chemical kinetics “rates of reactions”
DESCRIPTION
Chemical Kinetics “Rates of Reactions”. Reaction Rates. Average rate : Change of reactant or product concentrations over a specific time interval Initial rate : Rate at the beginning of a reaction. Question?. For the reaction: - PowerPoint PPT PresentationTRANSCRIPT
Chemical Kinetics“Rates of Reactions”
Reaction Rates
Average rate:
Change of reactant or product concentrations
over a specific time interval
Initial rate:
Rate at the beginning of a reaction.
For the reaction:
The dependence of the concentration of H2 on time is shown below. Is the reaction rate faster at point A or point B?
A, B, both rates are the same
Question?
Rate Determining Step
• slowest step in a multi-step mechanism
• the step which determines the overall rate of the reaction
Rate Law
• an expression which relates the rate to the concentrations and a specific rate constant
Rate Law
Reaction rate = k [A]m [B]n
where m => order with respect to A
n => order with respect to B
overall order = m + n
The empirical rate law for the reaction
is Rate = k[NO2][F2].
Which of the following mechanisms is consistent with this rate law?
A) NO2 (g) + F2 (g) <=> NO2F(g) + F(g) fast NO2(g) + F(g) --->NO2F(g) slow
B) NO2 (g) + F2(g) <==> NO2F(g) + F(g) slow NO2(g) + F(g) ---> NO2F(g) fast
C) F2(g) <==> F(g) + F(g) slow 2NO2(g) + 2F(g) ---> 2NO2F(g) fast
A, B, or C
Question?
Order of Reaction
• The exponent of the concentration for a reactant implies the number of molecules of that species involved in the rate determining step
• first order, exponent equals one
• second order, exponent equals two
Integrated Rate Laws
A ---> products
rate = - ([A]/t) = k[A]m
average rate
rate = - (d[A]/dt) = k[A]m
instantaneous rate
This plot of ln[H2O2] vs. time produces a straight line, suggesting that the reaction is first order.
Elementary Reactions• Molecularity is the number of particles coming
together in a reaction step.• unimolecular - rearrangement of a molecule• bimolecular - reaction involving the collision of
two particles• termolecular - reaction involving the collision of
three particles (statistically unlikely)
Unimolecular Reaction
Bimolecular Reaction
Collision Rate ModelThree conditions must be met at the molecular level if a reaction is to occur:
• the molecules must collide;
• they must be positioned so that the reacting groups are together in a transition state between reactants and products;
• and the collision must have enough energy to break bonds, form the transition state and convert it into products.
Transition State: Activated Complex or Reaction Intermediates
• an unstable arrangement of atoms that has the highest energy reached during the rearrangement of the reactant atoms to give products of a reaction
Activation Energy
the minimum energy required to start a reaction
Temperature & Reaction Rate
Catalyst• A substance which speeds up the rate of a
reaction while not being consumed
Homogeneous Catalysis - a catalyst which is in the same phase as the reactants
Heterogeneous Catalysis - a catalyst which is in the different phase as the reactants
catalytic converter– solid catalyst working on gaseous materials
Which curve illustrates the effect of a catalyst on the reaction diagram, given that it speeds up the rate of a reaction?
A, B, C, D
Question?
Reaction Mechanism
• A set of elementary reactions which represent the overall reaction
Car Catalytic Converter catalyst
H2O(g) + HCs -------> CO(g) + H2(g) (unbalanced)
catalyst
2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g)
catalyst
HCs + O2(g) --------> CO2(g) + H2O(g) (unbalanced)
catalyst
CO(g) + O2(g) --------> CO2(g) (unbalanced)
catalyst = Pt-NiOHCs = unburned hydrocarbons
Chain Mechanisms
chain initiating step - the step of a mechanism which starts the chain
chain propagating step(s) - the step or steps which keeps the chain going
chain terminating step(s) - the step or steps which break the chain
Chain Mechanismscombustion of gasoline in an internal
combustion engine
chain initiating step - additives which generate free radicals, particles with unpaired electrons
chain propagating step(s) - steps which generate new free radicals
chain terminating step(s) - steps which do not generate new free radicals
Enzymes
Any one of many specialized organic substances, composed of polymers of amino acids, that act as catalysts to regulate the speed of the many chemical reactions involved in the metabolism of living organisms.