Chap15_Zumdhal-1 - CHEMICAL KINETICS Chemical kinetics is...

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CHEMICAL KINETICS Chemical kinetics is the study of the rates of chemical reactions and the mechanisms by which they occur. THE RATE OF A REACTION The rate, at which a reaction proceeds, can be described in terms of the rate at which one of the reactants disappears, or the rate at which one of the products appears. Because rates may change as time passes, average rates of reactions are usually expressed as change in concentration over change in time and are in units of mols/L.s. Rate = concentration change/time change aA + bB cC + dD Rate = The reaction rate must be positive as it describes the forward reaction in which A and B are consumed to produce C and D. The rate of a chemical reaction changes with time. Therefore, the rate is usually expressed as the average rate during a given time interval. We can however express the instantaneous rate or the rate of a reaction at a particular time. The instantaneous rate is the slope of the tangent to the plot of concentration against time at Example In the reaction below, suppose that NH 3 is disappearing at a rate of 0.0816 M.s -1 . What are the rate of disappearance of O 2 , the rate of appearance of H 2 O, and the rate of the reaction? 4NH 3 (g) + 5O 2 (g) 4NO(g) + 6H 2 O(g) 1
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FACTORS THAT AFFECT REACTION RATES The rates of reactions depend upon four factors: the nature of the reactants, the concentration of the reactants, the temperature, and the use of a catalyst. NATURE OF THE REACTANTS This is a broad category that includes many properties of the reactants: For reactions involving solids, rate increases as particle size decreases and surface area increases (piece of chalk vs. powdered chalk reacting with HCl). The reaction between ions is more rapid than that between the corresponding neutral species (potassium sulfate and barium nitrate in aqueous solution vs. as solids). The chemical identity of elements and compounds affects the rate at which they react (Na in water vs. Ca vs. Mg) The physical state of reactants affects their rate of reactions (combustion of liquid gasoline vs. vapor gasoline). CONCENTRATIONS OF REACTANTS: THE RATE LAW EXPRESSION At constant temperature, as the concentrations change, the rate of a reaction changes. It is often useful to describe how the rate of a reaction depends on concentrations. This dependence of the rate on concentration is expressed in the rate law expression or simply rate law. The rate law is experimentally determined for each reaction: Rate = k[A] x [B] y x = order of the reaction with respect to A y = order of the reaction with respect to B x + y = overall order of a reaction k = the specific rate constant. Its units vary depending on the order of the reaction. It is unique for each reaction and changes with temperature. Its value changes depending on whether the reaction is catalyzed or not. Note that the order of the rate law expression, x and y, may or may not
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This note was uploaded on 05/06/2009 for the course CH 302 taught by Professor Holcombe during the Spring '07 term at University of Texas.

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Chap15_Zumdhal-1 - CHEMICAL KINETICS Chemical kinetics is...

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