AP Chapter 12 - Kinetics The study of reaction rates....

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1 Page 1 Kinetics s The study of reaction rates. s Spontaneous reactions are reactions that will happen - but we can’t tell how fast. s Diamond will spontaneously turn to graphite – eventually. s Reaction mechanism- the steps by which a reaction takes place. Review- Collision Theory s Particles have to collide to react. s Have to hit hard enough s Things that increase this increase rate s High temp – faster reaction s High concentration – faster reaction s Small particles = greater surface area means faster reaction Reaction Rate s Rate = Conc. of A at t 2 -Conc. of A at t 1 t 2 - t 1 s Rate = [A] t s Change in concentration per unit time s For this reaction s N 2 + 3H 2 2NH 3 s As the reaction progresses the concentration H 2 goes down C o n c e n t r a t i o n Time [H 2 ] N 2 + 3H 2 2NH 3 s As the reaction progresses the concentration N 2 goes down 1/3 as fast C o n c e n t r a t i o n Time [H 2 ] [N 2 ] N 2 + 3H 2 2NH 3 s As the reaction progresses the concentration NH 3 goes up 2/3 times C o n c e n t r a t i o n Time [H 2 ] [N 2 ] [NH 3 ] N 2 + 3H 2 2NH 3
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2 Page 2 Calculating Rates s Average rates are taken over long intervals s Instantaneous rates are determined by finding the slope of a line tangent to the curve at any given point because the rate can change over time s Derivative. s Average slope method C o n c e n t r a t i o n Time [H 2 ] t s Instantaneous slope method. C o n c e n t r a t i o n Time [H 2 ] t d[H 2 ] dt Defining Rate s We can define rate in terms of the disappearance of the reactant or in terms of the rate of appearance of the product. s In our example N 2 + 3H 2 2NH 3 s [H 2 ] = 3 [N 2 ] t t s [NH 3 ] = -2 [N 2 ] t t Rate Laws s Reactions are reversible. s As products accumulate they can begin to turn back into reactants. s Early on the rate will depend on only the amount of reactants present. s We want to measure the reactants as soon as they are mixed. s This is called the Initial rate method. s Two key points s The concentration of the products do not appear in the rate law because this is an initial rate. s The order (exponent) must be determined experimentally, s can’t be obtained from the equation Rate Laws
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3 Page 3 s You will find that the rate will only depend on the concentration of the reactants. (Initially) s Rate = k [NO 2 ] n s This is called a rate law expression. s k is called the rate constant. s n is the order of the reactant -usually a positive integer. 2 NO 2 2 NO + O 2 s The rate of appearance of O 2 can be said to be. s Rate' = [O 2 ] = k'[NO 2 ] t s Because there are 2 NO 2 for each O 2 s Rate = 2 x Rate' s So k[NO 2 ] n = 2 x k'[NO 2 ] n s So k = 2 x k' 2 NO 2 2 NO + O 2 Types of Rate Laws s Differential Rate law - describes how rate depends on concentration. s
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AP Chapter 12 - Kinetics The study of reaction rates....

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