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## Chapter 14

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**Chapter 14**Rates of Reactions Kinetics**I**I. Introduction A) Demonstrations B) Chemical Kinetics is the study of the rates (speeds) of chemical reactions and the mechanisms of chemical reactions.**C) The rate of a chemical reaction is a measure of how fast**reactants are consumed and/or how fast products are made. D) The mechanism of a reaction is a detailed description of the way a reaction occurs. It is a sequence of elementary steps which lead from reactants to products.**Mechanisms can be proven wrong through _____________,**but they can never be called _________________________ since they are, in general, educated guesses.**Practical reasons for studying kinetics: Some reactions we**would like to speed up:drug delivery, paint drying, destruction of air pollutants in auto exhaust breakdown of materials in landfills. Some reactions we would like to slow down: food decay, rubber decay, human aging, destruction of the ozone layer, rusting, corrosion**E) Some reactions take place in a fraction of a second and**other take many years. What variables affect the reaction rate? 1) The characteristics of the reactants and the products.**2) The concentration of the reactants – in some reactions**the rate is unaffected by the concentration of one of the reactants as long as it is there in some amount.**3) The presence of a catalyst, a substance that …**4)The temperature at which the reaction occurs. Increasing the temperature usually increases the rate. A general rule is … 5) The surface area of a solid reactant or catalyst affects the rate.**G) Reaction rate is the increase in molar concentration of**product of a reaction per unit time or the decrease in molar concentration of a reactant per unit time.**For example, for the reaction:**2 N2O5 4 NO2 + O2 The following table shows the concentration of N2O5 as a function of time at 45 oC.**1) The rate of reaction can be written in the form:**2) Usually the rate is a rapidly changing quantity, as the reaction proceeds the reactants are used up and there remains less and less material to undergo reaction.**3)Generally we obtain for a reactant a curve which resembles**the one below.**If we take the first 2 points from the table above, we can**find as avg. rate of decomposition of N2O5.**You should be able to see that the avg. rate is decreasing,**hence the curve. Even though we obtain a negative value for the rate, since N2O5 is decreasing, rates are reported as positive values.**Look at the graph again,**And make the time interval smaller and smaller, we can obtain an instantaneous rate.**The instantaneous rate is equal to the slope of the line at**that point. Calculus??? To what is the slope of the line at that point equal?**4) How is data obtained for a concentration curve?**a) Monitor a color change. b) Measure pressure if a gas is produced. c) Monitor a change in pH if an acid or base reaction.**5) A look at the change in rate over time for another**reaction: 2 NO2(g) 2 NO(g) + O2(g) at 300oC**H) What does the balanced equation tell us about rates?**The equation we will look at is: H2(g) + I2(g) ----> 2 HI(g) H2 and I2 must disappear at the same rate since 1 molecule of H2 reacts with 1 molecule of I2.**In the same amount of time, 2 molecules of HI must**appear.The rate of appearance of HI must equal twice the rate of disappearance of H2 (g) and I2(g).**The rate of disappearance of H2(g) = the rate of**disappearance of I2(g) = ½ the rate of appearance of HI.**In general, for the equation:**aA + bB -----> cC + dD To obtain a rate equation of the rates of the substances in relation to each other we divide through by the coefficients.**But we usually want the first reagent in terms of all the**others, so we multiply by the coefficient of the first reagent.**II. The Rate Law (Rate expression), Rate Constant Order of**Reaction A) The following equation has been studied in the gaseous state and the data at 250 K may be summarized as follows:**F2(g) + 2 ClO2(g) ---> 2 ClO2F(g)**B) From this data the answers to the following questions can be obtained:**1) What is the rate law of the reaction?**2) What is the order of the reaction? 3) What is the value of the rate constant k? 4) What is the rate of formation of ClO2F when [F2]is 0.010 mol/L and[ClO2] is 0.020 mol/L?**C) What is a rate law?**1) In 1864 it was discovered that the rate of a reaction is proportional to some power of the concentration of reactants at constant temperature.**rate law = rate equation = rate expression**k is the specific rate constant which is independent of concentration.**k depends on the nature of the reactants; fast reactions**have large k's and slow reactions have small k's ordinarily k ________________ with temperature.**THE EXPONENTS x AND y MUST BE EXPERIMENTALLY DETERMINED.**They are not automatically obtained from the balanced equation.**Some experimentally determined rate laws for equations are**as follows: a) 2 N2O5(soln) 2 N2O4(soln) + O2(g)**Notice that the exponent is NOT 2, the coefficient in the**balanced equation, but has been experimentally determined to be 1. The reaction order with respect to a given species equals the exponent of the concentration of that species in the rate law as determined experimentally.**The order of the above reaction with respect to N2O5 is 1.**The order of a reaction is the sum of the exponents of the reactants in a rate law. The order of the above reaction, since 1 is the only exponent, is 1 as well. It is a first order reaction.**This means that when we double the concentration of N2O5, we**double the rate of reaction OR if we halve the concentration, we halve the rate. b) For the reaction: 2 NO + O2 2 NO2 What is the situation here?**D) The answers to the questions then are obtained in the**following manner. The rate law for F2(g) + 2 ClO2(g) ---> 2 ClO2F(g) will look like the following.**Your job is to find the values of x and y from the**experimental data.**You need to do two division problems to find x and y.**What is the order of the reaction with respect to F2? _______ What is the order of the reaction with respect to ClO2? ______ What is the overall order of the reaction? __________**To find k, the rate constant, we take the experimentally**determined rate law, put the data in from one of the experiments and solve for k. The units of k are important. k = _________________**To find the rate of formation of ClO2F when [F2] is 0.010 M**and the [ClO2] is 0.020 M, we need to look at the relationship between the rate of formation of [ClO2], and the rate of disappearance of F2. Rate = __________________________**IV. A graphical method is often used to show the order of a**reaction, or from a graph we can obtain the order of a reaction. • For the general reaction of A ---> products • If the reaction is first order, we can write**When we divide both sides by [A] and multiply both sides by**dt we obtain the following: Those of you who have had calculus should recognize this as:**Which can be changed to the following in log to the base 10.****The above equations can be rewritten in a more familiar**form.**THIS MEANS THAT IF WE PLOT ln [A] vs. t AND OBTAIN A**STRAIGHT LINE THE RATE IS FIRST ORDER. Or plot log [A] vs. t.