Unit 6: Kinetics Copyright © The McGraw-Hill Companies, Inc.  Permission required for reproduction or display.

Chemical Kinetics Thermodynamics – does a reaction take place? Kinetics – how fast does a reaction proceed? Reaction rate is the change in the concentration of a reactant or a product with time (M/s). reactants products A B rate = - D[A] Dt D[A] = change in concentration of A over time period Dt rate = D[B] Dt D[B] = change in concentration of B over time period Dt Because [A] decreases with time, D[A] is negative.

A B rate = - D[A] Dt rate = D[B] Dt

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) red-brown Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) time 393 nm light Detector t1< t2 < t3 D[Br2] a D Absorption

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) slope of tangent slope of tangent slope of tangent average rate = - D[Br2] Dt = - [Br2]final – [Br2]initial tfinal - tinitial instantaneous rate = rate for specific instance in time

measure DP over time 2H2O2 (aq) 2H2O (l) + O2 (g) PV = nRT P = RT = [O2]RT n V [O2] = P RT 1 rate = D[O2] Dt RT 1 DP Dt =

Reaction Rates and Stoichiometry 2A B Two moles of A disappear for each mole of B that is formed. rate = - D[A] Dt 1 2 rate = D[B] Dt aA + bB cC + dD rate = - D[A] Dt 1 a = - D[B] Dt 1 b = D[C] Dt 1 c = D[D] Dt 1 d

13.1 Write the rate expressions for the following reactions in terms of the disappearance of the reactants and the appearance of the products:

13.2 Consider the reaction Suppose that, at a particular moment during the reaction, molecular oxygen is reacting at the rate of 0.024 M/s. At what rate is N2O5 being formed? At what rate is NO2 reacting?

Review of Concepts Write a balanced chemical equation for a gas-phase reaction whose rate is given by