Chapter Five Rates of Chemical Reaction. Weather can occur ? chemical thermodynamics How fast (reaction rate)? chemical kinetics. Chemical Reaction. 化学反应 速度的快慢 —— 主要决定于反应的 内在机理 。. 反应机理： 化学反应所经历的 途径 或 具体步骤 ， 又 称为 反应历程 。.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Weather can occur ?
chemical thermodynamics
How fast (reaction rate)?
chemical kinetics
Chemical
Reaction
)
51 Rates and Mechanisms of Chemical
Reactions
52 Theories of Reaction Rate
53 Reaction Rates and Concentrations
54 Effect of Temperature on Reaction
Rates
55 Effect of Catalyst on Reaction Rates
51 Rates and Mechanisms of Chemical Reactions
Reaction rate: changes in a concentration of a
product or areactant per unit time.
The rate is defined to be a positive number.
The unit of reaction rate is molL1S1,
molL1min1
or molL1h1 et al
Define reaction rate :
1. Average reaction rate
2N2O5 = 4NO2 + O2
average reaction rate is obtained by dividing the change in concentration of a reactant or product by the time interval over which the change occurs
0 : c1(H2O2) = 15.88103 mol/L
5 : c2(H2O2) = 12.8103 mol/L
So:
aA + bB
cC + dD
This equation can be used to establish the relationship between rate of change of one reactant or product to another reactant or product.
 c
dc
V = lim = 
t
dt
t0
Define reaction rate :
2. Instantaneous reaction rate
2N2O5 = 4NO2 + O2
The instantaneous rate of change at a point is the same as the slope of the tangent line. That is, it's the slope of a curve.
c
c
t
v
t
NO2 + CO NO + CO2
NO2 + NO2 NO3 + NO (slow)NO3 + CO NO2 + CO2 (fast)
This means that the rate of the overall reaction is dominatedby the rate of the first reaction, this is the ratedetermining step.
of two reactants to give an activated complex in an elementary reaction. NO(g) + O3(g) NO2(g) + O2(g)
_termolecular reaction: an elementary reaction involving the simultaneous collision of any combination of three molecules, ions, or atoms.
2 NO + H2 N2 + H2O2
The collision theory is based on the kinetic theory and assumes a collision between reactants before a reaction can take place.
1918 Lewis
The transition state theory suggests that as reactant molecules approach each other closely they are momentarily in a less stable state than either the reactants or the products.
Contents of Collision Theory:
reacting molecules must come so close that they
collide.
"effective" collisions : not every collision
between molecules creates products,
only few collisions between reactant
molecules will react.
HCl NH3
is the molecule have enough energy and can produce effective collision
Figure:As the activation energy of a reaction decreases, the number of molecules with at least this much energy increases, as shown by the yellow shaded areas.
Transition state theory (TST) is also called
activated complex theory.
reactants pass through highenergy transition
states before forming products, they are associated in an unstable entity called an activated complex,
then change into products.
Chemical reactions are faster when the concentrations of the reactants are increased .
Because more molecules will exist in a given volume. More collisions will occur and the rate ofa reaction will increase.
Each reaction has its own equation that gives its rate as a function of reactant concentrations.
this is called its Rate Law
A rate law shows the relationship between the reaction rate and the concentrations of reactants.
The rate of a reaction is proportional to the product of the concentrations of the reactants raised to somepower.
aA + bB cC + dD
v[A]m[B]n
v = k[A]m[B]n
v = k[A]m[B]n
aA + bB cC + dD
[A], [B] are the concentration of A and B;
mand nare themselves constants for a given reaction, it must be determined experimentally
in general, m and n are not equal to the
stoichiometric coefficientsa and b
The order of a reaction with respect toone of the reactantsis equal to the power to which the concentration of that reactant is raised in the rate equation.
The sum of the powers to which all reactant concentrations appearing in the rate law are raised is called theoverall reaction order.
For rate equationv = k[A]m[B]n
mis the order of the reaction with respect to A,nis the order of the reaction with respect to B.m+n isoverall order of the reaction
the exponents m and n are not necessarily related to the stoichiometric coefficients in the balanced equation, that is, in general it is not true that for a A + b B c C + d D, m a and n b
The rate law for the thermal decompositionof acetaldehyde (CH3CHO) CH3CHO(g) CH4(g) + CO(g)has been determined experimentally to bev =k[CH3CHO]3/2and not rate = k[CH3CHO]
Given the following data, what is the rate expression for the reaction between hydroxide ion and chlorine dioxide?
2ClO2(aq) + 2OH(aq) ClO3(aq) + ClO2(aq) + H2O
[ClO2] (mol.L1) [OH] (mol.L1) Rate (mol.L1 s1)
0.010 0.030 6.00104 v1
0.010 0.075 1.50103 v2
0.055 0.030 1.82102 v3
v = k[ClO2]m[OH]n
n=?
By inspection, n = 1
A firstorder reactionis a reaction whose rate depends on the reactant concentration raised to the first power.
A B
the rate equation is
Differential form:
 { lnc lnc0} = k1( t  0 )
c0 is the initial concentration of c (t =0).
c is the concentration of c at some time t.
The characteristics of firstorder reactions
2. The rate constant, k, has units of [time]1.
is the time it takes for the concentration of a reactant A to fall to one half of its original value.
By definition, when t = t1/2, , so
Example 52(a) What is the rate constant k for the firstorderdecomposition of N2O5(g) at 25 if the halflife of N2O5(g) at that temperature is 4.03104 seconds?
Solution: (a)

(b) Under these conditions, what percent of the N2O5 molecules will not have reacted after one day?
Solution: (b)
Putting in the value for k and substitutingt = 8.64104 seconds (one day has 86,400 seconds) gives
Therefore, 22.6% of the N2O5 molecules will not have reacted after one day at 25.
Example 53SO2Cl2 decomposes to sulfur dioxide and chlorine gas. The reaction is first order. If it takes 13.7 hours for a 0.250mol/L solution of SO2Cl2 to decompose into a 0.117mol/L solution, what is the rate constant for the reaction and what is the halflife of SO2Cl2 decomposition?
Solution:
The integrated rate law for a 2nd order reaction can be easily shown to be
1. A graph of 1/c against time is a straight line ,
the slope of which gives the rate constant for the reaction
2. The rate constant, k, has units of[c]1[t]1
3. The halflife of 2thorder reactions
t1/2=1 / kc0.
(c = 2/c0)
Note that the halflife of a secondorder reaction is easily shown to be notindependent of the initial concentration, as in the case of a firstorder reaction. This is one way to distinguish a firstorder reaction from a secondorder reaction.
Example 54 easily shown to be Butadiene(C4H6) dimerizes() to form C8H12. This reaction is 2nd order in butadiene. If the rate constant for the reaction is 0.84 Lmol1min1, how long will it take for a 0.500 mol/L sample of butadiene to dimerize until the butadiene concentration is 0.200 mol /L?
Solution:
511
t = 3.6 (min)
The characteristics of zeroorder reactions easily shown to be
c =  k0 t + c01. A graph of c against tis a straight line with a slope of k0.
2. The rate constant, k, has units of [c][ t ]1
3. The halflife of a zeroorder reaction is
Example 55 easily shown to be
The decomposition of HI into hydrogen and iodine on a gold surface is 0th order in HI. The rate constant for the reaction is 0.050molL1s1. If you begin with a 0.500mol/L concentration of HI, what is the concentration of HI after 5 seconds?
Solution:
c0= 0.500molL1k =0.050molL1 s1
t =5s
[HI] = 0.5000.0505
= 0.250(molL1)
54 easily shown to be Effect of Temperature on Reaction Rates
Chemical reactions are faster when the
temperature is increased. Why?
Figure: At a higher temperature,T2, more molecules have an energy greater than E a , as shown by the yellow shaded area.
When we increase the temperature, kinetic easily shown to be
energies and speeds will increase and so
the average energy of a collision will also
increase.
As a result, the energy of any particular
collision will be more likely to exceed the
activation energy.
Thus, chemical reactions are faster at higher
temperatures than at lower temperatures.
Rule of thumb easily shown to be
the rate of a chemical reaction will double for each 10 increase in the temperature.
The rate of a chemical reaction will easily shown to be double for
each 10 increase in the temperature.
54.1 Rule of Thumb(Vant Hoff Law)
T : vT = k T[A]a [B]b
(T + 10) : v(T+10) = k T+10[A]a [B]b
a A + b B = c C + d D
v(T+10) k (T+10)
=  =  = 24
v T k T
54.2 The Arrhenius Equation easily shown to be
In 1889 Svante Arrhenius showed that the dependence of the constant of a reaction on temperature can be expressed by the following equation, now known as the Arrhenius equation
The Arrhenius Equation: easily shown to be
Ea is the activation energy of the reaction (in kJ/mol)
R is the gas constant (8.314 JK1mol1)
T is the absolute temperature
e is the base of the natural logarithm scale
A represents the collision frequency, and is called
the frequency factor ().
AEa
Taking the natural logarithm of both sides, the equation becomes
or in terms of common logarithms :
The rate constants for the decomposition of acetaldehyde
CH3CHO(g) CH4(g) + CO(g)
were measured at five different temperatures. The data are show below. Plot lgk versus 1/Tand determine the activation energy (in KJ/mol) for the reaction.
T(K) 700 730 760 790 810
k [1/(mol/L)1/2s] 0.011 0.035 0.105 0.343 0.789
Answer: becomesWe need to plot lg k versus 1/T . From the given data we obtain
1/T(1/K) 1.43103 1.37103 1.32103 1.27103 1.23103
log k 1.96 1.46 0.979 0.465 0.103
The slope of the line is calculated from two pairs of coordinates:
lg k
1/T
Thus, a plot of lg becomesk versus 1/T gives a straight line
whose slope is equal to E a / 2.303R and whose
interceptwith the ordinate() is log A.
Ea = 2.303(8.314J/Kmol) (9.38103K)
= 1.80105J/mol
= 1.80102KJ/mol
According to this equation, we can calculate E aand k
T1 k1
T2 k2
(514)  (513)
The rate constant of a firstorder reaction is 3.46102 /s at 298 K. What is the rate constant at 350 K if the activation energy for the reaction is 50.2 KJ/mol?
Answer:
neither consumed nor produced in the reaction
MnO2
Catalysis becomes can be classified into
homogeneous() catalysis,
heterogeneous () catalysis
enzyme catalysis.
three types
The rate is very becomesSlow,
but it can be catalyzed by Ag+ or Mn2+:
25C becomes
e.g., H2 + CH2=CH2
no reaction (k very small)
25C
Pd
CH3CH3 rapid (k larger)
Ea (uncatalyzed)
Ea (catalyzed)
PE
C2H4
+
H2
C2H6
RC
catalysis
An average living cell may contain some 3000 different enzymes
E + S = ES P + E
activated complex
2. becomes(multielement catalysis)
,