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Chapter 15 Study Guide

Chapter 15 Study Guide - Chapter 15 Study Guide Spontaneous...

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Chapter 15 Study Guide Spontaneous does not mean fast . Chemical kinetics: the area of chemistry that concerns reaction rates The definition of reaction rate assumes that the volume remains constant. A more general definition of reaction rate is the change in the number of moles per unit of volume per unit of time. Reaction rate: the change in concentration of a reactant or product per unit time Rate = concentration of A at time t 2 - concentration of A at time t 1 = ∆[A] t 2 t 1 ∆t Difference in the rates of the forward and reverse reactions . The reaction rate depends only on the concentrations of the reactants. Rate law: an expression that shows how the rate of reaction depends on the concentration of reactants Rate constant k : the proportionality constant in the relationship between reaction rate and reactant concentrations Order n : the positive or negative exponent, determined by experiment, of the reactant concentration in a rate law 1. The concentrations of the products do not appear in the rate law because the reaction rate is being studied under conditions where the reverse reaction does not contribute to the overall rate. 2. The value of the exponent n must be determined by experiment; it cannot be written from the balanced equation. Differential rate law: an expression that gives the rate of a reaction as a function of concentrations; often called the rate law Rate law: rate that depends on concentration Integrated rate law: concentration depends on time Rate Laws 1) There are two types of rate laws: - The differential rate law (often called simply the rate law) shows how the rate of a reaction depends on concentrations. - The integrated rate law shows how the concentrations of species in the reaction depend on time. 2) Because we will typically consider reactions under conditions where the reverse reaction is unimportant, our rate laws will involve concentrations of reactants. 3) Because the differential and integrated rate laws for given reaction are related in a well-defined

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way, the experimental determination of either of the rate laws is sufficient. 4) Experimental convenience usually dictates which type of rate law is determined experimentally. 5) Knowing the rate law for a reaction is important mainly because we can usually infer the individual steps involved in the reaction from the specific form of the rate law. First Order: Rate = k [A]. Doubling the concentrations of A doubles the reaction rate. The value of the initial rate is determined for each experiment at the same value of t as close to t = 0 as possible. Initial rate: Instantaneous rate before the initial concentrations of reactants have changed significantly Overall reaction order is the sum of the orders for each reactant. Overall reaction order: the sum of the orders for each reactant In the field of kinetics the rate of this type of reaction is usually defined as Rate = - 1 d [A] a dt where a is the coefficient of A in the balanced equation. However, to avoid complications, we will leave out the factor of 1/ a
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