ch 11-12 Flashcards
chemical kinetics
the study of the speeds of chemical reactions and the factors that affect those speeds
homogeneous
reactants and products are all in the same phase
catalyst
a substance that increases the rate of a chemical reaction without being consumed like a reactant; changes the mechanism for the reaction; provides a lower Ea in the rate limiting step; does not change the products or their relative properties
factors that control the speed of reactions
properties of the reactants and products(structures and type of bonding); concentration of reactants and products; temperature; presence of catalyst; nature of the surface
rate
the change of some measurable quantity per unit of time
reaction rate
the change in the concentration of a reactant or product per unit of time; a positive quantity
average reaction rate
calculated from a change in concentration divided by a change in time
instantaneous reaction rate
reaction rate at a particular time after the reaction has begun
to find reaction rate
find the slope of the line tangent to the plot of the concentration versus time at the specific time plot of interest
effect of concentration on rate
for many reactions as reactant concentration changes the rate changes
rate=k[reactant]
rate law
mathematical expression that summarizes the relationship between concentrations and reaction rate
rate constant
k; proportionality constant relating reaction rate and concentrations of reactants and other species that affect the rate of a reaction
half life
the time it takes for one half of the reactant to be converted into product
zeroth order
t1/2=[A]0/2k
first order
t1/2=ln(2)/k
second order
t1/2=1/k[A]0
activation energy
Ea; the minimum energy that reactant molecules must have to be converted to product molecules
transition state (activated complex)
a molecular structure corresponding to the maximum of a plot of energy versus reaction progress
intermediate
an atom molecule or ion that is produced in one step of a reaction mechanism and used up in later step
elementary reactions
are the reactions that occur at the molecular level
unimolecular reactions
involve a single molecule or particle that rearranges into one or more different particles; cis-2-butene to trans-2-butene; always first order
bimolecular reactions
involving two molecules or particles that collide and rearrange; iodide ions reacting with methly bromide; always second order
Arrhenius equation
gives us the relationship between the rate constant, temperature and activation energy
k=Ae^-Ea/RT
frequency factor
how often a collision occurs in the correct orientation
complex reactions
do not occur as written; progress by a series of elementary steps
reaction mechanisms
the combination of elementary steps that comprise a complex reaction
rate-limiting step
the slowest elementary reaction in a complex reaction; the overall reaction will occur at the same rate
heterogeneous catalyst
in a different phase from the reaction mixture
homogeneous catalyst
in the same phase as the reaction mixture; usually more active; usually less stable
enzymes
highly efficient biochemical catalysts; most active homogeneous catalysts
chemical equilibrium
the point in a chemical reaction where reactant and product concentrations are constant; dynamic, independent of the direction of the approach, unaffected by the presence of catalysts
dynamic equilibrium
reactants convert to products; products can also react to make reactants; equilibrium occurs when [A],[B],[C],[D] stop changing; species do not stop forming or being destroyed, rate of formation=rate of removal, concentrations are constant
equilibrium constant
Kc=kfoward/kreverse=[C]^c[D]^d/[A]^a[B]^b
Kp=PcCPdD/PaAPbB
Kc>1
reaction is strongly product favored; very little reactant; forward reaction only; goes to completion
Kc
strongly reactant favored; very little product forms; no reaction
Kc=1
reactants and products present at equilibrium
reaction quotient
Q; can predict the direction of a reaction; whenever system is at equilibrium Kc=Q
Q
Q must increase;; make more product; move foward
Q>K
Q must decrease; less product; move back
lechatelier’s principle
if the conditions of a system at equilibrium are changed so that it is no longer at equilibrium, the system wil react to reach a new equilibrium in a way that partially counteracts the change; system at equilibrium resist change
Qc>Kc
shift to the left to consume some product and form more reactant
changing pressure or volume
K does not change if P or V do not change; equilibrium position may change
Volume doubled
lower concentration; more molecule, increase concentration; shift toward products
pressure doubled
removing molecules, decrease P; shift toward reactants
changing temperature
increase- minimize by removing heat; forward reaction is endothermic; equilibrium shift forward
decrease-offset by adding heat; reverse reaction is exothermic; equilibrium shift to left
exothermic reactions
Kc decreases as T increases; less product favored
endothermic reactions
Kc increases as T increases; more product favored
