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C. Y. Yeung (CHW, 2009)

Energy Profile and Reaction Mechanism. Energy Profile and Reaction Mechanism. How does the reaction take place???. p.01. C. Y. Yeung (CHW, 2009). +. C. A. B. +. A. B. C. p.02. How does a reaction occur?. A + B  C. K.E.  E a. Single Stage Rxn. collide in right orientation.

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C. Y. Yeung (CHW, 2009)

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  1. Energy Profile andReaction Mechanism Energy Profile and Reaction Mechanism How does the reaction take place??? p.01 C. Y. Yeung (CHW, 2009)

  2. + C A B + A B C p.02 How does a reaction occur? A + B  C K.E.  Ea Single Stage Rxn collide in right orientation Multi-Stage Rxn Intermediate How to know that …. ?

  3. Step 1: A + B  Intermediate (r.d.s.) Step 2: Intermediate + A C p.03 From the Chemical&DifferentialRate Eqns ! e.g. 2A + B  3C If after Kinetics Studies, it was found that … Rate = k[A][B] ** a “A” molecule and a “B” molecule are involved in the “rate-determining step”. i.e. The reaction is a 2-step reaction!

  4. Step 1: A + A  Intermediate (r.d.s.) Step 2: Intermediate + B C p.04 e.g. 2A + B  3C If after Kinetics Studies, it was found that … Rate = k[A]2 ** Two “A” molecules are involved in the “rate-determining step”. i.e. The reaction is a 2-step reaction!

  5. Step 1: B  Intermediate (r.d.s.) Step 2: Intermediate + 2A C p.05 e.g. 2A + B  3C If after Kinetics Studies, it was found that … Rate = k[B] ** Only one “B” molecule is involved in the “rate-determining step”. i.e. The reaction is a 2-step reaction!

  6. 2A + B  3C p.06 e.g. 2A + B  3C If after Kinetics Studies, it was found that … Rate = k[A]2[B] ** ALL the molecules are involved in the “rate-determining step”. i.e. The reaction is a Single-step reaction!

  7. Transition state higher Ea: r.d.s. (slower step) Rate = k[A][B] Transition state Rate = k[A]2 Rate = k[B] Rate = k[A]2[B] p.07 2A + B  3C Energy Profile ?? intermediate

  8. p.08 p. 79 Q.9 (1999 --- Differential Rate Eqn. and Energy Profile) (a) Rate = k[A] (c) A + B  product i.e. Only a “A” is involved in r.d.s.

  9. p.09 Catalyst changes the energy profile of rxn! A + B  C If no catalyst … A + B  C, with high Ea. With catalyst (X)… A + X intermediate, with a lower Ea. (r.d.s.) intermediate + B  C + X(becomes a 2-stage rxn)

  10. (slow) [r.d.s.] (fast) Only 1 transition state 2 transition states DH is not affected by catalyst p.10 Energy Profileswith / withoutCatalyst …

  11. p.11 p. 79 Q.11 (2001 --- Differential Rate Eqn. and Energy Profile) (a) Rate = k [I(g)]2 [Ar] (c) 2 I  I2 i.e. Two “I” atoms and one “Ar” atom are involved in r.d.s. 3 possible energy profiles!!

  12. p.12 3 Different Stories are possible!

  13. p.13 I(g) + I(g) + Ar  I2(g) + Ar*(g) Role of Ar: acts as a third body to absorb energy from the colliding I(g) atoms.

  14. p.14 Step 1: I(g) + Ar(g)  I-Ar(g) [fast] Step 2: I-Ar(g) + I  I2 + Ar(g) [slow] Role of Ar: homogeneous catalyst

  15. p.15 Step 1: 2I(g) + Ar(g)  I2Ar(g) [slow] Step 2: I2Ar(g)  I2 + Ar(g) [fast] Role of Ar: homogeneous catalyst

  16. formed quickly! OH 3 Br2 3 HBr + + OH Br Br Br p.16 Expt. 9 Activation Energy 5 Br- + BrO3- + 6 H+ 3 Br2 + 3H2O e.g. 3 moles are form in 42 s All the 3 moles of Br2 reacts with phenol.  does not bleach methyl red indicator. e.g. 1 mol At 42.1s, a new Br2 is formed, which will not react with phenol, but bleach methyl red!

  17.  rate  1/t p.17 The time required for bleaching Methyl Red is recorded. data treatment ...? i.e. time required , rate 

  18.  k  1/t ln (1/t) = – + lnA 1 1 T T Ea Ea ln k = – + lnA R R p.18 rate  1/t Therefore …

  19. p.19 Assignment p.74 Q.11, 12 [due date: 2/3(Mon)] Lab Report: Expt. 9 Determination of Ea [due date: 3/3(Tue)] Quiz on Chemical Kinetics (Ch. 13-15) [9/3(Mon)]

  20. p.20 Next …. Chemical Equilibria & Keq (p. 88-100)

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