Reaction Rates & Equilibrium Rates of Reaction

Collision Theory Rate: measure of speed of any change that occurs within an interval of time Expressed in amount of reactant changing per unit time Collision Theory: atoms, ions, and molecules can react forming product when they collide Must have enough kinetic energy or they bounce apart unchanged

Chemical Reaction Activation Energy: the minimum energy that colliding particles need to react Activated Complex: unstable arrangement of atoms that forms momentarily at the peak of activation-energy barrier. Lifetime: 10-13s Transition state

Factors Affecting Ration Rate Temperature Hi temp  >KE more collisions inc. rxn. rate Concentration [Hi]  more collision  inc. rxn. rate Particle Size Smaller particle size  greater surface area Greater surface area, increase exposure to collision, and inc. rxn. rate Catalyst: increases a reaction without being changed Inhibitor: interferes with a catalyst Reaction slows or even stops

Reversible Reactions and Equilibrium

Reversible Reaction Reaction in which the conversion from reactant to products and products to reactants happens simultaneously 2SO2 + O2 ↔ 2SO3 Chemical Equilibrium: when the forward and reverse rates are equal Reactions, fwd. & rev., still occur No net change in concentration

Catalyst: speeds up the fwd. and rev. reactions equally Irreversible reactions: one set of components is completely converted to a new substance Catalyst: speeds up the fwd. and rev. reactions equally Don’t effect equilibrium concentrations

LE CHÂTELIER’S PRINCIPLE When stress is applied to a system, the system will responds to relieve the stress Shift equilibrium position Stresses: Concentration Temperature Pressure

LE CHÂTELIER’S PRINCIPLE Concentration H2CO3 ↔ CO2 + H2O Add CO2 Shifts left Remove CO2 Shifts right Add H2O

Affects of Temperature Increasing the temp.  shift in the direction that absorbs heat 2SO2 + O2  2SO3 + heat Add heat Shifts left Cool Rxn. Shifts right

Affects of Pressure Pressure changes affect gaseous systems that have unequal # of moles of reactants and products Want to optimize room N2(g) + 3H2 (g)  2NH3(g) Increase pressure Shifts right Decrease pressure Shifts left

Equilibrium Constant Keq: ratio of product concentration to reactant concentration at equilibrium raised to the power = # of moles Only the concentration of gases and substances in solution are shown Concentration for pure liquids and solids are not shown

Equilibrium Constant aA + bB  cC + dD Keq >1: favors products Keq <1: favors reactants

Example Problems A liter of gas mixture at equilibrium contains 0.0045mol of N2O4 and 0.030mol of NO2. Write the Keq expression and determine its value. N2O4  2NO2

Calculating an Equilibrium Constant HI placed in a sealed container and comes to equilibrium; equilibrium reaction is: 2HI(g) H2(g) + I2(g) Equilibrium concentrations: [HI] = 0.54 M [H2] = 1.72 M [I2] = 1.72 M Substitute concentrations: Keq = [H2] [I2] [HI]2 Keq= [1.72] [1.72] = 2.96 [0.54]2 0.29 = 10.1 or 1.0 x 101 2 significant figures