Chemistry - Kinetics Test

Kinetics

Measures time required for a rxn to occur

What can kinetics of a rxn tell us?

• how long it will take to complete a rxn

• how chemicals react to form products (mechanism)

• effects of catalysts & enzymes

• how to control a rxn

Thermodynamic Stability

substance doesn’t spontaneously decompose at room temp

Kinetic Stability

activation energy for decomposition is so high that the rxn goes too slowly for a change to be detected

Factors Influencing Rxn Rates

1. reactant structure (polar v. nonpolar)

   1. physical state of reactants (vapor v. liquid)

2. concentration of reactants (medications)

   1. surface area (sugar cube v. crystals)

3. temp (hypothermia & metabolism)

4. catalyst (H₂O₂ & blood)

concentration

• more reactants

• more collisions

• increase of reactant concentration then increase # of collisions & increase rxn rate

• more surface area increases collisions

temperature

• higher temp increases collisions (more energy)

catalyst

adding catalyst increases collisions (lower activation energy, don’t need to have as much energy to collide)

homogeneous catalyst

in same phase as reactants

heterogeneous catalyst

• not in same phase as reactants

• are surface on which substances can react

Rate Law Expression

• rate = k(H₂)(I₂)

• rxn rate varies directly as product of concentrations of reactants

specific rate constant (k)

• depends on size, speed, and kind of molecule involved in rxn

• large is products form quickly

• small is products for slowly

Rate Law

rate of disappearance of A is _____ to concentration of A

proportional

order of a rxn

power to which concentration of a reactant must be raised to give the experimentally observed relationship between concentration and rate

rate law based on 3 things:

1. concentration (molarity) of each of the reactants

2. power to which each of the reactants is raised (calculated from experimental data; rxn order is SUM of all of reactant orders)

3. value of k (rate constant)

elementary rxn

• rxn in which reactants are converted to products in single step

• one activation energy peak

• peaks are energies of activated complexes, valleys are energy of intermediate

intermediate

product of ONE of the steps in a rxn mechanism

equilibrium

state where forward & reverse conditions occur at same rate

dynamic equilibrium

when is equilibrium reached?

where rates of forward & reverse rxns are the same

equilibrium expression

equilibrium constant

LeChatelier’s Principle

• stress causes shift in equilibrium (temp, concentration, pressure)

• pressure only affects gases (increase in pressure causes equilibrium to shift to side with least moles of gas)

• shift towards decrease; shift away from increase