CHEM 116 Rates of Reaction

Transcription

1 UMass Boston, Chem 6 CHEM 6 Rates of Reaction FSG is cancelled Lecture Prof. Sevian today (Oct 4) in order to keep both FSG sections at the same pace (since there was no school yesterday) Today s agenda Graphing concentration data to determine reaction rates (ch. 4) Initial rate of change vs. average rate of change Concentration vs. time for one reactant that is measured Concentration vs. time for other reactants and for products Determining the order of a reaction from concentration vs. time data Tables of initial reaction rates to determine the order of reaction for different species involved in a reaction H. Sevian

2 UMass Boston, Chem 6 Kinetics: Key terms What makes kinetics difficult is that there are a lot of new ideas and new vocabulary all introduced at the same time. Becoming familiar with the vocabulary will help immensely. Rate law Concentration Catalyst Overall order of reaction Order with respect to a particular reactant Rate constant Half-life Collision model Activated complex/transition state Activation energy Arrhenius equation Reaction mechanism Elementary step in reaction mechanism Rate-determining step Molecularity of a step Equilibrium Kinetics The study of how quickly (or slowly) reactions occur Using chemistry in the world: How do you produce chemicals? What kinds of chemical reactions are there? What properties do materials have and why do they have those properties? How much of a given chemical can you produce? How can you control it? Stoichiometry How fast do reactions occur? How much energy does it take to make a reaction occur, or how much energy does a reaction give off? How pure can you make the chemical? (Are there competing reactions?) Scaling up reactions to useful proportions How dangerous are specific chemicals? LD5 of the desired products Are there any byproducts or left over reactants? Are these dangerous? H. Sevian

3 UMass Boston, Chem 6 What is rate of a reaction? Comparison: What is rate of travel? How fast you travel. distance traveled Δd d d rate of travel ( speed) time of travel Δt t t Rate of a reaction is how fast the reaction happens Measured mathematically as change of chemical concentration Δ[A] [A] [A] rate of reaction ( rate) time elapsed Δt t t What if you couldn t measure reactant concentration but you could measure how much product is created? The class did four different experiments Control (same for everyone): tablet Alka-Seltzer in ml of room temperature water You Sent a member of your group to get a set of materials, noted the Group # Set contained: cups, water, Alka-Seltzer tablets You measured time in seconds (digital watch or second hand on clock in front) A) tablets instead of Groups,, 3 and 4 Same control Experiment: tablets instead of tablet C) Powdered tablet Groups 9,, and Same control Experiment: crushed A-S tablet in same amount of room temp water B) Higher temperature Groups 5, 6, 7 and 8 Same control Experiment: tablet A-S in ml of very warm water D) Twice as much water Groups 3, 4, 5 and 6 Same control Experiment: tablet A-S in ml (twice as much water as the control) H. Sevian 3

4 UMass Boston, Chem 6 Results of Alka-Seltzer Experiments In one Alka-Seltzer tablet, there are 96 mg of the limiting reagent, sodium bicarbonate (f.w. 84.). Thus the average rate of the reaction was calculated as the change in reactant molarity (from M of reactant present at the start, down to zero M at the end) divided by the time for the reaction to reach apparent completion (in seconds). Expt # Control A) x tablets B) warm HO C) crushed D) x HO Average time (s) mg of lim reagent mmol lim reagent ml water Initial concen (M) Avg Rate (M/s) Rate (change in molarity of limiting reactant that reacted) / (average time for the reaction to reach apparent completion) Extra credit assignment (due at beginning of class (:) on Tues Oct 4) Make a table like the following and fill it out In the Results column, explain in words what the results were and how they compare to the control Your answers in the Why I think that happened column do not need to be correct, but they do need to be what you think. Results of this experiment in class Why I think that happened A) Two tablets instead of one B) Warm water instead of room temp water C) Tablet crushed first before adding to water D) Twice as much water used H. Sevian 4

5 UMass Boston, Chem 6 Alka-Seltzer: Bicarbonate + acid solution NaHCO ( aq ) + Citric acid ( aq) Na - citrate ( aq) + HO ( l) + CO ( 3 g ) in the Alka-Seltzer in the Alka-Seltzer What did you vary in the experiment (compared to the control)? Amount of NaHCO 3 (Alka-Seltzer) and amount of citric acid doubled Higher initial reactant concentration Temperature raised all chemicals have higher KE Powdered tablet Increased surface area for reaction to occur Volume of water increased Lower initial reactant concentration Did the rate of the reaction change? Why? change of chemical concentration Δ[A] [A] [A] rate of reaction ( rate) time elapsed Δt t t Compare tablets vs. tablet Back of the envelope calculation Two tablets in ml water Took about 7 sec for reaction to be complete ( tablets tablet). L Δconc tablet tablet rate ( tablets).9 Δt 7 s 7 L s L s One tablet in ml water (control) Took about 6 sec for reaction to be complete ( tablet tablet). L Δconc tablet tablet rate ( tablet).7 Δt 6 s 6 L s L s H. Sevian 5

6 UMass Boston, Chem 6 tablets (higher conc) vs. tablet (control) Graphical explanation Say. tablet/ml is the smallest concentration that your eye can tell there is still rxn happening Important! Rate of reaction is the slope of concentration vs. time plot concentration (tablet/ ml) NaHCO3 + citric acid --> sodium citrate + HO + CO Apparent rate for tablets in ml water initial [A] tablet initial [A] tablets Apparent rate for tablet in.5.5 ml water time (minutes) tablets (higher conc) vs. tablet (control) vs. twice as much water (lower conc.) Graphical explanation Say. tablet/ml is the smallest concentration that your eye can tell there is still rxn happening concentration (tablet/ ml) NaHCO3 + citric acid --> sodium citrate + HO + CO initial [A] tablet initial [A] tablets initial [A] tablet in tw ice as much w ater.5.5 time (minutes) H. Sevian 6

7 UMass Boston, Chem 6 Average vs. instantaneous rxn rate Average rxn rate change of chemical concentration between beginning of rxn and end Δ[A] rate of reaction ( rate) total time elapsed Δt Instantaneous rxn rate (tangent to curve) change of chemical concentration over a short period of time Δ[A] rate of reaction ( rate) the short time elapsed Δt Usually in chemistry, we are interested in the instantaneous rxn rate at the very beginning (at t ) INITIAL REACTION RATE Note that the same comparison exists with initial rate as with average rate Average vs. initial rxn rate NaHCO3 + citric acid --> sodium citrate + HO + CO Instantaneous rate at t (initial rate) Note: Explaining experiment (B) the temperature dependence of rates will happen when we talk about Collision Theory. concentration (tablet/ ml) Average rate initial [A] tablet initial [A] tablets.5.5 time (minutes) H. Sevian 7

8 UMass Boston, Chem 6 The rate of a reaction changes over time because reactants get used up From Chemistry & Chemical Reactivity 5 th edition by Kotz / Treichel. C 3. Reprinted with permission of Brooks/Cole, a division of Thomson Learning: Fax Comparing concentrations of chemicals in a reaction A --> B A --> C concentration (mol/l) [A] [B] concentration (mol/l) [A] [C] time (hours) time (hours) H. Sevian 8

9 UMass Boston, Chem 6 Example: Reaction rates and stoichiometry Similar to Practice Exercise, p. 58 Compare the rates of appearance or disappearance of each chemical in the decomposition of NOCl. NOCl (g) NO (g) + Cl (g) If the initial rate of disappearance of NOCl is mol/(l h), what is the initial rate of appearance of Cl? 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 H. Sevian 9

10 UMass Boston, Chem 6 Rate law must be determined experimentally Some reactions and their experimentally determined rate laws N O 5 (g) 4 NO (g) + O (g) NO (g) + N O 5 (g) 3 NO (g) NO (g) + F (g) NO F (g) NO (g) + O (g) NO (g) Rate k [N O 5 ] Rate k [N O 5 ] Rate k [NO ] [F ] Rate k [NO] [O ] CH 3 CHO (g) CH 4 (g) + CO (g) Rate k [CH 3 CHO] 3/ Important: Order of rate determined by experiment may not match stoichiometric coefficients 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 initial reaction rate The equations which we will study all talk about initial reaction rate, which is the instantaneous reaction rate measured at the beginning of a reaction (at t) 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 H. Sevian

11 UMass Boston, Chem 6 Order of reaction rate Rate of reaction can depend on how much reactant is present in more than one way Zero order: rate [A] First order: rate [A] Second order: rate [A] concentration (mol/l) Conclusion: Unless you have a zeroorder reaction rate, it is difficult to tell from the concentration vs. time graph what order the reaction rate is. Zero vs. first vs. second order rates first order second order zero order.5.5 time (hours) What does order mean? Zero order rate k [A] if you double [A] rate stays the same [A] rate stays same if you triple A, [A] 3 rate 3 stays same First order rate k [A] if you double [A] rate doubles [A] rate if you triple A, [A] 3 rate 3 Second order rate k [A] if you double [A] rate quadruples [A] rate rate 4 if you triple A, [A] 3 rate 3 rate 9 H. Sevian

12 UMass Boston, Chem 6 Two ways to figure out the dependence of the rate on concentration of chemicals in the reaction. Graph a version of concentration vs. time and examine mathematical shape of curve Zero order rate d dt k[ A] [ A] k at time t when t d t [ A] k dt [ A] [ A] k t y a + bx t First order rate k[ A] [ ] [ ] d A k A dt Second order rate k A d dt [ ] [ A] k[ A] at time t when t [ A] [ A] k dt [ A] ln[ A] k t. Measure initial rate at many different initial concentrations and compare d at time t when t t ln y a + bx d[ A] t k dt [ A] [ A] [ A] t t y a + bx k t Method : Comparing graphs [A] t Time (min) [H O ] (M) Key question is: which one is a straight line? ln ([ ] ) A t [ A] t H. Sevian

13 UMass Boston, Chem 6 Method : Determining rate law by measuring initial rate at various concentrations Example Given the following measurements of initial rate of reaction under various conditions of initial concentrations of reactants, determine: the rate law the value of the rate law constant, k the units of k Experiment [NO] Initial Concentration (M) [O ] Initial rate (M/s) Iodine clock reaction H O + 3 I - + H + I H O Hydrogen peroxide And then, two things can happen: Tri-iodide gets reduced back to iodide ion by thiosulfate I S O 3-3 I - + S 4 O 6 - Tri-iodide ion Reactants Iodide ion Thiosulfate ion Hydronium ion Iodide ion Tri-iodide ion When the thiosulfate gets used up, tri-iodide reacts with starch to form blue starch-triiodide complex I starch blue complex Signal that all the tri-iodide is gone Water H. Sevian 3