ek reversible reaction reactants aur products ke mixture par settle hoti hai.

"Dynamic" = molecules abhi bhi react kar rahe hain (motion kabhi nahi rukti). "Equilibrium" = net change zero hai (concentrations freeze ho gayi hain).

Concentrations constant KYU ho jaati hain? (Scratch se Derivation)

Aao ise build karte hain, memorise nahi karte. Ek simple reversible reaction lo: $A ⇌ B$

Step 1 — Dono rates likho. Ek elementary step ke liye, rate ∝ concentration.

Forward rate: $r_{f} = k_{f} [A]$ — Kyun? Sirf A hi aage ja sakta hai, isliye yeh depend karta hai ki kitna A hai.
Backward rate: $r_{b} = k_{b} [B]$ — Kyun? Sirf B hi wapas ja sakta hai.

Step 2 — Dekhte hain time ke saath kya hota hai. Shuru mein hum pure A daalte hain:

$[A]$ bada hai → $r_{f}$ bada hai.
$[B] = 0$ → $r_{b} = 0$ .

Isliye initially A → B dominate karta hai. Jaise jaise reaction chalta hai:

$[A]$ girta hai → $r_{f}$ girta hai.
$[B]$ badhta hai → $r_{b}$ badhta hai.

Yeh step kyun? Hum dono rates ko ek closing scissor ki tarah ek doosre ki taraf aate dekh rahe hain.

Step 3 — Milne ka point. Kisi ek moment mein $r_{f} = r_{b}$ . Tab se: $rate of A consumed = rate of A produced$ isliye $[A]$ ab change nahi ho sakta, aur na hi $[B]$ . Yehi equilibrium hai.

Step 4 — Equilibrium relationship nikalo. Rates ko equal karo: $k_{f} [A]_{e q} = k_{b} [B]_{e q}$

Yeh kyun important hai: yeh prove karta hai ki equilibrium concentrations kinetics se fix hoti hain, aur yeh explain karta hai ki $K_{c}$ ek given temperature par constant kyun hota hai (kyunki $k_{f}, k_{b}$ us temperature par constants hain).

Figure — Reversible reactions and dynamic equilibrium

Dynamic equilibrium ko KAISE pehchanen (4 conditions)

Worked Examples

Common Mistakes (Steel-manned)

Recall Feynman: ek 12-saal ke bacche ko samjhao

Socho ek busy pond hai jisme do taalaab ek stream se jude hain. Machhliyan baye talaab se daaye talaab mein jaati hain, aur doosri machhliyan daaye se baye wapas aati hain. Agar har minute same number dono taraf cross kare, toh har talaab mein machhliyon ki sankhya badalni band ho jaati hai — bhale hi machhliyan hamesha tairi rehti hain! Yehi dynamic equilibrium hai: sab kuch abhi bhi ho raha hai, lekin totals change karne band ho jaate hain. Yeh sirf tabhi kaam karta hai jab koi machhli talaab se bahar nahi kood sakti (ek closed system).

Flashcards

What defines a reversible reaction?

Woh jisme products wapas react karke same conditions mein reactants bana sakti hain;

⇌

se likhi jaati hai.

What is dynamic equilibrium?

Ek closed system mein woh state jahan forward aur backward reaction rates equal hon, isliye concentrations constant rehti hain jabki dono reactions jaari rehti hain.

Why is equilibrium called "dynamic"?

Kyunki molecules abhi bhi dono directions mein react kar rahe hain; sirf net change zero hai.

What are the four signs of dynamic equilibrium?

Closed system; constant macroscopic properties; equal nonzero forward/backward rates; dono directions se reach hone layak.

Derive the equilibrium ratio for

A ⇌ B

Set

k_{f} [A] = k_{b} [B]

⇒

[B] / [A] = k_{f} / k_{b} = K_{c}

Are reactant and product concentrations equal at equilibrium?

Nahi — rates equal hoti hain, concentrations nahi.

What does an isotope-labelling experiment prove about equilibrium?

Ki reactions jaari rehti hain (label doosri side par appear hota hai), isliye equilibrium dynamic hai static nahi.

Effect of a catalyst on the equilibrium position?

Koi nahi — yeh dono directions ko equally speed up karta hai, same concentrations par equilibrium jaldi reach hota hai;

K_{c}

unchanged rehta hai.

Why must the system be closed to reach equilibrium?

Agar products escape kare, toh backward rate kabhi forward rate se match karne tak nahi badh sakti, isliye rates kabhi balance nahi hote.

On what does

K_{c}

depend?

Sirf temperature par (kyunki

k_{f}

aur

k_{b}

temperature par depend karte hain).

Connections

Equilibrium constant Kc and Kp — woh constant jo is note mein derive hota hai, quantified.
Le Chatelier's principle — equilibrium disturb hone par kaise shift karta hai.
Rate of reaction and rate constants — woh $k_{f}, k_{b}$ jo derivation ki buniyaad hain.
Collision theory — kyun rate ∝ concentration.
Effect of temperature on equilibrium — kyun $K_{c}$ temperature-dependent hai.
Catalysis — kyun catalysts equilibrium ko move nahi karte.