Rates of Reaction and Equilibrium

Rates Measurement of change that occurs within an interval of time Collision Theory – Particles that contain enough kinetic energy will collide and stick together causing reactions Activation energy – The minimum amount of energy particles must have to react

Activation Energy Activation NRG Reactants NRG released by rxn Products

Things that affect rxn rates Temperature – increasing temp. increases motion of particles Concentration – cramming more particles in a fixed space increases the frequency of collision Particle size – smaller size = larger surface area and more area to react Catalyst – substance that increases rxn rate without being used up

Activation Energy w/ catalyst

A B Chemical Equilibrium State in which the forward and reverse reactions take place at the same rate A B 1% 99%

Le Châtelier’s Principle If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.

Concentration Changing the concentration will force the equilibrium to shift. CO + 2 H2 ⇌ CH3OH Adding more carbon monoxide will force an equilibrium shift towards the product

Temperature Increasing the temp. causes equilibrium to shift in the direction that absorbs heat. N2 + 3 H2 ⇌ 2 NH3 ΔH = −92kJ This is an exothermic reaction when producing ammonia. If you lower the temperature, the equilibrium would shift in such a way as to produce heat. Since this reaction is exothermic to the right, it would favor the production of more ammonia

Pressure Changing pressure causes an equilibrium shift. N2 + 3 H2 ⇌ 2 NH3 ΔH = −92kJ An increase in pressure causes the reaction to shift to the side with the fewer moles

Equilibrium Constants Keq aA+bB cC + dD Keq >1, products favored at equilibrium Keq <1, reactants favored at equilibrium [C]c [D]d Keq = [A]a [B]b

Spontaneous RXNs Rxns that occur naturally and favor the formation of products. All spontaneous rxns release “free energy” Free Energy – energy that is available to do work. This energy is not 100% efficient, some energy is lost as friction and heat. Nonspontaneous rxns – rxns that do not favor the formation of products

Entropy Measurement of disorder in a system. The law of disorder – processes move in the direction of maximum disorder or randomness The size and direction of heat changes and entropy changes together determine whether a rxn is spontaneous.

Entropy Tidbits Entropy of a gas is greater than the entropy of the liquid or solid Entropy increases when a substance is divided into parts (dissociation) Entropy tends to increase in chemical reactions in which the product molecules out number the reactant molecules Entropy tends to increase when temperature increases

Is Reaction Spontaneous? Heat Change Entropy Spontaneous? Decreases (Exo) Increases Yes Increases(Endo) Only if entropy is more favorable than Δ heat Decreases Only if Δ heat is more favorable than entropy No

Standard Entropy ΔS0 = S0 (products) – S0 (reactants) J/K • mol The theoretical entropy of a crystal at 0K is 0 ΔS0 = S0 (products) – S0 (reactants) J/K • mol

Gibbs free-energy change ΔG = ΔH – TΔS The maximum amount of energy that can be coupled to another process to do useful work. ΔG is negative in spontaneous reactions