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Chemical Kinetics

Chemical Kinetics. The area of chemistry that concerns reaction rates and reaction mechanisms. Reaction Mechanism. The reaction mechanism is the series of elementary steps by which a chemical reaction occurs.

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Chemical Kinetics

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  1. Chemical Kinetics The area of chemistry that concerns reaction rates and reaction mechanisms.

  2. Reaction Mechanism The reaction mechanism is the series of elementary stepsby which a chemical reaction occurs. • The sum of the elementary steps must give the overall balanced equation for the reaction • The mechanism must agree with the experimentally determined rate law

  3. Rate-Determining Step In a multi-step reaction, the slowest stepis the rate-determining step. It therefore determines the rate of the reaction. The experimental rate law must agree with the rate-determining step

  4. Identifying the Rate-Determining Step For the reaction: 2H2(g) + 2NO(g)  N2(g) + 2H2O(g) The experimental rate law is: R = k[NO]2[H2] Step #1 H2(g) + 2NO(g)  N2O(g) + H2O(g) Step #2 N2O(g) + H2(g)  N2(g) + H2O(g) req. #1 sum of elementary steps must give overall balanced equation for the reaction.

  5. Identifying the Rate-Determining Step 2H2(g) + 2NO(g)  N2(g) + 2H2O(g) The experimental rate law is: R = k[NO]2[H2] Which step in the reaction mechanism is the rate-determining (slowest) step? Step #1 H2(g) + 2NO(g)  N2O(g) + H2O(g) Step #2 N2O(g) + H2(g)  N2(g) + H2O(g) req. #2? agree with the experimental rate law? yes, agrees with the experimental rate law

  6. Identifying Intermediates For the reaction: 2H2(g) + 2NO(g)  N2(g) + 2H2O(g) Which species in the reaction mechanism are intermediates (do not show up in the final, balanced equation?) Step #1 H2(g) + 2NO(g)  N2O(g) + H2O(g) Step #2 N2O(g) + H2(g)  N2(g) + H2O(g) 2H2(g) + 2NO(g)  N2(g) + 2H2O(g)  N2O(g) is an intermediate

  7. Collision Model Key Idea: Molecules must collide to react. However, only a small fraction of collisions produces a reaction. Why?

  8. Collision Model Collisions must have sufficient energy to produce the reaction (must equal or exceed the activation energy). 1. Colliding particles must be correctly oriented to one another in order to produce a reaction. 2.

  9. Factors Affecting Rate Increasing temperature always increases the rate of a reaction. • Particles collide more frequently • Particles collide more energetically

  10. Endothermic Reactions

  11. Exothermic Reactions

  12. The Arrhenius Equation • k = rate constant at temperature T • A = frequency factor • Ea = activation energy • R = Gas constant, 8.31451 J/K·mol

  13. The Arrhenius Equation, Rearranged • Simplifies solving for Ea • -Ea / R is the slope when (1/T) is plotted against ln(k) • ln(A) is the y-intercept • Linear regression analysis of a table of (1/T) vs. ln(k) can quickly yield a slope • Ea = -R(slope)

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