Sequential Reactions and Intermediates (25.7)

1. Sequential Reactions and Intermediates (25.7) • Sequential reactions (elementary) involve multiple reactions in which one or more intermediates are formed • The differential form of the rate is written with respect to product formation • Rate law only involves intermediate since it is the only species that generates products • Intermediate concentration is extremely hard to measure, so we need to relate it to quantities we can measure • We should be able to measure reactant concentration, so we rely on the fact that A only decays in one way • Since the intermediate only forms from the reactant, we can express its concentration in terms of A • The first reaction creates I, the second reaction depletes it (hence the negative sign)

2. Sequential Reactions and Rate Determining Steps (25.7) • Concentration of product can be determined using a simple mass balance principle • The sum of concentrations of all substances at any time must be equal to the initial concentration of reactant • The size of the rate constantsis an indication of how quickly each part of the reaction proceeds • If kI is much larger than kA, the intermediate does not last very long and thus does not build up in the reaction • If kA is much larger than kI, then the reactant decomposes quickly and a significant amount of intermediate will form • The rate-determining (or rate-limiting) step is the slowest step in the mechanism and it dictates how quickly products will form • If first step is rate-limiting, the reaction looks like it is one step (i.e., decay of reactant and formation of product follow first order kinetics) • If second step is rate-limiting, reaction follows first-order for intermediate

3. Parallel Reactions (25.8) • Parallel reactions involve the reactant decaying into more than one product • The rate of decay of the reactant is related to the rate constants of both processes • The rates of formation of each product has a simple form due to the simplicity of the differential rate equation • The product concentration (P = B or C) differs based on the rate constant • The ratio of concentrations is related to the ratio of the rate constants for a parallel set of reactions • The overall yield (ϕ) of a product in the reaction is the ratio of the concentration of the product of interest over the sum of all product concentrations

4. Reversible Reactions and Equilibrium (25.10) • Reversible reactions are ones in which the reactants can be generated from products • Each direction of the reaction has a rate constant associated with it • If the reaction starts with only A, after a certain length of time the concentrations of A and B stabilize (i.e., equilibrium is obtained) • At equilibrium, rate of change of A and B are zero • Equilibrium constant (K) can be expressed as a ratio of rate constants

5. Concentration Profile for Sequential Reactions

6. Rate-Limiting Behavior in Sequential Reactions

7. Concentration Profile for Parallel Reactions

8. Concentration Profile for Reversible Reaction