CH 302 1st Edition Lecture 19Outline of Last Lecture I Clicker Questions II. KineticsIII. Reaction RatesIV. Method of Initial ratesOutline of Current Lecture I. Integrated Rate LawsII. First OrderIII. Zero OrderIV. Second Order V. Pseudo First OrderCurrent LectureIntegrated Rate Laws:- For several special cases of rate laws, we can integrate the rate law to yield an equation of the concentration of a particular species as a function of time- In general, these ideas are most useful when the rate of the reactions depends only one one chemical species. - This could result from the mechanism of the reaction be true under all conditions for that reaction. - Alternatively, we can create situations in which we essentially remove the concentration dependence of one of the reactants by maintaining it at a constant value. - Finally, these ideas are assume the system is far from equilibrium and that any "backward" reaction (products to reactants) can be ignored.First Order:- By far the most important cases are first order reactions. - When a reaction is overall first order with respect to one of the reactants, then the rate of the reaction is simply proportional to the amount of that reactant.- Nuclear decay is an excellent example of a first order process. - The rate of decay is simply proportional to the amount of the radioactive isotope. - This is due to the fact that any isotope has the same chance of decaying at any given time. - Thus the more isotopes there are, the more that can decay (and as time goes on the fewer there are, the fewer that can decay).Zero Order:- If a reaction is zeroth order overall, it means that the rate of the reaction is independent of all of the concentrations of the reactant.- This means that the rate of this reaction will not change as the reaction proceeds. - Typically as a reaction progresses, the concentrations of the reactants decreases and the rate decreases. - However, for zeroth order reactions, this is not true, the rate is constant in time.Second Order:- A much more limited case, is that of second order reactions. - This is highly limited as more 2nd order reactions are the result of bimolecular steps occurring in a reaction. - This typically involves a collision between two different molecules. - This results in a reaction being 2nd order overall, but one that is first order with respect to two different reactants. - In the unusual case that a reaction is second order with respect to a single reactant and zeroth order with respect to all other reactants, we can again come up with an integrated rate law.Pseudo First Order:- A very important case is that of pseudo-first order kinetics. - This is when a reaction is 2nd order overall but is first order with respect to two reactants. This is a very common kinetic scheme.- rate=k[A][B] where A and B are some generic reactants. - Now the kinetics of this reaction can be a bit complicated. - The initial rate depends on both A and B and as the reaction proceeds both A and B are changing in concentration and affecting the rate.- When trying to understand this reaction, we can as experimentalists try to set up conditions that simplify things. - The easiest way to do this is to try to eliminate the concentration dependence of one of the reactants. - We can do this by making the initial concentrations of one of the reactants very very high compared to the
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