Chapter 12 kinetics Flashcards
kinetics
the study of the ratewith which a chemical reaction occurs, the factorsthat affect the reaction rate, and the mechanismof reaction.
concentrations as a Function of Time
The reactantconcentration decreases(consumed), and the productconcentration increases(formed) with time.2NH3(g) →N2(g)+ 3H2(g)
rate of a reaction
is a measure of the speedat which a reaction occurs (mol/L • s)– always positive; samefor reactants or products.– expressed as the change in concentration of reactants (−)or products (+) divided by the change in tim
Generic Reaction:
aA+ bB→cC+ dDA and B are reactants, C and D are products;a, b, c, and dare the stoichiometric coefficients
average rate of reaction:
ate of a chemical reaction computed as the ratio of a change in concentration to the time interval over which the change occurred
nstantaneous rate of reaction:
ate of a chemical reaction at any instant in time, determined by the slopeof the line tangential to a graph of concentration as a function of time.2NH3(g) → N2(g)
nitial rate
instantaneous rate of a chemical reaction at t = 0 s (immediately after the reaction has begun)
Rate law:
mathematical expressions that describe the relationship between the rateof a chemical reaction and the concentration of its reactants
•n= 0
zero order
The unit of k: M • s−1
rate= k (A)o = k
n=1
1st order
units: s-1
rate = k(a)1=k(a)
n=2
2nd order
unitz; : M−1• s−1
rate= k(a)2
Determining a rate law from initial rates:
- Select two sets of experimental rate data that differ in the concentration of only one reactant 2. Set up a ratioof the two rates and the two rate laws. 3. After cancelingterms that are equal, we are left with an equation that contains only one unknown4. We then solve this equation for the order.
•The rate laws
shows the relationship between therateof areaction and theconcentrationsof the reactants.– How reaction rate depends on concentration of reactant
rate= k(a)n
•The integrated rate law
for a chemical reaction is an equation that relates the concentration of a reactant to elapsed timeof reaction– How concentration of reactants depends on reaction time. – The integrated forms of the rate laws
half-life (t1/2) of
f a reaction is the time required for one-half of a given amount of reactant to be consumed.
half life zwero order
depeding on initual concentration
half life 1st order
independent of intial concentration
half life seconf order
depending on intial concentration
collision theory
model that emphasizes the energyand orientationof molecular collisionsto explain and predict reaction kinetics
Three postulates for collision theory:
- The rate of a reaction is proportional to the rate of reactant collisions: ∝ࢋ࢚ࢇ࢘ ࢚ࢉࢇࢋ࢙࢙࢘ࢉ #ࢋ࢚2. The reacting species must collide in an orientationthat allows contact between the atoms that will become bonded together in the product.3. The collision must occur with adequate energyso that the electrons of reactants can be rearranged and form new bonds and new chemical species.
Activated complex:
unstable combination of reactant species representing the highest energy state of a reaction system– transition state, reaction intermediate, not final product
Activation energy (Ea)
The minimumenergy necessary to form a product during a collision between reactants.
Arrhenius equation:
mathematical relationship between the rate constant and the activation energy of a reaction
- Ea: activation energy (J/mol)
- R: 8.314 J/(mol • K)
- T: temperature in kelvins
- A: frequency factorࢋ
Collision theory:
model that emphasizes the energyand orientationof molecular collisionsto explain and predict reaction kinetics
Overall reaction
shows the startingsubstances and the endingsubstances.
elementary reaction.
Overall reaction can be broken down into several steps, and each step is called
Reaction Mechanism
s the process, or pathway, by which a reaction occurs
Reaction intermediate
is a species that are produced in one step and consumed in a subsequent step (eg: O). – It doesn’t show up in the overall reaction
Molecularity
of an elementary reaction is the number of reactantspecies (atoms, molecules, or ions).
–Unimolecular:
oneparticle; A-products
–Bimolecular
twoparticles (same or different); A+ B =products
Termolecular:
threevparticles (same or different); A+ B + C =products
Rate-determining step
s the slowestelementary reaction in a reaction mechanism; determines the rate of the overall reaction– like the narrowest section on a freeway
To validate a mechanism, threeconditions must be met:
The elementary steps must sum to the overall reaction.2.The rate law predicted by the mechanism must be consistent with the experimentally observed rate law.3.Each elementary step should have an order of three or less
Catalyst:
a substance that increasesthe rate of a chemical reaction but is not consumed by the reaction.– Catalysts function by providing an alternative mechanism that has a lower activation energy, and therefore a higher reaction rate.
