Kinetics and Equilibrium Slide 3 / 119 Slide 4 / 119 Kinetics - PDF document

Slide 1 / 119 Slide 2 / 119 Kinetics and Equilibrium Slide 3 / 119 Slide 4 / 119 Kinetics Think About It... In kinetics we study the rate at which a chemical process occurs. Besides the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs). This is similar to the first chapter in 9th grade physics: kinematics, which also had to do with the rate of change. The reaction that occurs in an airbag must happen in 0.030 seconds. 2NaN 3 (s) --> 2Na(s) + 3N 2 (g) Slide 5 / 119 Slide 6 / 119 The Physical State of the Reactants Factors That Affect Reaction Rates In order to react, molecules must come in contact with each other. The Physical State of the Reactants The more homogeneous the mixture of reactants, the faster The Concentrations of the Reactants the molecules can react. Temperature The Presence of a Catalyst Surface area This proves to us something that is fairly fundamental to our understanding of Chemistry. In order for the particles of two substances to react these particles need to be in actual physical proximity to one another.

Slide 7 / 119 Slide 8 / 119 The Concentrations of the Reactants Temperature At higher temperatures, reactant molecules: As the concentration of Air hole have more kinetic reactants increases, so does · energy the likelihood that reactant move faster molecules will collide. · collide both more · often and with greater The flame from the Bunsen burner shows the variation in energy 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. Recall that blue light has more energy than red. Since the rate of reaction increases, the rate of energy being released also increases, hence the blue color. Slide 9 / 119 Slide 10 / 119 The Presence of a Catalyst The Presence of a Catalyst Catalysts speed up Catalysts are not reactions by changing consumed during the the mechanism of the course of the reaction. reaction. If they are chemically changed at some point in the reaction they are Catalysts are very changed back to their specific in their original state during direction and reaction some other part of the involved. A catalyst will reaction. not effect the reactions Palladium is a rare metal and is more expensive than equilibrium, only the Platinum. However, because it is not "used up" in a speed at which that reaction, a single piece of palladium can be used again and again (without wasting this VERY expensive metal) equilibrium is reached. as a catalyst in certain organic reactions. Slide 11 / 119 Slide 12 / 119 1 Why does a higher temperature cause a reaction Surface Area to go faster? When more surface area of a reactant is exposed, a reaction will occur faster. A Only because there are more collisions per second. More surface area allows for more interaction B Only because collisions occur with greater energy. between particles. C There are more frequent collisions and they are of greater A steel bar will not ignite if energy. exposed to a flame. In answer contrast, a steel wool pad will burn and ignite quickly due to the steel being able to access oxygen more readily. click here to watch steel wool burn

Slide 13 / 119 Slide 14 / 119 2 Why does a higher reactant concentration make a 3 Why does a greater surface area cause a reaction faster? reaction to proceed faster? A Only because there are more collisions per second. A Because there are more collisions per second. B Only because collisions occur with greater energy. B Because collisions occur with greater energy. C There are more frequent collisions and they are of greater energy. C There are more frequent collisions and they are of greater energy. answer answer Slide 15 / 119 Slide 16 / 119 Reaction Rates 4 What happens to a catalyst in a reaction? 0 s 20 s 40 s answer A It is unchanged. Rates of reactions can be B It is incorporated into the products. determined by monitoring the A C It is incorporated into the reactants. change in concentration of B either reactants or products as D It evaporates away. a function of time. 0.25 mol A 1 mol A 0.5 mol A 0.5 mol B 0.75 mol B A --> B Slide 17 / 119 Slide 18 / 119 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 The average rate of the 50 0.0905 1.9 x10 -4 Time , t(s) [C 4 H 9 Cl](M) reaction over each interval 00.0 0.0820 1.7 x10 -4 is the change in 50. 0 0.0741 1.6 x10 -4 In this reaction, the 0.0 0.1000 concentration divided by 200. 0 0.0671 1.4 x10 -4 50 0.0905 concentration 300.0 0.0549 1.22 x 10 the change in time: -4 00.0 0.0820 400.0 0.0448 1.01 x10 of butyl chloride, -4 50.0 0.0741 500.0 0.0368 0.88 x10 -4 C 4 H 9 Cl, was 200.0 0.0671 800.0 0.0200 0.560 x10 -4 300.0 0.0549 10000 0 measured at various 400.0 0.0448 times. 500.0 0.0368 This is the same as velocity in physics; 800.0 0.0200 [C 4 H 9 OH] or the first derivative in calculus. 10000 0 average rate = t

Slide 19 / 119 Slide 20 / 119 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) Instantaneous A plot of [C 4 H 9 Cl] vs. time rate at t=0s for this reaction yields a Time , t(s) [C 4 H 9 Cl](M) Average Rate (M/s) curve like this. Note that the average -2 [C 4 H 9 Cl] (M) x 10 0.0 0.1000 rate decreases as the The slope of a line tangent 50 0.0905 1.9 x10 -4 reaction proceeds. 00.0 0.0820 1.7 x10 -4 to the curve at any point is Instantaneous 50. 0 0.0741 1.6 x10 -4 rate at t=600s the instantaneous rate at This is because as the 200. 0 0.0671 1.4 x10 -4 that time. In calculus this 300.0 0.0549 1.22 x 10 -4 reaction goes forward, 400.0 0.0448 1.01 x10 -4 slope is called the there are fewer collisions 500.0 0.0368 0.88 x10 -4 derivative. between reactant 800.0 0.0200 0.560 x10 -4 10000 0 molecules. Time (s) This is the same as a negative acceleration in physics; The slope of the line on the velocity vs. time curve is acceleration in or a negative second derivative in calculus. physics; The tangent to the curve of the first derivative of a function is the second derivative in calculus. Slide 21 / 119 Slide 22 / 119 Reaction Rates Reaction Rates and Stoichiometry 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) All reactions slow down In this reaction, the ratio of Instantaneous Instantaneous over time. C 4 H 9 Cl to C 4 H 9 OH is 1:1. rate at t=0s rate at t=0s Thus, the rate of Therefore, the best -2 [C 4 H 9 Cl] (M) x 10 -2 [C 4 H 9 Cl] (M) x 10 disappearance of C 4 H 9 Cl is indicator of the rate of a the same as the rate of reaction is the Instantaneous Instantaneous appearance of C 4 H 9 OH. instantaneous rate near the rate at t=600s rate at t=600s beginning of the reaction. At this time we assume we are looking at the rate of the forward reaction unneffected by the reverse The symbol indicates there is a Time (s) Time (s) reaction(s) small change being measured. Slide 23 / 119 Slide 24 / 119 5 Which expression represents a reaction rate? 6 Which set of units represents a reaction rate? A time / mass A s/ g answer answer B concentration / time B M/ s C energy / time C s D time / energy D J/ s E s/ J

Slide 25 / 119 Slide 26 / 119 Concentration and Rate One can gain information about the rate of a reaction by seeing how the rate changes with changes in concentration. Slide 27 / 119 Slide 28 / 119 Slide 29 / 119 Slide 30 / 119 Rate Laws Rate Laws Rate = k [NH 4 + ] [NO 2 − ] A rate law shows the relationship between the reaction rate and the concentrations of reactants. the reaction is: The exponents tell the order of the reaction with respect to First-order with respect to [NH 4 + ] and each reactant. First-order with respect to [NO 2 − ].