Chapter 15 Kinetics • 15.1 Rates of Chemical Reactions • 15.2 Reaction Conditions and Rate • 15.3 Effect of Concentration on Reaction Rate • 15.4 The Relationship between Concentration and Time • 15.5 A Molecular View of Reactions • 15.6 Reaction Mechanisms
Rates of Chemical Reactions • Rate= Change of Quantity / Time • For example, Δ distance / time = velocity • Rate of reaction = -Δ[N2O] / Δ time • Collision theory states three conditions that must be met for a collision to occur • The molecules must collide • They must collide with sufficient energy • They must collide with the correct orientation with respect to each other
Things That Affect Rate k = Ae(-Ea)/(RT) Where A is the percent of properly aligned collisions, Ea is activation energy, T is temperature, and R is 8.314 JK-1 mol-1 • Concentration of Reactants • this is because the rate is proportional to the concentration of reactants, so as the reaction proceeds and the reactants are used up, rate decreases • Temperature • T is proportional to KE which is proportional to velocity (squared). As T increases, molecules have more frequent, more forceful collisions, speeding up the reaction rate • Catalysts • Catalysts provide an alternative path for reactions, one with much lower activation energy. Therefore, catalysts speed up the rate of reaction. However, they speed up the forward and reverse reactions equally, so there is no change in the k.
Writing Rate Equations • Rate of a Reaction = K*[A]a[B]b • Where A and B are reactants of the reaction • And a and b are determined by analysis of experimental data 3x=9=32 9x 3x .5x .5x Therefore, HgCl2 is first order while C2O42- is second order. Rate = k[HgCl2][C2O42-]2
Relationship Between Concentration And Time K = Slope Note: Any combinations not shown are not linear
Reaction Mechanisms • Consider Br2 + 2NO 2BrNO • Possible elementary steps of mechanism: • Step 1: Br2 + NO Br2NO FAST • Step 2: NO + Br2NO 2BrNO SLOW • The slowest step is the rate determining step. Therefore, rate is set equal to k2[NO][Br2NO]. However, Br2NO is an intermediate. We must find rate in terms of the products. • k1[Br2][NO] = k-1[Br2NO] so [Br2NO] = k1[Br2][NO]/k-1 • Substitute: Rate = k2[NO](k1[Br2][NO]/k-1) = K[Br2][NO]2 Note: We can assume that the rate equations for elementary steps of a reaction are based on stoichiometry