Reaction Rates and Chemical Equilibria Bettelheim, Brown, Campbell and Farrell Chapter 7

Chemical Kinetics Chemical kinetics:Chemical kinetics: the study of the rates of chemical reactions Example, the concentration might increase from 0 to 0.12 mol/L over a 30 minute time period

Reaction Rates The rates of chemical reactions are affected by several factors –Molecular collisions –Activation energy –Nature of the reactants –Concentration of the reactants –Temperature –Presence of a catalyst

Molecular Collisions –Two species, A and B (molecules or ions), must collide in order to react –Most collisions do not result in a reaction effective collision –A collision that does result in a reaction is called an effective collision –Effective collisions must have Enough energy to reach the activation energy Correct orientation of A and B at the time of collision

Molecular Collisions E a Activation energy:E a Activation energy: the minimum energy required for a reaction to take place –Most reactions involve breaking covalent bonds initially –Energy is required to break covalent bonds –Energy comes from the collision between A and B

–If the collision energy is large, there is sufficient energy to break the necessary bonds, and reaction takes place –If the collision energy is too small, no reaction occurs

Molecular Collisions Orientation at the time of collision –Colliding particles must be properly oriented for bond breaking and bond making –In reaction between H 2 O and HCl, the oxygen of H 2 O must collide with the H of HCl so that the new O-H bond can form and the H-Cl bond can break

Energy Diagrams Exothermic

Energy Diagrams Endothermic

Energy Diagrams Transition state:Transition state: maximum on an energy diagram

Factors Affecting Rate

Catalyst:Catalyst: a substance that increases the rate of a chemical reaction without itself being used up

Reversible Reactions Equilibrium:Equilibrium: a dynamic state in which the rate of the forward reaction is equal to the rate of the reverse reaction –No change in concentration of either reactants or products –Reaction is still taking place but the rates of the two reactions are equal

Equilibrium Constant, K For the general reaction –the equilibrium constant is

Equilibrium Constant Expression K eq aA + bB → cC + dD reactants products K eq = [C] c [D] d [A] a [B] b For equilibrium constant expressions, we use coefficients as the powers.

Equilibrium Constants Problem:Problem: when H 2 and I 2 react at 427°C, the following equilibrium is reached –Equilibrium concentrations are [I 2 ] = 0.42 mol/L, [H 2 ] = mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K –Solution:

Equilibrium Constants Problem:Problem: when H 2 and I 2 react at 427°C, the following equilibrium is reached –Equilibrium concentrations are [I 2 ] = 0.42 mol/L, [H 2 ] = mol/L, and [HI] = 0.76 mol/L. Using these values, calculate the value of K –Solution:

Coefficients from different sources Equilibrium constants: coefficients are the powers in equation Reaction Rate equations: coefficients are NOT the powers (must be determined experimentally)

What does Keq mean? At equilibrium: Keq > 100: mostly products present Keq < 0.01: mostly reactants present 0.01 < Keq < 100: significant amounts of both products and reactants present Keq value changes at different temperatures

Equilibrium and Rates There is no relationship between a reaction rate and the value of K –Reaction rate depends on the activation energy of the forward and reverse reactions; these rates determine how fast equilibrium is reached but not its position

LeChatelier’s Principle: LeChatelier’s Principle: when a stress is applied to a chemical system at equilibrium, the position of the equilibrium shifts in the direction to relieve the applied stress

Le Chatelier’s Principle When system at equilibrium is “disturbed” or “stressed” the system moves in the direction that relieves the stress Types of “stress”: Add or remove reactant Add or remove a product Change temperature

Ways to stress system Increase the concentration of a chemical Decrease the concentration of a chemical Heat the system (add heat to the system) Cool the system (remove heat energy)

Le Chatelier’s Principle 2 O 2 + N 2 ↔ 2 NO 2 colorless colorlessred/brown Add O 2 : Turns Redder Concentration increases on left System makes more product to relieve stress System “shifts” to right

Le Chatelier’s Principle 2 O 2 + N 2 ↔ 2 NO 2 colorless colorlessred/brown Remove O 2 : Turns less red Concentration decreases on left Some product changes to reactants to relieve stress System “shifts” to left

Can treat heat as if it were a reactant or product for LeChatelier A + B ↔C + heat