Rates of Chemical Reactions Activity

Rates of Chemical Reactions Activity - Rates of Chemical...

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Unformatted text preview: Rates of Chemical Reactions Why? Chemical kinetics is the part of chemistry that looks at the speed at which reactants are converted into products. Knowledge of the reaction mechanism and the factors that affect the rate of reaction make it possible to control reaction conditions to maximize the efficiency of industrial processes. Understanding the kinetics of metabolism also allows drug developers to determine optimal dosage amount and timing. Learning Objectives • Calculate the instantaneous rate by expressing rate in terms of changes in concentration over time. • Explain how the concentration of reactions affects the rate • Determine an expression for the rate law based on initial rates of reaction at different concentrations. Information When a chemical reaction occurs, reactants are consumed and products are produced. The rate of a chemical reaction depends on how quickly reactants are consumed or, alternately, how quickly products are formed. By convention, rates of changes in concentration are always reported as positive numbers. (Note: square bracket notation is often used to indicate concentration in molarity) ∆ !"#$%#&% !"#$ = − ∆!"#$ or ∆ !"#$%&'( !"#$ =  ∆!"#$ A better measure of the rate is the instantaneous rate of reaction, generally written as ! !"#$%#&% !"#$ = − !" In order to have a general rate of reaction that does not depend on which species in the reaction we are measuring, you have to take into account the stoichiometry of the reaction. ∆ !"#$%#&% !"#$!"!"#$%&'( =  − !"#$%!!"#$%&!'!"#$$%!%#&' ×∆!"#$ The value of the instantaneous rate of reaction (for reactants with a stoichiometric coefficient of one in the balanced chemical equation) can be obtained by plotting the concentration of the reactant versus time, drawing a tangent line to the curve, and determining the slope of the line. 256 ChemActivity K1 Introduction to Chemical Kinetics Consider This … Figure 1. Nitrite concentration ⇄ ! ! + 2! !(ℓ) the reaction of ! !"! !" + !"! !" versus time for ! ammonium ion !with nitrite ion ! ! ! !"! ! = – .0050! 0 !"! !+= 0.100! (NO2)o = 0.00500 M (NH4 )o = 0.100 M 0.005 at t = 0 s: – d(NO–) 2 dt = 1.35 × 10 –7 M/s nitrite concentration (M) 0.004 0.003 at t = 75,000 s: d(NO– ) 2 –8 – = 1.80 × 10 M/s dt 0.002 0.001 0.000 0 50000 100000 time (s) 150000 and J.J. Farrell, Chemistry: A Guided Inquiry, 2nd Ed., John Wiley & Sons, R.S. Moog Key Q 57. This Inc., 2002, CA uestionsmaterial is used by permission of John Wiley & Sons, Inc. 1. What is the rate of reaction at t = 0 sec? Critical Thinking Questions 10. What is the ratetof reaction reaction at t = 75,000 sec? 2. What is he rate of at t = 0 s? 1.35 × 10–7 M/s 3. How does the rate of reaction change as the concentration of nitrite decreases? 11. What is the rate of reaction at t = 75,000 s? 1.80 ×E10–8 M/sthe value of the rate of reaction at t = 175,000 sec. Explain your 4. stimate reasoning. 12. How does the rate of reaction change as (NO–) decreases? 2 The rate of reaction gets smaller as (NO2–) decreases. 13. Estimate the value of the rate of reaction at t = 175,000 s. Explain your reasoning. The rate of reaction approaches zero at 175,000 s. This makes sense because (NO2–) becomes constant as it approaches zero. ChemActivity K1 Introduction to Chemical Kinetics 257 CModel This The Effect of Concentration on Reaction Rate. onsider 2: … ! !+ → !"! !" + !"NH4 (aq) !! NO–+ 2!! !(ℓ)N2(g) + 2 H2O(l) ! !" ⇄ + ! 2(aq) ← Table Initial reaction rates for several experiments Table 2. 1: Initial reaction rates for several experiments at 25°C. at 25° C . Experiment Initial Initial Initial Rate Concentration Concentration of Reaction + (M /s) of NH4 (M) of NO– (M) 2 1 0.100 0.0050 1.35 × 10–7 2 0.100 0.010 2.70 × 10–7 3 0.200 0.010 5.40 × 10–7 Information Key Questions 5. For the three experiments in Table 1: OftenWhich experiment has constant temperature) is found to be proportional to the a. the rate of reaction (at the fastest initial rate of reaction? concentration of a reactant raised to some power (usually an integer such as 0, 1, 2, ...). For example, if b. Which experiment has tthe slowest initial rate of reaction? rate = k (R)x hen, x k ( R) 2 initial rate2 (R) x = think the initial rates of reaction are different in the three = 2 c.initial rate1you k (R)x Why do (R)1 experiments? 1 where k is called the rate constant and k = f(T) initial ratej = the initial rate of experiment j (R)j = the initial concentration of the reactant R for experiment j The r ate law for a particular reaction is an empirically determined expression describing how xperiments 1 and depends 6. Comparing ethe rate of reactions 2 only: upon the concentrations of reactants and products (and possibly catalysts). Althoughf this ! tnot same? the case, often the rate law is a. Are the initial concentrations o !"! he always for a reaction such as equation (2) has the form γ α β δ (9) What is the rate =of the (B) (C) (D) ratio k(A) concentrations expressed as a fraction? ! ! where k is !"! !"# ! / !"! !"# ! a constant (at a given temperature) known as the rate constant, and the exponents are the order of the reaction with respect to each of the components. For example, if the exponent for component i is 1, then the reaction is said to be first-order in i. ! b. Are the initial concentrations of !"! the same? What is the ratio of the concentrations expressed as a ! ! fraction? !"! !"# ! / !"! !"# ! c. Are the initial rates of reaction the same? What is the ratio of the rates of reaction expressed as a fraction? initial rate2 / initial rate1 d. Based on the answers to parts a ­c above, can you determine whether the initial rate of reaction depends on the initial concentration of ammonium ions? Why or why not? e. Based on the answers to parts a ­c above, can you determine whether the initial rate of reaction depends on the initial concentration of nitrite ions? Why or why not? f. Based on your analysis, does the rate of reaction appear to be proportional to the concentration of any species raised to a power? Explain your reasoning. Consider This … Often the rate of reaction is found to be proportional to the concentration of reactant raised to some power, usually an integer (0, 1, 2, …). For example, if rate of reaction = k [R]x then, !"!#!$% !"#$!"# ! !"!#!$% !"#$!"# ! !!! = ! ! !"# ! = ! !"# ! ! !"# ! ! !"# ! ! The relationship between the rate of reaction and the concentration of reactants is known as the rate law. An example of a typical rate law is ! ! rate of reaction = ! !"! ! !"! ! where k is the proportionality constant known as the rate constant, and x and y are the orders of reaction for the ammonium ion and nitrite ion, respectively. The rate constant is characteristic of a particular reaction at a given temperature. These exponents are called the order of the reaction with respect to each species. The overall order of the reaction is the sum of the powers. For example, if x = 3, then we say the reaction is third order with respect to the ammonium ion. The rate constant and the exponents (or orders) can be determined by EXPERIMENT only! Key Questions 7. Based on your answers to CTQ 6, what is the order of the reaction with respect to the ammonium ion? 8. Based on the information in Table 1 (particularly the information for experiments 2 and 3), what is the order of the reaction with respect to nitrite ion? 9. Based on your answers to questions 7 and 8 and the general form of the rate law … a. What is the value of the rate constant for Experiment 1? b. What is the value of the rate constant for Experiment 2? c. What is the value of the rate constant for Experiment 3? d. Compare the three values. Explain why the relative values are reasonable. 10. Outline a general procedure for determining the rate law expression for a reaction. ...
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This note was uploaded on 10/11/2011 for the course CHEM 012 taught by Professor Mounaamaalouf,reneescole, during the Spring '11 term at Iowa State.

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