Kinetics: Part I: Rates of Reaction Jespersen Chapter 14 Sec 1 & 2 Dr. C. Yau Spring 2013

Chemical Kinetics "Chemical Kinetics" refer to how fast a reaction runs. Gen Chem I – studied stoichiometry We predicted how much product will be made. We assumed reaction has finished. Actually some reactions are very slow and this can be a reason why sometimes % yield is low.

Chemical Kinetics In this chapter, we study the rates of reaction. It's like going on a trip. Stoichiometry is like noting at the end of the trip how many miles we have traveled. Kinetics is like noting how fast, in miles per hour, we are traveling during the trip. The speed may fluctuate during the trip.

Chemical Kinetics We measure how fast a reaction runs by measuring… the rate of formation of a product or the rate of disappearance of a reactant reactant product.

Factors Affecting Reaction Rate Chemical nature of the reactants Bond strengths General reactivity (instability of reactants) e.g. Reactivity of Group IA metals increases as you go down the column (Li, Na, K, Rb, Cs). Na in water K in water Rb & Cs in water Why is there such a trend? What is the reaction of these metals with water? Figure 13.2 The chemical nature of reactants affects reaction rates. (a) Sodium loses electrons easily, so it reacts quickly with water. (b) Potassium loses electrons even more easily than sodium, so its reaction with water is explosively fast. Particle size and reaction rate. An explosion in this grain elevator in New Orleans, Louisiana, killed 35 people in December 1977.

Factors Affecting Reaction Rate (contd.) 2. Ability of the reactants to come in contact with each other Physical state (consider solid + solid; solid +liquid etc.) Surface area for liquids, solids, and heterogeneous mixtures Amount of Mixing Particle shape/size Concentration of reactants Molarity for solutions (higher conc means reactants are closer to each other.) Pressure effects for gases Volume effects for gases Temperature Catalysts Figure 13.4 Effect of concentration on rate. Steel wool, after being heated to redness in a flame, burns spectacularly when dropped into pure oxygen.

Which of the following would speed a reaction? stirring it dissolving the reactants in water, if ionic adding a catalyst grinding any solids boil off some of the solvent all of these

Measuring Rates Can be measured using any substance in the reaction. We choose one that can be easily detected. Units: M/s or Measured in three ways: instantaneous rate initial rate (most often used) average rate Figure 13.5 Effect of time on concentration. The data for this plot of the change in the concentration of HI with time for the reaction 2HI(g) ® H2(g) + I2(g) at 508 °C are taken from Table 13.1. The slope is negative because we’re measuring the disappearance of HI. But when its value is used as a rate of reaction, we express the rate as positive, as we do all rates of reaction. We often select the substance to monitor due to its ease of detection. For example, the disappearance of a vivid color tells us that that particular reactant is consumed and that the reaction is complete.

Fig. 14.5 p.643: Instantaneous Rate 2HI(g) H2(g) + I2 (s) At time 100s, the rate is determined by the slope of the tangent at 100s. The graph is for disappearance of HI, thus slope is negative. BUT rate is always given as a positive number: Rate = (.027M)/110s = 2.5x10-4 M/s

Why is the graph not linear? What happens to the rate as time progresses? What happens to the slope as time progresses? Why? As HI is consumed, there are less HI to collide with each other to form the products…. & reaction slows down.

Instantaneous Rates Changes With Time At 100s, rate =2.5x10-4M/s At 250s, rate = 1 x10-4M/s As time progresses, reaction slows down. Slope= -0.01M/100s rate=1x 10-4 M/s

Initial Rate of Reaction "Initial Rate of Reaction" is the instantaneous rate at time zero. A tangent is drawn at time zero & the slope is determined. See Example 14.2 p.643 Do Practice Exercises 3 & 4 p.644

Instantaneous Reaction Rates Instantaneous rate: the slope of the tangent to the curve at any specific time Initial rate determined at the initial time NO2 appearance 0.005 0.01 0.015 0.02 0.025 0.03 0.035 200 400 600 800 Time (s) [NO2] Instantaneous rate: Very fast. Much smaller reproducibilities for all of the same reasons as above. It is imperative that the same time be used, so auto mixers and cell concentration measurements must be made. Chem FAQ: Estimate the initial rate of a reaction from experimental data. Why does this graph look so different from the previous graphs?

Average Reaction Rates Average rate of reaction: the slope of the line connecting the starting and ending coordinates for a specified time frame NO2 appearance 0.005 0.01 0.015 0.02 0.025 0.03 0.035 200 400 600 800 Time (s) [NO2] e.g. Average rate for time period of 200-600 seconds is measured thus... Average Rate: Faster, because we can choose any time interval. Less reproducible because the (error in time/time frame) factor is larger. Requires that we be able to measure concentration. Requires that we use the same time interval reproducibly.

What is the average rate of B between 10 and 40 s? -0.006 M/s +0.006 M/s -0.002 M/s +0.002 M/s can’t tell from the information 10 20 30 40 Time Elapsed in Reaction Progress (s) Concentration of B (M) 0.0 0.1 0.2 0.3 0.4 X (10s, 0.20M) (40s, 0.03M) X Is B a reactant or product?

Rates And Stoichiometry Example 14.1 p. 640 Butane, C4H10, the fuel in cigarette lighters, burns in oxygen to give carbon dioxide and water. If, in a certain experiment, the butane concentration is decreasing at a rate of 0.20 mol L-1 s-1, what is the rate at which the oxygen concentration is decreasing, and what are the rates at which the product concentrations are increasing? We typically describe rates as positive numbers, no matter on which substance we base the measurement. Chem FAQ: How is the rate at which a reactant disappears related to the rate at which a product appears? Do Practice Exercise 1 & 2 on p. 641.

In the reaction: 2A + 3B →5D we measured the rate of disappearance of substance A to be 3.5×10-5M/s. What is the rate of appearance of D? In the reaction 3A + 2B →C, we measured the rate of B. How does the rate of C relate? Rate of appearance of D = RD = 8.8×10-5 M/s RC=1/2 RB Rate of formation of C should be slower than rate of consumption of B (by ½).

In the reaction 2CO(g) + O2(g) →2CO2(g), the rate of the reaction of CO is measured to be 2.0 M/s. What would be the rate of the production of CO2? the same twice as great half as large you cannot tell from the given information