SANTOSH CHEMISTRY DEPT Chemical kinetics SANTOSH CHEMISTRY DEPT

Chemical Kinetics Chemical kinetics - speed or rate at which a reaction occurs How are rates of reactions affected by Reactant concentration? Temperature? Reactant states? Catalysts?

The Instantaneous Reaction Rate Consider the following reaction A + B  C Define the instantaneous rate of consumption of reactant A, A

Reaction Rates and Reaction Stoichiometry Look at the reaction O3(g) + NO(g) ® NO2(g) + O2(g)

Another Example 2 NOCl (g)  2 NO + 1 Cl2 (g) WHY? 2 moles of NOCl disappear for every 1 mole Cl2 formed.

The General Case a A + b B ® c C + d D rate = -1 d[A] = -1 d[B] = +1 d[C] = +1 d[D] a dt b dt c dt d dt Why do we define our rate in this way? removes ambiguity in the measurement of reaction rates Obtain a single rate for the entire equation, not just for the change in a single reactant or product.

Alternative Definition of the Rate Rate of conversion related to the advancement of the reaction, . V = solution volume

An Example Examine the following reaction 2 N2O5 (g)  4 NO2 (g) + O2 (g) N2O5 NO2 O2 Initial nø Change -2 +4 + Final nø - 2 4 

Note – constant volume system The N2O5 Decomposition Note – constant volume system

The Rate Law Relates rate of the reaction to the reactant concentrations and rate constant For a general reaction a A + b B + c C ® d D + e E

Rate Laws (Cont’d) The only way that we can determine the superscripts (x, y, and z) for a non-elementary chemical reaction is by experimentation. Use the isolation method (see first year textbooks).

The Concentration Dependencies of the Species The amounts of the reactants are related to the reaction rates as follows.

Temperature Dependence of Reaction Rates Reaction rates generally increase with increasing temperature. Arrhenius Equation A = pre-exponential factor Ea = the activation energy

Reactions Approaching Equilibrium Examine the concentration profiles for the following simple process. A ⇌ B

Approaching Equilibrium Calculate the amounts of A and B at equilibrium.

The Equilibrium Condition At equilibrium, vA,eq = vB,eq.

Elementary steps and the Molecularity Kinetics of the elementary step depends only on the number of reactant molecules in that step! Molecularity  the number of reactant molecules that participate in elementary steps

The Kinetics of Elementary Steps For the elementary step unimolecular step For elementary steps involving more than one reactant a bimolecular step

The Kinetics of Elementary Steps For the elementary step unimolecular step For elementary steps involving more than one reactant a bimolecular step

The Reaction “Yield” We can calculate the amount of material produced from the competing reactions. kJ = the rate constant for the reaction J.

Activated Complex Theory Consider the following bimolecular reaction Presume that the reaction proceeds through the transition state?

The Activated Complex The species temporarily formed by the reactant molecules – the activated complex. A small fraction of molecules usually have the required kinetic energy to get to the transition state The concentration of the activated complex is extremely small.

Transition State Theory (TST) TST pictures the bi-molecular reaction proceeding through the activated complex in a rapid-pre-equilibrium.

TST (II) From the thermodynamic equilibrium constant for the formation of AB↕

TST (IV) Assume that we can obtain the Gibbs energy of activation from K↕. The Eyring Equation!

TST (V) Hence, substituting for the Gibbs energy of activation in terms of the enthalpy and entropy of activation.

TST (VI) Expressing the enthalpy of activation in terms of the activation energy.

TST (VII) The final result for a bimolecular gas phase reaction.

TST (VIII) The final result for a unimolecular gas phase reaction (or any solution reaction).

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