Factors That Affect Reaction Rates Presence of a Catalyst Surface - PDF document

kinetics­presentation­Yoos.notebook October 15, 2012 Kinetics Chemical Kinetics • In kinetics we study 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). Factors That Affect Reaction Rates Factors That Affect Reaction Rates • The Physical State of the Reactants • Physical State of the Reactants • The Concentrations of the Reactants • In order to react, molecules must come in contact with each other. • Temperature • The more homogeneous the mixture of • The Presence of a Catalyst reactants, the faster the molecules can react. • Surface area Factors That Affect Reaction Rates Factors That Affect Reaction Rates The Concentrations of the Reactants Temperature As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. At higher temperatures, reactant molecules • have more kinetic energy Air hole • move faster • collide more often and with greater energy. Why do we keep food in fridge? The flame from the Bunsen burner shows the variation in color as more air is mixed with the natural gas. As more air is mixed with the natural gas, the rate of combustion reaction will increase and the flame gets more blue in color. 1

kinetics­presentation­Yoos.notebook October 15, 2012 Factors That Affect Reaction Rates Factors That Affect Reaction Rates Presence of a Catalyst Surface Area • Catalysts speed up reactions by changing the • When more surface area of a reactant is mechanism of the reaction. exposed, a reaction will occur faster. • Catalysts are not consumed during the course • A steel bar will not ignite of the reaction. if exposed to a flame. In contrast, a steel wool pad will burn and ignite quickly. 1 Which of the following 2 Food spoils at a faster rate at 25 degrees F does not play a part in compared to 5 degrees C. determining the rate of a reaction? Yes A Temperature No B Concentration of the reactants C Catalyst D Surface area of the solid E Equilibrium constant 3 Why does a higher 4 Why does a higher reactant temperature cause a concentration make a reaction to go faster? reaction faster? Only because there are more Only because there are more A A collisions per second. collisions per second. Only because collisions occur with Only because collisions occur with B B greater energy. greater energy. There are more frequent collisions There are more frequent collisions C C and they are of greater energy. and they are of greater energy. 2

kinetics­presentation­Yoos.notebook October 15, 2012 5 Why does a greater surface 6 What happens to a catalyst area cause a reaction to in a reaction? proceed faster? Only because there are more A It is unchanged. A collisions per second. B It is incorporated into the products. Only because collisions occur with B greater energy. C It is incorporated into the reactants. There are more frequent collisions D It evaporates away. C and they are of greater energy. Reaction Rates 7 If a catalyst is used in a reaction_________. 0 s 20 s 40 s A the energy of activation increases. B different reaction products are obtained. C the reaction rate increases. A D it evaporates away. B 1 mol A 0.5 mol A 0.25 mol A 0.75 mol B 0.5 mol B Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time. Reaction Rates Reaction Rates C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Time , t(s) [C 4 H 9 Cl](M) 0.0 0.1000 50 0.0905 C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) 00.0 0.0820 50.0 0.0741 Time , t(s) [C 4 H 9 Cl](M) Average Rate (M/s) 200.0 0.0671 300.0 0.0549 400.0 0.0448 0.0 0.1000 500.0 0.0368 50 0.0905 1.9 x10 ­4 800.0 0.0200 00.0 0.0820 1.7 x10 ­4 10000 0 50. 0 0.0741 1.6 x10 ­4 In this reaction, the concentration of butyl chloride, 200. 0 0.0671 1.4 x10 ­4 C 4 H 9 Cl, was measured at various times. 300.0 0.0549 1.22 x 10 ­4 400.0 0.0448 1.01 x10 ­4 500.0 0.0368 0.88 x10 ­4 800.0 0.0200 0.560 x10 ­4 10000 0 [C 4 H 9 Cl] Average Rate = t The average rate of the reaction over each interval is the change in concentration divided by the change in time: 3

kinetics­presentation­Yoos.notebook October 15, 2012 Reaction Rates Reaction Rates C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Time , t(s) [C 4 H 9 Cl](M) Average Rate (M/s) 0.0 0.1000 • A plot of [C 4 H 9 Cl] vs. 50 0.0905 1.9 x10 ­4 Instantaneous time for this reaction 00.0 0.0820 1.7 x10 ­4 rate at t=0s 50. 0 0.0741 1.6 x10 ­4 yields a curve like this. 200. 0 0.0671 1.4 x10 ­4 • The slope of a line [C 4 H 9 Cl] (M) x 10 ­2 300.0 0.0549 1.22 x 10 ­4 tangent to the curve at 400.0 0.0448 1.01 x10 ­4 500.0 0.0368 0.88 x10 ­4 any point is the Instantaneous 800.0 0.0200 0.560 x10 ­4 instantaneous rate at rate at t=600s 10000 0 that time. • Note that the average rate decreases as the reaction proceeds. WHY? Time (s) Reaction Rates and Stoichiometry Reaction Rates C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) • In this reaction, the ratio of C 4 H 9 Cl to C 4 H 9 OH is 1:1. Instantaneous Instantaneous • Thus, the rate of • All reactions slow rate at t=0s rate at t=0s disappearance of C 4 H 9 Cl down over time. [C 4 H 9 Cl] (M) x 10 ­2 [C 4 H 9 Cl] (M) x 10 ­2 is the same as the rate of • Therefore, the best appearance of C 4 H 9 OH. indicator of the rate of Instantaneous a reaction is the rate at t=600s Instantaneous instantaneous rate rate at t=600s near the beginning of the reaction. [C 4 H 9 Cl] [C 4 H 9 OH] ­ Rate = = Time (s) t t Time (s) 8 Which substance in the reaction below either 9 Which of the following expressions could appears or disappears the slowest? represent a reaction rate? 4NH 3 + 7O 2 4NO 2 + 6H 2 O A time/mass concentration/time B A NH 3 energy/time C O 2 B D time/energy H 2 O C NO 2 D The rates of appearance are the same as the rates of E disappearance 4

kinetics­presentation­Yoos.notebook October 15, 2012 11 A flask is charged with 0.124mol of A and allowed 10 Which of the following set of units represents a to react to form B according to the reaction reaction rate? A(g) B(g). the following data are obtained for [A] as the reaction proceeds. The average rate of g/s A disappearance of A between 10 s and 20 s is B mol/s __________ mol/s. s C D j/s s/j E A 2.2 10 ­3 B 1.1 10 ­3 C 4.4 10 ­4 D 454 E 9.9 10 ­3 13 2­ , reacts with the The peroxydisulfate ion, S 2 O 8 12 A flask is charged with 0.124 mol of A and allowed iodide ion in aqueous solution via the reaction: to react to form B according to the reaction 2­ (aq) + 3I ­ ­­­> 2SO 4 (aq) + I 3 ­ (aq) S 2 O 8 A(g) →B(g). The following data are obtained for [A] as the reaction proceeds: The following data are collected for the concentration of iodide ion: The average rate of disappearance of between 400.0 s and 800.0 s is __________ M/s. A 2.8 x 10 ­5 A B 1.4 x 10 ­5 B 590 5.8 x 10 ­5 C C 3.6 x 10 ­4 D D 2.6 x 10 ­4 E E Reaction Rates and Stoichiometry Reaction Rates and Stoichiometry • To generalize, then, for the reaction • What if the ratio is not 1:1? 2HI (g) H 2 (g) + I 2 (g) aA + bB cC + dD [HI] [I 2 ] Rate = ­ 1 = [C] [A] [B] [D] ­1 ­1 ­1 ­1 2 t t Rate = = = = a c t t d t b t For every 1 mole of I 2 that is produced, 2 moles of HI must decompose. So the rate of disappearance of HI is twice as fast as the appearance of I 2 or the appearance of H 2 . 5