# Chemistry Chapter 14 Chemical Kinetics

## Terms

undefined, object
copy deck
What is chemical kinetics?
The area of chemistry concerned with the speeds, or rates, at which chemical reactions occur.
Describe the four factors governing reaction rates?
-the physical state of the reactants (reaction rates increase with the increase contact area between reactants)
-the concentrations of the reactants (reaction rates increase with increased concentrations)
-the temperature at which the reaction occurs (reaction rates increase as temperature increase)
-the presence of a catalyst (increased if there is one)

notes: on the molecular level, reaction rates depend on the frequency of collisions between molecules. The greater the frequency the greater the reaction rate
How does increasing the partial pressure of the reactive components of a gaseous mixture affect the rate at which the components react with one another?
Increasing the partial pressure of a gas increases the number of molecules in a given volume and therefore increases the concentration of the gas. Rates normally increase with increasing reactant concentrations.
Define reaction rate :
It is the speed of a chemical reaction; it is the change in the concentration of reactants or products per unit time. (M/s)
What is the formula for average rate of apperance?
Average rate of apperance of B = change in concentration B / change in time
What is the formula for average rate of disappearance?
Average rate of disappearance of A = - (change in concentration of A)/ change in time
(a)How is the rate at which ozone disappears related to the rate at which oxygen appears in the reaction 2O3(g) -> 3O2(g)
(b)If the rate at which O2 appears is 6e-5 M/s at a particular instant, at what rate is O3 disappearing at this same time?
(a)-1/2 &[O3]/&t = (1/3)&[O2]/&t
(b)-1/2 &[O3]/&t = (1/3)&[O2]/&t --> -&[O3]/&t = (2/3)&[O2]/&t

(2/3)(6e-5) = 4e-5 M/s
The decomposition of N2O5 proceeds according to the folowing equation:
2(N2O5)(g) -> 4(NO2)(g) + O2(g)
If the rate of decomposition of N2O5 at a particular instant in a reaction vessel is 4.2e-7 M/s, what is the rate of appearance of (a) NO2
(a)8.4e-7 M/s
(b)2.1e-7 M/s
What is the rate law?
An equation that relates the reaction rate to the concentrations of reactants and sometimes of products also.
What is the rate constant?
A constant of proportionality between the reaction rate and the concentrations of reactatns that appear in the rate law
Define reaction order :
The power to which the concentration of a reactant is raised in a rate law.
The experimentally determined rate law for the reaction 2NO(g) + 2H2(g) -> N2(g) + 2H2O(g) is rate = k[NO]^2[H2]. (a) What are the reaction orders in the rate law? (b) Does doubling the concentration of NO have the same effect on rate as doubling the
(a)order 2 for [NO], order 1 for [H2]
(b)No they do not have the same effect. Doublbing [NO] makes rate increase 4 fold where as doubling [H2] only doubles rate.
Define first-order reactions
A reaction whose rate depends on the concentration of a single reactant raised to the first power.

rate = k[A]
What is the formula for first order reaction when you have the intial concentration and want to find the concentration at another time?
ln([A]/[Ao])= -kt
or
ln[A] - ln[Ao] = -kt
The decomposition of a certain inseticide in water follows first order kinetics with a rate constant of 1.45yr^-1 at 12C. A quantity of this insecticide is washed into a lake on June 1, leading to a concentration of 5e-7 g/cm3. Assume that this average t
(a)
ln[insecticide] = -(1.45yr^-1)(1yr) + ln(5e-7)

[insecticide] = e^-15.96 = 1.2e-7

(b)ln(3e-7) = -(1.45yr^-1)(t) + ln(5e-7)
t = -[ln(3e-7) - ln(5e-7)]/1.45yr^-1
t = -(-15.02 + 14.51)/1.45 = .35yr
The decomposition of dimethyl ether, (CH3)2O, at 510C is a first order process with a rate constant of 6.8e-4s^-1

(CH3)2O(g) -> CH4(g) + H2(g) + CO(g)

If the intial pressure of (CH3)2O is 135 torr, what is its partial pressure af
51 torr.
Define second-order reactions
Reactions whose rate depends on the reactant concentraiton reaised to the second power or on the concentrations of two different reactants raised to the first power.
What is the equation for a second order reaction?
1/[A] = kt + 1/[Ao]
What are the x and y axis of second order reaction?
x = time
y = 1/[A]
What are the x and y axis of first order reaction?
x = time
y = ln[A]
Define half-life
It is the time required for the concentration of a reactant to reach one-half of its inital value.
How does the half-life of a second order reaction change as the reaction proceeds?
As the reaction proceeds, the concentration of the reactant decreases. Thus, according to Equation 14.16, the half-life increases during the course of the reaction.
What is the central idea behind the collision model?
Molecules must collide in order to react.
Define activation energy, Ea
It is the minium energy required to initate a chemical reaction.
Define transitin state
The paricular arrangement of reactant and product molecules at the point of maximum energy in the rate-determining step of reaction.
Why isn't colision frequency the only factor affecting a reaction rate?
THe molecules must not only merely collide in order to react, but they must collide in the proper orientation and with an energy greater than the ativation energy for the reaction.
What is the Arrhenius equation and what are the three factors it is based on?
k = Ae^(-Ea/RT)

1)the fraction of molecules possessing an energy of Ea or greater
2)the number of collisions occuring per second
3)the fraction of collisions that have the appropriate orientation.
What is the gas law constant?
8.314 J/mol-k
The ___ the activation energy, the faster the reaction
lower
Define elementary reactions
A process in a chemical reaction that occurs in a single event or step. An overall chemical reaction consists of one or more elementary reactions or steps.
Define molecularity of the reaction?
The number of molecules that participate as reactants in an elementary reaction.
Define unimolecular
If a single molecule is invovled in an elementary reaction.
Define bimolecular
elementary reactions involving collisions of two reactant molecules. ex. reactions between NO and O3.
Define termolecular
elementary reactions involving collisions of three reactant molecules.
What is the molecularity of this elementary reaction?

NO(g) + CO(g) --> NOCl(g) + CO2(g)
Because the elementary reaction involves two molecules, it is bimolecular
What is chemical kinetics?
The area of chemistry concerned with the speeds, or rates, at which chemical reactions occur.

36