Notes

Outline

Chemical Kinetics Part I-Kinetic Rate Law

Types of Reaction Rates Chemical Kinetics-Study of reaction rates of Chemical and Physical Processes Reaction Rate-Change in the concentration of a reactant or product per unit time Types of Reaction Rates 1. Instantaneous Rate 2. Average Rate

Average Rates For reaction:  H2O2 -à H2O  +  ½ O2 Product Formation R avg = D[O2] / Dt = [O2]final – [O2]initial / tf-ti Reactant Disappearence R avg =  - D[H2O2] / Dt =  - [H2O2]final – [H2O2]initial / tf-ti Problem

T-13.5  Calculation of Average Rate

T-133 Rate Data for Hypothetical Reaction

T-135   Butyl Chloride Conc vs Time:Instantaneous Rate

Factors Affecting Reaction Rates Solute-Solvent Interactions Concentration of Reactants (in rate determining step) Temperature Change Effect of a Catalyst

Solute-Solvent Interactions Affecting Rates Gaseous state produces the largest Reaction rates compared to same reactants in liquid state Solid State of reactants produce the slowest rates when compared to liquid or gaseous states Relative Rates Gaseous > solution> liquid> solid Reaction Rates increase with increasing entropy(degrees of freedom) of reactants

Affect of Reactant Concentrations on Rates: Kinetic Rate Law For a reaction process:  aA  +  bB --à  cC R~(Conc A) m R~(Conc B) n Therefore:  R ~ (Conc A) m (Conc B) n R = k[A]m [B]n  where k = rate constant and m and n are rate orders Combined rate order = m + n

Rate Orders (m and n) First order Kinetics in respect to a component Doubling the concentration doubles the rate R1 = k[A]   if [A] becomes [2A] then R2 = k[2A] = 2k[A] R2 = 2R1 Second Order kinetics Doubling the concentration will quadruple the rate R1 = k[A]2  if [A] becomes [2A] then R2 = k[2A]2 = 4k[A]2 R2 = 4R1 Third Order kinetics Doubling the concentration will raise the rate by a factor of 8

Rate Constant Proportionality Constant of the Kinetic Rate Law Relationship

Defining a Rate Law Expression Determine Rate orders of the expression from lab data collected Determine Rate constant, k using lab data collected Plug into the general rate Expression the rate orders and rate constant to give most definitive expression

T-136 Determining Rate Orders Using Experimental Data

Determining Rate Constant Using lab Data

Concentration-Time Relationship for First Order System Log vs time is linear follows slope intercept formula Y = mx + b Log [A]t / log [A]0 = - kt / 2.303 or                                   ln [A]t / ln [A]0 = - kt   where [A]t = final conc;              [A]0 = initial conc; k = rate constant; t= time of reaction Derivation(optional) Problem

T-137  Graphical Representation of First Order System

T-13 Derivation of First Order Equation

Half Life of First Order System Half Life-The time it takes for half of the remaining reactant to undergo its change to product [A]t = [A]0 / 2 t ½ = .693 / k Derivation(optional) Problem

T-138 Derivation of Half Life Equation of First Order System