Ch 2 exam Flashcards
the half life of a ____ order reaction is constant.
first
t1/2 = ln 2/k
the half life of a reaction depends on the ____
order of the reaction
rate of a first order reaction
rate = k [A]
rate of a zero order reaction
rate = k
i.e. rate = k[A]^0 = k
the half life of a zero order reaction DOES DEPEND on ____
the initial concentration of the reactant
t1/2= [A]0/2k
the half life of a second order reaction DOES DEPEND on ____
the initial concentration of the reactant
t1/2 = 1/k*[A]0
collision theory
reactants must collide in order to react with each other
1) rate of reaction is proportional to the rate of reactant collisions
2) molecules must be oriented properly when they collide
3) molecules must have adequate kinetic energy to react (activation energy)
Ea
activation energy
minimum energy necessary to form a product during a collision between reactants
catalyst
a substance that increases the rate of a chemical reaction by lowering the activation energy without itself being consumed
catalyst is regenerated in the process
provide an alternate reaction pathway with a lower activation energy
homogeneous catalyst
in the SAME PHASE as the reactants. it speeds up the reaction by forming a reactive intermediate.
the reactive intermediate reacts further to form a product and regenerate the catalyst. the reactive intermediate is formed but immediately consumed.
heterogeneous catalyst
in a different phase from that of the reactants.
usually the catalyst is a solid and the reactants are gas or liquid
enzyme
biological HETEROGENEOUS catalyst
reaction mechanism
the series of reactions that occurs to produce the overall observed reaction
most reactions occur in a series of small reactions involving one, two, or (at most) three molecules.
knowing the rate law of the reaction helps us determine the sequence of reactions in the mechanism
intermediates
materials that are products in an early mechanism step, but then reactants in a later step
molecularity
the number of reactant particles in an elementary step
unimolecular = one particle
bimolecular = two particles (can be the same type of particle)
termolecular = three particles
rate law for an overall reaction must be determined ____
experimentally
rate determining step
slowest step
the rate law of the rate determining step determines the rate law of the overall reaction
chemical equilibrium
rates of both reaction and reverse reaction are equal
no further changes in concentration of either gas occurs unless the temperature or the volume of the container is changed
does NOT mean the amount of reactants and products are equal (just rates)
this is DYNAMIC equilibriumr
reaction quotient, Q
mathematically expression of the the amounts of reactants and products present at any given point in a reversible reaction
can vary prior to equilibrium
can be calculated at any point in the reaction
often calculated at the start of the reaction using initial concentrations
Qc vs Qp
Qc = amounts expressed in concentration; Molarity
Qc = [C]^c[D]^d / [A]^a[B]^b
Qp = partial pressures
Qp = P(C)^cP(D)^d / P(A)^aP(B)^b
equilibrium constant, K
the value of Q when the reaction is at equilibrium
Kc = Qc at equilibrium
K is unitless
law of mass action
states that the values of the equilibrium constant expressions are a constant for a particular reaction, at a given temperature
the relationship between the chemical equation and concentrations of reactants and products
The value of the equilibrium constant K is independent of:
the starting amounts of reactants and products
The value of the equilibrium constant K is dependent on:
the temperature of the system
the magnitude of an equilibrium constant indicates:
the extent of a reaction
K and Q are
unitless
If K is very small:
the mixture contains mostly reactants at equilibrium
If K is very large:
the mixture contains mostly products at equilibrium
The value of K gives no indication as to:
whether the reaction is fast or slow
K (backward) =
1/K (forward)
Q<K
reaction forms products;
shift R
Q>K
reaction forms reactants;
shift L
Q=K
equilibrium
homogenous equilibrium
all the reactants and products are present in the same phase
all ___ and ___ phase species appear in the Kc expression
aqueous and gas
K = 1
doesn’t favor reactants or products
K «_space;1 (much smaller than 1)
reverse rxn is favored
Q»1 (much larger)
reaction has more products than reactants
the relationship between Kp and Kc
Kp = Kc (RT) ^n
heterogeneous equilibrium
reaction mixture that contains reactants and products that are in two or more different phases
Pure solids and pure liquids DO NOT appear in the K expression
equilibrium is independent of the amount of solid or liquid present as long as at least some is present in the reaction mixture
le Chatelier’s principle
when a chemical system at equilibrium is disturbed, it returns to equilibrium by counteracting the disturbance
disturbance causes a change in Q. reaction will shift to re-establish Q= K
ways equilibrium can be disturbed
- adding or removing a reactant or product
- changing the pressure
- changing the temperature
adding or removing a pure liquid or solid:
has no effect on the system unless all the liquid or solid is removed
pressure changes will effect the equilibrium if:
the reaction contains gas phase species AND n is NOT equal to 0
exothermic reaction will shift
left
endothermic reaction will shift
right
