Environmental Chemistry

3. Chemical Kinetics The study of the rates and mechanisms of chemical reaction. Rate of reaction - the amount of chemical change that takes place in a given interval of time.

The Rate of Chemical Reaction The rate at which reactants are consumed or products are produced in a chemical reaction Will a reaction occur? Collision Theory for a reaction to occur: -reactant particles must collide -collision must have a certain minimum amount of energy: Activation Energy -reactants may require a specific orientation

Potential Energy Diagrams Activation Energy, Ea Avg. Energy of Products, PEP E E energy absorbed during the reaction Avg. Energy of Reactants, PER Endothermic Reaction reaction progress 4

Rate Law For General Reaction: aA + bB  cC + dD The rate law generally has a form Rate = k[reactant 1]m[reactant 2]n For the reaction above, Rate = k[A]a[B]b where k in the rate law is called the rate constant

Order of Reaction The sum of all the exponents of the concentration terms in the rate equation

Sample Problem 12 How long will it take the carbon monoxide (CO) concentration in room to decrease by 99 percent after the source of carbon monoxide is removed and the windows are opened? Assume the first order rate constant for removal (due to dilution by incoming clean air) is 1.2/hr. No chemical reaction occurring.

Half-Life (t½) It is defined as the time required for the concentration of a chemical to decrease by one-half (for example, [C] = 0.5[C]0).

Sample Problem 13 Subsurface half-lives for benzene, TCE, and toluene are listed as 69, 231, and 12 days, respectively. What are the first-order rate constant for all three chemicals.

Sample Problem 14 After a Chernobyl nuclear accident, the concentration of 137Cs in milk was proportional to the concentration of 137Cs in the grass that cows consumed. The concentration in the grass was, in turn, proportional to the concentration in the soil. Assume that the only reaction by which 137Cs was lost by soil was through radioactive decay and the half-life for this isotope is 30 years. Calculate the concentration in milk shortly after the accident was 12,000 bequerels (Bq) per liter. (Note: A bequerel is a measure of radioactivity; 1 bequerel equals 1 radioactive disintegration per second.)

Effect of Temperature on Rate Constants Arrhenius equation k = Ae –(Ea/RT) Where: A – preexponential factor (same as k) Ea – activation energy (kcal/mole) R - gas constant T - temperature (K)

Sample Problem 15 The rate constant for carbonaceous biochemical oxygen demand (CBOD) at 20oC is 0.1/day. What is the rate constant at 30oC? Assume Ea = 1.072.

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