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1、5/26/2019,1,Chemical Kinetics,Concerns reaction rates: Speed with which reactants are converted to products,http:/ explode/give bright colors due to very fast chemical reactions The rusting of a bridge is also a chemical reaction, but it is very slow. The eroding of a mountain takes even longer,5/26
2、/2019,3,Rate: the speed at which something happens,Average rate or speed = quantity t Ratechem reaction = reactant or product = x mol unit time Ls Always positive = molar concentration Forward Rate: reactants products Reverse Rate: products recombine to become reactants Net Rate: forward rate - reve
3、rse rate Average Rate: speed of entire reaction from start to finish Instantaneous Rate: speed of reaction at one moment in time,5/26/2019,4,The Collision Theory,5/26/2019,5,Atoms, ions, and molecules (reacting substances) must collide in order to react:,5/26/2019,6,Activated complex: temporary, uns
4、table arrangement of atoms that may form products or may break apart to reform reactants,Reacting substances must collide with sufficient energy to form the activated complex,http:/ reactant molecules collide w/enough energy, orbitals overlap Transition state Reactants broken down into particles Not
5、 reactants or products As likely to form reactants as products Reactant molecules undergo further change Rearrangement of molecular structure Breaking of existing bonds Making new ones,5/26/2019,8,Enough kinetic energy for reactants to leap over reaction barrier?,5/26/2019,9,Exothermic reactions rel
6、ease energy and form products at lower energy level. Endothermic reactions absorb energy and form products at lower energy level.,H = (-),5/26/2019,10,Covers period of time between mixing of reactants and point at which chemical reaction stops or reaches equilibrium,How fast chemicals react and fact
7、ors that influence rate allows chemists to exercise precise control over chemical reactions,5/26/2019,11,Chemical Kinetics,Understand steps by which reaction takes place Called reaction mechanism,5/26/2019,12,Rate of chemical reaction dictates whether reaction can occur Knowing rate law (expression
8、relating rate to concentrations of reactants) helps chemist adjust reaction conditions to get more suitable rate If 2 reactions competing for single reagent, knowing rate law lets you favor exclusive formation of single product,5/26/2019,13,5/26/2019,14,5/26/2019,15,Measurement of reaction rate base
9、d on rate of appearance of product or disappearance of reactant,Determined by measuring concentration of one or more chemicals at different times during course of reaction Use kinetic curve (concentration vs. time curve) If reactant measured, decrease in w/time is recorded Slope of curve at point in
10、 time from kinetic curve determines rate of product production (or reactants disappearance),5/26/2019,16,Change can be Disappearance of reactants Decrease Rate has negative sign Appearance of product Increase Rate has positive sign Always define rate as positive quantity indicate concentration in mo
11、l/L,5/26/2019,17,A B Rateaverage = -A = +B t t RateA disappearing = RateB appearing Rate of appearance (or disappearance) of substance is divided by its stoichiometric coefficient aA + bB cC + dD Rate = -A = -B = +C = +D at bt ct dt Get same reaction rate no matter which is measured,5/26/2019,18,Rat
12、e is change in of some species w/respect to time, then measure of all species at multiple times to determine rate.,Initial H2/I2 equal at all times and initial product is zero,5/26/2019,19,Rate of formation of HI is twice rate of disappearance of H2 or I2 at any given time Rate slows in time due to
13、decreasing of reactants Stated mathematically, relationship between formation of products and disappearance of reactants for reaction is H2 + I2 2HI,5/26/2019,20,Instantaneous rate Rate not constant Decreases w/time ( change with time) Rate measured for given point in time Rates in tables: average r
14、ate over specific time period Determined by finding slope of line tangent to point representing particular time Rate = -(slope of tangent line) Slope of tangent line = change in y/change in x = dA /dt (derivative): rates measured over time intervals that approach 0 second,5/26/2019,21,Decomposition
15、of NO2 2NO2(g) 2NO(g) + O2(g),5/26/2019,22,2NO2(g) 2NO(g) + O2(g) Rate of consumption of NO2 = Rate of production of NO = 2(Rate of production of O2) because rate of production of NO is twice that of O2 NO2 = NO = (O2 ) 2t 2t t (multiply all by 2) NO2 = NO = 2(O2 ) t t t,5/26/2019,23,5/26/2019,24,ht
16、tp:/ affecting rate of reaction,5/26/2019,26,Nature of reactants,Some naturally occur faster than other Number of reacting species State of reacting species (particles forming solids move slower than gases/in solution) Complexity of reaction,5/26/2019,27,Change in concentration of reactants, reactan
17、ts increases rate For reaction to occur, particles must collide If higher, chance of collision greater Cases where this doesnt affect rate of reaction Where catalyst is already working as fast as it can Steps in certain multi-step reactions likely to have widely different rates (some fast/some slow)
18、,5/26/2019,28,Overall rate governed by how fast A splits up to make X and Y (rate determining step) Increasing A increases chances of step happening Increasing B speeds up 2nd step which is “waiting around” for 1st to happen, so hardly makes difference to overall rate,5/26/2019,29,Temperature,Partic
19、les only react when they collide Most particles dont have energy needed to react Increasing that # increases rate of collisions/probability of favorable collisions Increasing temperature always increases reaction rates Decreasing temperature always decreases reaction For many reactions occurring at
20、around room T, rate of reaction doubles for every 10C (9/11C rise in temperature # degrees needed to double rate changes gradually as T ,5/26/2019,30,Entropy & temperature,Place 3 drops of blue food coloring in 3 flasks. Place 3 drops of yellow food coloring in 3 more. Keep 1 of each at room tempera
21、ture, and warm 1, and cool the other. Using note cards, invert the blue flask over the yellow and remove the card. Secure the flask with a clamp. Allow the flasks to stand and record the time necessary for both flasks to become the same shade of green. Is the entropy greatest when the colors of the
22、flasks are different or the same? Explain. Explain the relationship between diffusion and entropy. What influence does an increase/decrease in temperature have on the diffusion observed?,5/26/2019,31,Particles present have very wide range of energies For gases, shown on Maxwell-Boltzmann Distributio
23、n Plot # particles having each particular energy Area under curve = total # particles present,5/26/2019,32,For reaction to occur, particles must collide with energies activation energy Many dont have enough energy to react when they collide Either change shape of curve, or move activation energy to
24、left,5/26/2019,33,Pressure,Increasing pressure increases rate of reaction Particles are closer together Have increased probability of favorable collisions,5/26/2019,34,Use a Catalyst,Substance which speeds up reaction Chemically unchanged at end of reaction Usually increases rate , but some can decr
25、ease it Provide alternative pathway w/lower activation energy,http:/ 17.1-.2, pp. 529-541, 17.4, pp. 546-547 Q pp. 554-555, #34, 37, 48-49, 62, 66, 71,Homework:,5/26/2019,37,Rate Laws,The rate of a chemical reaction dictates whether a reaction can occur,5/26/2019,38,Rate Laws: An introduction,Chemic
26、al reactions are reversible With time, enough products accumulate Reverse reaction becomes important Concentration of reactants depends on difference in rates of forward and reverse reactions Focus on reaction soon after reactants are mixed, before products build up to significant levels Reaction ra
27、te depends only on of reactants,5/26/2019,39,Rate = kAn Rate law depends on of reactants Must be determined by experiment Proportionality constant “k“-rate constant Relates and orders to rate of reaction Constant value at fixed T for given reaction Units of rate constant depend on order of reaction
28、“n“-order of reactant Can be integer (including zero) or fraction,5/26/2019,40,Important Rate law points,products do not appear Reaction rate studied under conditions where reverse reaction does not contribute to overall rate Value of exponent “n“ must be determined by experiment Cannot be written f
29、rom balanced equation Differential Rate Law (Rate law) - expresses how rate depends on concentration Integrated rate law - expresses how concentration depends on time,5/26/2019,41,Possible to write rate law that relates reaction rate to concentration of reactants Rate = kAxByCz x/y/z: reaction order
30、s (reactant with which it is associated) May be zero, positive, negative, integer, or fraction Must be determined by laboratory experiments No relationship to stoichiometric coefficients of balanced chemical equation,5/26/2019,42,If reaction is zero order with respect to reactant Rate does not depen
31、d on concentration of that reactant Doubling concentration will not increase or decrease rate,concentration-time graph,rate-concentration graph,5/26/2019,43,If first order with respect to reactant Rate directly proportional to concentration of that reactant Doubling concentration doubles rate,concen
32、tration-time graph,rate-concentration graph,5/26/2019,44,If second order with respect to a reactant Rate is directly proportional to square of concentration of that reactant Doubling concentration quadruples rate,rate-concentration graph,concentration-time graph,5/26/2019,45,Overall order of a react
33、ion: sum of individual reaction orders (above reaction-overall order is x + y + z).,Easier to use rate-concentration graph rather than concentration-time graph to decide if reaction is first or second-order,5/26/2019,46,5/26/2019,47,Units used:,Rate measured in change in concentration per second - u
34、nits are mol dm-3 s -1 Units for concentrations of A/B are mol dm-3, but if orders are more or less, they change If order is 0, just ignore them If order is 1, its mol dm-3 If order is 2, units are mol dm-32 = mol2 dm-6,5/26/2019,48,5/26/2019,49,http:/ + NO2- N2 + 2 H2O,rate = kNH4+1NO2-1 or simply
35、kNH4+NO2-,1.35 x 10-7 M/s = k(0.100 M)(0.005 M) solving, k = 2.7 x 10-4/Ms or 2.7 x 10-4 L/mols,5/26/2019,54,NO2 + CO NO + CO2,rate = kNO22CO0 or simply kNO22,0.18 M/s = k(0.10 M)2(0.0010 M)0 k = 18/Ms or 18 L/mols,5/26/2019,55,2 NO2 + 2 H2 N2 + 2 H2O,rate = kNO22H21 or simply kNO22H2,1.3 x 10-5 M/s
36、 = k(0.0050 M)2(0.0020 M)1 k = 2.6 x 102/M2s,5/26/2019,56,Example,The initial rate of the reaction BrO3-(aq) + 5 Br-(aq) + 8 H+(aq) 3 Br2(l) + H2O(l) has been measured at the reactant concentrations shown (in mol/L): Experiment BrO3- Br- H+ Initial rate (mol/Ls) 1 0.10 0.10 0.10 8.0 x 10-4 2 0.20 0.
37、10 0.10 1.6 x 10-3 3 0.10 0.20 0.10 1.6 x 10-3 4 0.10 0.10 0.20 3.2 x 10-3 Rate = kAxBy so Rate = kBrO3-1Br-1H+2 Rate = 1 + 1 + 2 = 4,5/26/2019,57,Example,The reaction of iodide ion with hypochlorite ion, OCl- (which is found in liquid bleach), follows the equation OCl- + I- OI- + Cl- It is a rapid
38、reaction that gives the following rate data: Initial Concentrations Rate of Formation (mol/Ls) OCl- I- Cl- 1.7 X 10-3 1.7 X 10-3 1.75 X 104 3.4 X 10-3 1.7 X 10-3 3.50 X 104 1.7 X 10-3 3.4 X 10-3 3.50 X 104 Determine value of rate constant. Rate = kAxBy 1.75 x 104 = k(1.7 x 10-3)1 (1.7 x 10-3)1 rate
39、= 6.06 x 109 L/mol sec OCl- I- Rate = 1 + 1 = 2,5/26/2019,58,Homework:,Read 17.3, pp. 542-545 Q pp. 554-556, #56, 69-70, 74,5/26/2019,59,Reaction Mechanisms- how atoms or molecules interact to make products,Complex reaction-two or more elementary steps Mechanism may never be proven Cannot see chemic
40、al reaction Based on scientific evidence available for reaction Guess at identity of many intermediates Intermediates usually so reactive, cant be isolated Produced in one elementary step and consumed in another Does not appear in overall reaction Mechanism for decomposition of ozone, O3,5/26/2019,6
41、0,Combine to produce path molecules take from reactant(s) to product(s) in chemical reaction-sum is overall reaction,Represents single collision or vibration that leads to chemical change,Determines overall rate of reaction Reaction can only go as fast as its slowest step-highest activation energy,5
42、/26/2019,61,5/26/2019,62,Use rate laws to determine mechanism for chemical reaction A + B Y + Z Not sure how process actually occurs, but overall reaction rate expressed by Rate = kB Predict if following mechanism is valid or invalid Step 1: 2A Y + D Slow process Step 2: B + D A + Z Fast process,5/2
43、6/2019,63,Reaction mechanisms must conform to rules Overall equation for reaction needs to equal sum of elementary steps 2A Y + D B + D A + Z 2A + B + D Y + D + A + Z A + B Y + Z (So far, looks good) Rate law for rate-determining step should match rate law for overall reaction 2A Y + D Rate = kA2 (d
44、oesnt match rate = kB)-invalid,5/26/2019,64,Rate determining step,Overall reaction 2 NO2 + F2 2 NO2F Mechanism (two elementary steps) NO2 + F2 NO2F + F (slow) F + NO2 NO2F (fast) Rate determining step (RDS) determines overall reaction rate law Rate overall rate for rds step = k1 NO2 F2,5/26/2019,65,5/26/2019,66,5/26/2019,67,5/26/2019,68,5/26/2019,69,5/26/2019,70,Homework:,Read 17.4, pp. 546-549 Q pp. 554-556-#57, 76 Do test, pg. 557 Use link for quiz and submit as before. http:/