Environmental Environmental ChemistryChemistry

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

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Potential Energy Diagrams

reaction progress

EActivation Energy, Ea

Avg. Energy of Products, PEP

Avg. Energy of Reactants, PER

Endothermic Reaction

E energy absorbed during

the reaction

Rate LawFor General Reaction:

aA + bB cC + dD

The rate law generally has a formRate = 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

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Order of Reaction• The sum of all the exponents of the

concentration terms in the rate equation

Sample Problem 12How 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 ConstantsArrhenius equation

k = Ae –(Ea/RT)

Where:A – preexponential factor (same as

k)Ea – activation energy (kcal/mole)R - gas constantT - temperature (K)

Sample Problem 15The 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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