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14 Chemical Kinetics - 14 Chemical Kinetics 14.1 Reaction...

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© David Hanson, Stony Brook University, 2008 Reproduction or distribution by any means is prohibited by law. 1 14. Chemical Kinetics 14.1. Reaction Rate The average rate of a chemical reaction is defined as the change in the concentration of a reactant or product over some period of time multiplied by the reciprocal of the stoichiometric coefficient in the balanced reaction equation. For a reactant, the concentration is decreasing so the change is multiplied by -1 to make the rate a positive quantity. For example, for the generic reaction aA + bB cC +dD (14.1.1) any of the following equalities gives the average rate of the reaction at time t. 1 [ ] 1 [ ] 1 [ ] 1 [ ] Rate t t t t A B C D a t b t c t d t     (14.1.2) where [A] t gives the change in concentration of A during the time interval t, which begins at time t. [ ] [ ] [ ] t t t t A A A t t  (14.1.3) The instantaneous rate of a reaction is the value obtained using Eq. 14.1.2 as the time interval t approaches 0. The instantaneous rate can be found from the slope of the tangent line at any point on a graph of concentration vs time. The instantaneous rate also can be estimated by selecting the time interval t to be as small as possible. If the reaction occurs in a homogeneous medium such as a gas or a solution, the rate depends on four factors: the properties of the reactants and products, the concentrations of the reactants and occasionally the products, the temperature, and the presence of a catalyst. If the reaction occurs in a heterogeneous medium such as on a surface or in a solid, the rate also depends on the area of the surface and the nature of the heterogeneous medium.
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© David Hanson, Stony Brook University, 2008 Reproduction or distribution by any means is prohibited by law. 2 14.2. Practice Questions 14.2.1. The following reaction produces ammonia, an important industrial chemical, from nitrogen and hydrogen. If nitrogen is consumed at the rate of 0.2 mol L -1 s -1 , what is the rate at which ammonia is produced? N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 14.2.2. Using the following data, calculate the average rate of the ammonia synthesis reaction three times: once using the nitrogen concentrations, once using the hydrogen concentrations, and once using the ammonia concentrations. Enter your results in the table, be sure to include the units. If you do the calculations correctly, you should obtain the same positive value for the reaction rate in all three cases. Time [N 2 ] M [H 2 ] M [NH 3 ] M 10 s 1.25 0.50 0.05 20 s 1.20 0.35 0.15 Calculated Rate 14.3. Rate Laws Rate laws for a chemical reaction show how the reaction rate depends on the concentrations of reactants. A rate law for a reaction can be identified experimentally, e.g. by the method of initial rates.
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