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Chemical Kinetics Chemistry, The Central Science , 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com and Bruce E. Bursten Chapter 14 Chemical Kinetics Chemical Kinetics 1 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Kinetics • Chemical Kinetics is the study of the rate at which a chemical process occurs. • Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs). Chemical Kinetics 2 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Factors That Affect Reaction Rates • Physical State of the Reactants In order to react, molecules must come in contact with each other. If the reaction is happening between a solid and a liquid it will react only on the surface. The more homogeneous the mixture of reactants, the faster the molecules can react. Chemical Kinetics 3 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Factors That Affect Reaction Rates • Concentration of Reactants As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. Chemical Kinetics 4 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Factors That Affect Reaction Rates • Temperature At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. Chemical Kinetics 5 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Factors That Affect Reaction Rates • Presence of a Catalyst Catalysts speed up reactions by changing the mechanism of the reaction. Catalysts are not consumed during the course of the reaction. Chemical Kinetics 6 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time. Chemical Kinetics 7 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates → C4H9Cl (aq ) + H 2O(l) C4H9OH (aq ) + HCl (aq ) In this reaction, the concentration of butyl chloride, C4H9Cl, was measured at various times. Chemical Kinetics 8 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com The average rate of the reaction over each interval is the change in concentration divided by the change in time: [X] -[ X ] ∆[X] Average rate = t2 t1 = • ∆ t2 -t 1 t ∆[C H Cl] Average rate = 4 9 ∆t Chemical Kinetics 9 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates → C4H9Cl (aq ) + H 2O(l) C4H9OH (aq ) + HCl (aq ) [0.0905] -[ 0.1000 ] 0.0095 Average rate = = -= − 1.9 x 10 -4 50-0 50 Chemical Kinetics 10 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates → C4H9Cl (aq ) + H 2O(l) C4H9OH (aq ) + HCl (aq ) • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, there are fewer collisions between reactant molecules. Chemical Kinetics 11 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com • calculate the average rate of appearance of B over the time interval from 0 to 40 s. Chemical Kinetics 12 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates → C4H9Cl (aq ) + H 2O(l) C4H9OH (aq ) + HCl (aq ) • A plot of concentration vs. time for this reaction yields a curve like this. • The slope of a line tangent to the curve at any point is the instantaneous rate at that time. Chemical Kinetics 13 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates → C4H9Cl (aq ) + H 2O(l) C4H9OH (aq ) + HCl (aq ) • All reactions slow down over time. • Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning. Chemical Kinetics 14 Average vs. instantaneous rxn PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com rate • Average rxn rate change of chemical concentrat ion between beginning of rxn and end ∆[A] rate of reaction (rate ) = = total time elapsed ∆t • Instantaneous rxn rate (tangent to curve) change of chemical concentrat ion over a short period of time ∆[A] rate of reaction (rate ) = = the short time elapsed ∆t Usually in chemistry, we are interested in the instantaneous rxn rate at the very beginning (at t = 0) Chemical Kinetics INITIAL REACTION RATE 15 Average vs. instantaneous rxn PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com rate NaHCO3 + citric acid --> sodium citrate + H2O + CO2 initial [A] = 1 tablet 2 initial [A] = 2 tablets Instantaneous 1.8 rate at t = 0 1.6 (initial rate) Average rate 1.4 1.2 1 0.8 0.6 concentration (tablet/100mL) concentration 0.4 0.2 0 0 0.5 1 1.5 2 Chemical time (minutes) Kinetics 16 Comparing concentrations PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com of chemicals in a reaction A --> B A --> 2 C [A] 2 2 [B] 1.8 [A] 1.8 [C] 1.6 1.6 1.4 1.4 1.2 1.2 1 1 0.8 0.8 0.6 0.6 concentration (mol/L) concentration0.4 (mol/L) 0.4 0.2 0.2 0 0 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Chemical time (hours) Kinetics time (hours) 17 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com What we’re after in kinetics • We want to be able to understand and predict how much reactants remain (or products get formed) in a certain amount of time • Change with time is rate • Rate means calculus must be used, so we need to be careful which rate we are talking about • We will use instantaneous rate at the start of a reaction (initial rate) • Several factors affect the rate of a reaction Initial concentration Temperature Amount of reactant exposed (surface area) Presence of a catalyst • A theory (equations) that predicts kinetics must explain these Chemical Kinetics 18 Calculate the instantaneous rate of disappearance of C 4H9Cl at t = 0 (the PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com initial rate). Chemical Kinetics 19 Determine the instantaneous rate of disappearance of C4H9Cl at t PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com = 300 s. Chemical × –4 Kinetics Answer: 1.1 10 M/s 20 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates and Stoichiometry → C4H9Cl (aq ) + H 2O(l) C4H9OH (aq ) + HCl (aq ) • In this reaction, the ratio of C 4H9Cl to C 4H9OH is 1:1. • Thus, the rate of disappearance of C4H9Cl is the same as the rate of appearance of C 4H9OH. -∆[C H Cl] ∆[C H OH] Rate = 4 9 = 4 9 ∆t ∆t Chemical Kinetics 21 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates and Stoichiometry • What if the ratio is not 1:1? → 2 HI (g) H2(g) + I 2(g) in such a case , 1 ∆[HI ] ∆[H] ∆ [ I ] Rate = - = 2= 2 2t∆ ∆ t ∆ t Chemical Kinetics 22 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Reaction Rates and Stoichiometry • To generalize, then, for the reaction aA + bB cC + dD 1 ∆[A] 1 ∆[B] 1 ∆[C] 1 ∆[D] Rate = = = = − a ∆t − b ∆t c ∆t d ∆t Chemical Kinetics 23 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Key points about kinetics so far • Study the vocabulary • Reaction rate has weird units • Be clear whether you are talking about average reaction rate or instantaneous reaction rate • The equations which we will study all talk about instantaneous reaction rate, most easily measured at the beginning of a reaction (initial rate) • Relating rates at which products appear (+) and reactants disappear (-) has to do with inverse stoichiometric coefficients • Reaction rate depends on several factors: initial concentration, temperature, surface area of reactants exposed, presence of a catalyst Chemical Kinetics 24 (a) How is the rate at which ozone disappears related to PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com the rate at which oxygen appears in the reaction (b) If the rate at which O 2 appears, Δ[O 2]/Δ t, is 6.0 × 10 –5 M/s at a particular instant, at what rate is O 3 disappearing at this same time, –Δ[O 3] / Δ t? Chemical Kinetics 25 SAMPLE EXERCISE 14.3 continued PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com The decomposition of N 2O5 proceeds according to the following equation: × –7 If the rate of decomposition of N 2O5 at a particular instant in a reaction vessel is 4.2 10 M/s, what is the rate of appearance of (a) NO 2, (b) O 2? Answers: (a) 8.4 × 10 –7 M/s, (b) 2.1 × 10 –7 M/s Chemical Kinetics 26 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Concentration and Rate One can gain information about the rate of a reaction by seeing how the rate changes with changes in concentration. Chemical Kinetics 27 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Concentration and Rate + − NH 4 (aq ) + NO 2 (aq ) N2(g) + 2 H 2O(l) + Comparing Experiments 1 and 2, when [NH 4 ] doubles, the initial rate doubles. Chemical Kinetics 28 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Concentration and Rate + − NH 4 (aq ) + NO 2 (aq ) N2(g) + 2 H 2O(l) Likewise, comparing Experiments 5 and 6, − when [NO 2 ] doubles, the initial rate doubles. Chemical Kinetics 29 PDF Created with deskPDF PDFWriter -Trial::http://www.docudesk.com Concentration and Rate • This means ∝ + Rate [NH 4 ] ∝ − Rate [NO 2 ] ∝ + − Rate [NH ] [NO 2 ] or + − Rate = k [NH 4 ] [NO 2 ] • This equation is called the rate law , and k is the rate constant .
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