chemical kinetics rates of reaction反应的化学.ppt

* * * * * * * * * * * * * * * Bimolecular Reaction Bimolecular Reaction O3 + NO O2 + NO2 Reaction Energy Diagrams Forward reaction: Ea = 10kJ/mol Exothermic Backward reaction Ea=? Exo or Endo? Increasing T will speed up most reactions. Higher T = higher average Ek for the reactants. = larger fraction of the molecules can overcome the activation barrier. 25°C kinetic energy number of molecules Ea 75°C Many more molecules have enough E to react at 75°C, so the reaction goes much faster. Factors Affecting Reaction Rates Temperature and Reaction Rate Rule of thumb – approximately double the rate for every 10 K. 250 300 350 400 T (K) 0.00 0.10 0.20 0.30 k (L mol-1 K-1 ) T (K) k (L mol-1 K-1) 273 4.18 x 10-5 290 2.00 x 10-4 310 2.31 x 10-3 330 1.39 x 10-2 350 6.80 x 10-2 370 2.81 x 10-1 I- + CH3Br ? Br - + CH3I Rate = k[I-]m[CH3Br]n The Arrhenius equation shows how k varies with T -Ea / RT k = A e A Frequency factor. How often a collision occurs with the correct orientation. (1/4 for I- + CH3Br) e-Ea/RT Fraction of the molecules with enough Ea to cross the barrier. T Temperature Must be in Kelvin. R Gas law constant 8.314 J K-1 mol-1. Temperature and Reaction Rate exponential relationship 250 300 350 400 T (K) k (L mol-1 K-1 ) Temperature and Reaction Rate rate = k[I-]m[CH3Br]n rate = (Ae-Ea/RT)[I-]m[CH3Br]n Can rearrange…. ln = – k1 k2 1 T2 1 T1 Ea R k depends on T, Ea (catalyst) k increases, when T increases k increases, when Ea decreases OWL 13.5a OWL link Presence of a catalyst. A catalyst is a substance that increases the rate of a reaction without being consumed in the overall reaction. The catalyst generally does not appear in the overall balanced chemical equation (although its presence may be indicated by writing its formula over the arrow). Factors Affecting Reaction Rates Catalysts A catalyst increases the rat