Principles of Reactivity: Chemical Kinetics. Chapter 15. Kinetics. Chemical kinetics is the study of the rates of chemical reactions. Macroscopic level: rates of reactions, reaction rate, factors affecting rates
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2NOCl(g) 2NO(g) + Cl2(g)
rate of reaction = k[N2O5]
where k is the rate constant.
aA + bB xX
the rate equation is
rate = k[A]m[B]n
where m and n are experimentally determined exponents NOT necessarily the stoichiometric coefficients (usually positive whole numbers)
2NO(g) + Cl2(g) 2NOCl(g)
Rate = k[NO]2[Cl2]
This reaction is second order in NO, first order in Cl2, and third order overall.
Compare experimental evidence to figure out the order respective to reactants.
2NO(g) + O2(g) 2NO2(g)
was measured at 25oC for various initial concentrations of NO and O2. Data are collected in the table on p. 683. Determine the rate equation from these data. What is the value of the rate constant and what are the appropriate units of k?
Pt(NH3)2Cl2(aq) + H2O(l) [Pt(NH3)2(H2O)Cl]+(aq) + Cl-(aq)
and the rate equation is
rate = k[Pt(NH3)2Cl2]
Calculate the rate of reaction when the concentration of Pt(NH3)2Cl2 is 0.020M. What is the rate of change in the concentration of Cl- under those conditions?
ln([R]t/[R]o) = -kt
[R]o and [R]t are concentrations of reactant at time t = 0 and at a later time, t. The ratio of concentrations is the fraction of reactant that remains after a given time has elapsed.
1/[R]t – 1/[R]o = kt
same symbolism applies and k has units of L/mol∙time.
[R]0 – [R]t = kt
where the units of k are mol/(L∙time).
[R]t = -kt + [R]o
ln[R]t = -kt + ln[R]o
1/[R]t = kt + 1/[R]o
t1/2 = 0.693/k
half life is independent of concentration!
t1/2 = [R]o/2k
t1/2 = 1/(k[R]o)
0.0016 year -1. In contrast, radioactive iodine-125, which is used for studies of thyroid functioning, has a rate constant for decay of 0.011 day-1.
a. What are the half-lives of these isotopes?
b. Which element decays faster?
c. If you begin a treatment with iodine-125, and have 1.6e15 atoms, how many remain after 2.0 days?
k = Ae-Ea/RT
ln k2/k1 = - Ea/R[1/T2 – 1/T1]
The rate constant k = 4.5 x 103 s-1 at 274K and 1.00 x 104 s-1 at 283K. What is the energy of activation?
2NO(g) + 2H2(g) N2(g) + 2H2O(g)
and one possible mechanism is
N2O2(g) + H2(g) N2O(g) + H2O(g)
N2O(g) + H2(g) N2(g) + H2O(g)
What is the molecularity of each of the three steps? What is the rate equation for the third step? Show that the sum of these elementary steps gives the net equation.