CHEMICAL KINETICS Detailed Class Notes (Hinglish Version) Teacher ka Message: "Ek baat meri fir sun lo bhai, jitni janta abhi class mein hai, main aapse chahunga last tak bane rahein. Dekho fayda tabhi hai is notes ka jab aap ise pura dhyaan se padhein. Jo bacha in concepts ko imandaari se padhega, main guarantee de sakta hu wo JEE Mains ka jo bhi question aayega use chutki mein nipta ke aayega!" 1. Introduction: Thermodynamics vs. Chemical Kinetics Agar tum logo ko yaad ho, Thermodynamics ka naam suna hai tum logo ne? Arey munna, suna hai kya? Thermodynamics ki ek sabse badi limitation hua karti thi. Thermodynamics ye toh bata deta hai ki ek particular reaction possible (feasible) hai ya nahi Lekin, agar koi chemical reaction ho rahi hai toh wo kitne time mein hogi , kis rate (speed) se hogi... iske baare mein Thermodynamics humein koi information nahi deta. Ye information kaun deta hai? Chemical Kinetics! Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 1 Concept Highlight: Thermodynamics humein Feasibility ke baare mein batata hai, aur Chemical Kinetics humein Rate (Speed) ke baare mein batata hai. Chemical Kinetics chemistry ki wo branch hai jo Rate of Reactions, factors affecting rate, aur reaction mechanism se deal karti hai. 2. Rate ke Basis par Reactions ke Types Reactions ki speed ek jaisi nahi hoti. Inhe hum 3 categories mein divide kar sakte hain: Very Fast Reactions: Ye reactions chutki mein aise-aise ho jati hain. Fraction of seconds mein complete ho jati hain. (Example: Ionic reactions). Very Slow Reactions: Sir, ek reaction aisi hoti hai bhaiya jisko saalon lag jate hain. Jaise aapka iron (loha) hota hai na, iron pe jung lag jati hai (Rusting). Bahut slow process hai. Moderate Reactions: Na bahut slow aur na hi bahut fast. • • • Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 2 DHYAN RAKHEIN! Ye jo chapter hota hai na dada, isme na toh 'Very Fast' ke baare mein padha jata hai, aur na hi 'Very Slow' ke baare mein. Is chapter mein hum sirf aur sirf Moderate Reactions ki speed aur unki mechanism ke baare mein padhte hain. 3. Rate of Reaction (ROR) Kya Hai? Sabse pehla question tumhare dimaag mein aayega, "Sir, Rate of Reaction aakhir hoti kya hai?" Rate of reaction ki bahut hi aasaan si definition hai. Bhai, reaction mein kya change hota hai? Sir, reaction mein change hoti hai Concentration ... kiski? Reactant aur Product ki! Toh, Unit time mein Reactant aur Product ki concentration mein jo change hota hai, use hi Rate of Reaction kehte hain. Khallas! Mathematical Definition Rate = Change in Concentration / Time Taken Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 3 Reactant aur Product ki kahani Chalo ek example lete hain. Let's say ek reactant R convert ho raha hai product P mein: R → P Maan lete hain, Time t = t 1 par, Reactant ki concentration R 1 thi aur Product ki P 1 thi. Thode time baad, Time t = t 2 ho gaya. Tab Reactant R 2 aur Product P 2 ho gaya. Agar main aapse puchu, batao Rate of Disappearance of Reactant kya hoga? (Kyunki reactant dreere-dheere gayab ho raha hai) Aap bologe, sir: (R 2 − R 1 ) / (t 2 − t 1 ) • • Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 4 NEGATIVE SIGN KA LOGIC Hum jaante hain ki time ke saath Reactant ki value kam hoti hai. Matlab R 2 hamesha R 1 se chota hoga. Toh R 2 − R 1 ka answer Negative aayega. Arey dada, bol na yaar! Negative rate ka koi real life significance thodi na hai (speed minus mein thodi hoti hai). Isliye, us negative value ko positive banane ke liye, formula mein pehle se hi ek Minus (-) sign lagaya jata hai. Rate of Disappearance (Reactant ke liye) − Δ[R] / Δt Ye minus sign sirf ye batata hai ki Reactant ki concentration time ke saath decrease kar rahi hai. Wahi dusri taraf, Product appear ho raha hai. Product ki concentration badh rahi hai ( P 2 > P 1 ). Toh iska rate positive hoga. Rate of Appearance (Product ke liye) + Δ[P] / Δt Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 5 4. Rate of Reaction (ROR) Ki Unit (Matrak) Sir, ek baat mujhe aur batao, unit kya hogi? Arey bhaiya, rate hota kya hai? Concentration upon time hota hai. Humne padha hai ki Concentration ki unit hoti hai Mol per Liter (mol L -1 ) . Aur time ki unit hum Seconds (s) ya Minutes lete hain. General Unit Unit = mol L −1 s −1 Aap mol/L ko Molarity (M) bhi likh sakte ho, toh M s -1 bhi sahi hai. Ghabrane ki zarurat nahi hai. Gases Ke Liye Unit (For Gaseous Reactions) Agar socho sir, reaction gaseous hai... toh kya karein? Sari duniya jaanti hai ki PV = nRT hota hai. Agar main isko aise likh du: P = (n/V)RT Yahan n/V (moles / volume) hi toh concentration (C) hoti hai! Matlab Gases ke liye concentration ki jagah hum Pressure (P) ka use kar sakte hain. Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 6 Gases ke liye ROR ki Unit atm s −1 ya atm min −1 Pressure ki aur bhi units hoti hain, par sabse famous atm hi hoti hai. 5. Stoichiometric Coefficients ka Laphda (Sabse Important Concept) Sir, kuch badhiya example do jisse pura concept clear ho jaye! Aa jao, yahan bahut logo ko confusion hota hai aur galati karte hain. Apna dhyaan screen par rakhna, idhar-udhar muh mat faado bas! Example 1: H 2 + I 2 → 2HI Agar main aapse puchu: Rate of Disappearance of H 2 ka expression likho, toh kya likhoge? Rate of Disappearance of H 2 = − Δ[H 2 ] / Δt Agar main aapse puchu: Rate of Appearance of HI likho? Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 7 Rate of Appearance of HI = + Δ[HI] / Δt BADI GALATFAIMI (BIG MISCONCEPTION) Bahut saare logo ke dimaag mein confusion hota hai ki yahan HI ke aage '2' laga hai, toh usko Half (1/2) likho. Nahi mere bhai! Jab aap individual 'Rate of Appearance' ya 'Disappearance' ka expression likhte ho, toh kabhi bhi coefficient se divide nahi kiya jata! Toh sir, wo Half (1/2) kab aata hai? Arey munna, jab tum puri Reaction ki Rate (ROR) ka expression likhte ho tab wo divide hota hai. Dekho kaise: Rate of Reaction (ROR) ROR = − Δ[H 2 ]/Δt = − Δ[I 2 ]/Δt = + 1/2 Δ[HI]/Δt Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 8 Aisa Kyun Hota Hai? (Reasoning): Socho, jab H 2 ka 1 mole disappear hota hai, toh I 2 ka bhi 1 mole disappear hota hai. Lekin HI ke 2 mole ban jate hain! Yani HI ke banne ki speed (rate of appearance), H 2 ke gayab hone ki speed se do guna (2 times) hai. Agar in sabko equation mein "Equal (Barabar)" rakhna hai, toh HI wale part ko 2 se divide karna padega tabhi toh equations balance hongi! Samjhe? Example 2: N 2 + 3H 2 → 2NH 3 Ise imandaari se khud likhne ki koshish karo! Main directly Rate of Reaction (ROR) ka final expression likh raha hu: ROR = − Δ[N 2 ]/Δt = − 1/3 Δ[H 2 ]/Δt = + 1/2 Δ[NH 3 ]/ Δt Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 9 General Reaction: aA + bB → cC Agar koi aisi general reaction aa jaye jahan small letters (a, b, c) coefficients hain: Rate of Disappearance of A: − Δ[A] / Δt Rate of Disappearance of B: − Δ[B] / Δt Rate of Appearance of C: + Δ[C] / Δt Aur agar koi pure reaction ki ROR puche: ROR = − 1/a (Δ[A]/Δt) = − 1/b (Δ[B]/Δt) = + 1/c (Δ[C]/Δt) 6. Average vs. Instantaneous Rate Rate ko hum log Physics ki tarah do alag-alag tareeke se padhte hain: ek hota hai Average Rate aur ek hota hai Instantaneous Rate. Average Rate (R avg ): Jab ek lambe time interval (jaise t = 5 second se leke t = 20 second tak) ke beech mein rate find kar rahe ho, toh usko Average Rate bolenge. Ise Delta ( Δ ) se show karte hain. • • • 1. Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 10 Instantaneous Rate (R inst ): Agar koi aapse kahe, "Bata time t = 10 second (ek particular second) pe rate kitna tha?" Toh isey Instantaneous bolte hain. Yahan time interval bahot chota hota hai ( Δt → 0 ). Ise hum Derivative ( d ) se show karte hain. Isko find karne ke liye hum Concentration-Time graph mein ek Tangent (sparsh rekha) draw karte hain, aur uska Slope hi humein Instantaneous rate deta hai. 7. JEE aur Board Level Ke Important Questions Lo ji, ab theory khatam. Ab thode questions niptaate hain jisse tumhe samajh aaye ki exam mein kya laphda hota hai. Calculate tum khud kar lena, tumhari calculation ke charche door-door tak hain! 2. Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 11 Question 1: Chintu sa asaan sawal Ek reaction mein Reactant ki concentration 0.03 M se badalkar 0.01 M ho jati hai 25 minutes mein. Average rate of reaction find karo (min aur second dono mein). Solution: Average Rate = − (C 2 − C 1 ) / Δt Sir, C 1 = 0.03 thi aur C 2 = 0.01 ho gayi. Time laga 25 minutes. R = − (0.01 − 0.03) / 25 R = − (− 0.02) / 25 = 0.02 / 25 M min -1 Second mein convert karna hai? Sir, 1 min mein 60 seconds hote hain. Toh time ko 60 se multiply kar do denominator mein. R = 0.02 / (25 × 60) = 0.02 / 1500 M sec -1 Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 12 Question 2: Mistake wala Question For a reaction: 2A → Product . Concentration of A decreases from 0.5 M to 0.4 M in 10 minutes. Calculate the Rate of Reaction during this interval. Isme Galati Karoge Tum: Bahut saare log directly likh dete hain: −(0.4 - 0.5) / 10 = 0.1 / 10 = 0.01 . Bhai, ye expression "Average Rate" ka nahi hai! Tumne sirf Rate of Disappearance of A nikal diya. Reaction dekho, usme '2A' likha hai. Usko kaun dekhega? Sahi Solution: Sahi Rate ka expression hoga: ROR = − 1/2 × (Δ[A] / Δt) ROR = − 1/2 × (0.4 − 0.5) / 10 ROR = − 1/2 × (− 0.1) / 10 = 0.1 / 20 = 0.005 M min -1 Aise banti hai baat! Khush hoke directly answer mat nikal dena bhai. Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 13 Question 3: JEE Main 27 August 2021 Ek chemical reaction mein Rate of Appearance of Cr 2 (SO 4 ) 3 diya gaya hai 2.67 M min -1 Reaction ka expression is tarah ka banta hai: 3 moles of Reactant ( C 2 H 6 O ) forms 2 moles of Product ( Cr 2 (SO 4 ) 3 ). Rate of disappearance of C 2 H 6 O find karo. (Nearest integer mein) Solution: Sabse pehle Rate of Reaction ka expression likh lo. − 1/3 × Δ[C 2 H 6 O]/Δt = + 1/2 × Δ[Cr 2 (SO 4 ) 3 ]/Δt Humein question mein Δ[Cr 2 (SO 4 ) 3 ]/Δt ki value 2.67 de rakhi hai. Humein find kya karna hai? Rate of disappearance of C 2 H 6 O, matlab sirf − Δ[C 2 H 6 O]/Δt (yahan 1/3 mat lagana, kyunki individually pucha hai). Toh 3 ko udhar bhej do! Disappearance = 3/2 × (2.67) = 1.5 × 2.67 = 4.005 Question mein pucha hai "Nearest Integer". Toh answer ho gaya 4 Sir apna kaam ban gaya, bahut hi aasan tha! Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 14 Question 4: JEE Main 2021 (Unit Ka Laphda) For a chemical reaction, it was found that the concentration of B increases by 0.2 M in 30 minutes. Find the average rate of reaction (in hours). Solution: "Increases by 0.2" ka matlab hai Change in concentration ( ΔC ) = 0.2 M Time laga = 30 minutes. Par answer humein Hours (h) mein chahiye. Yahi fasate hain ye log! Time in hours = 30 / 60 = 0.5 hr (yaani half hour). Rate = ΔC / Δt = 0.2 / 0.5 = 0.4 M hr -1 Sir ye toh direct aa gaya. 0.4 ko aise bhi likh sakte hain: 4 × 10 -1 Answer is 4. Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 15 Final Note: Bas dhyaan rakhna, jaldi mein gadbad nahi karni hai. Insan do hi time pe sabse zyada galti karta hai, ya toh bahut zyada khush ho, ya bahut zyada udaas. Reaction aur unit dekhe bina answer tick mat karna. Ab milte hain next lecture mein! Class 12 PCM - Chemical Kinetics (Hinglish Notes) Page 16