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Understanding Reaction Rates: Chemical Kinetics with Concentration and Time

Explore the concept of reaction rates in chemical kinetics, determined by changes in reactant or product concentrations over time. This explanation utilizes the reaction between butyl chloride (C4H9Cl) and water (H2O), illustrating how to calculate average and instantaneous rates. Learn how to interpret graphs, slopes, and the significance of decreasing rates as reactions progress. Through practical examples, discover key methodologies to determine reaction dynamics, including a comparison of average rates and instantaneous rates. Enhance your understanding of stoichiometry and reaction behavior over time.

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Understanding Reaction Rates: Chemical Kinetics with Concentration and Time

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  1. Typical Graphs

  2. Rate of Reaction = Chemical Kinetics Δ [Concentration] Δ Time • Rate of Rxn = = Slope

  3. Reaction Rates Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.

  4. Watch This!

  5. Par Example C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) In this reaction, [the concentration]of butyl chloride, C4H9Cl, was measured at various times.

  6. Average rate = Average rate = [.10 - .0905] [50 – 0] [C4H9Cl] t Reaction Rates The average rate of the reaction over each interval is the change in concentration divided by the change in time: = 1.9 x 10 -4

  7. AVERAGE RATE CHANGES! • It is not constant. • What’s happening to the average rate?

  8. Practice Example 14.1 p. 560

  9. Reaction Rates • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, there are fewer collisions between reactant molecules.

  10. Change of Rate over Time Practice Example p. 598 #14.4 YES! Linear Function with positive slope. b. Yes! The slope = 0 indicating that the reaction is over evidenced by no change in [M].

  11. Instantaneous Rate of Change • Instantaneous Rate of Change = slope of tangentline to curve at a point “t” @ t = 0, initial rate

  12. Think of it this way! • Your instantaneous rate is • You drove 98 miles to Charlotte in 2 hours. • Your average rate is 49 mi/hr.

  13. Reaction Rates p. 561 C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • A plot of [C4H9Cl] vs. time for this reaction yields a curve like this. • The slope of a line tangent to the curve at any point is the instantaneous rate at that time= RATE @ instant. • Examine the slope at t = 0 vs. slope at t = 600 s. • Which is greater? Steeper Slope

  14. What’s happening over time? Slope is decreasing. Rate is decreasing. Reaction is slowing.

  15. Reaction Rates C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • All reactions slow down over time. • Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning of the reaction.

  16. How do calculate instantaneous rate? • NON-Calculus Method • Find slope of line at point: HOW??? • USE GRAPH! • Draw in tangent line • Calculate ~ slope • Approximation of actual slope of tangent line to curve @ t = seconds • Calculus Method • In order to find the ACTUAL slope of tangent line at t = X seconds • MUST know function • DON’T know function • IF we knew the function, THEN we could use the 1st derivative to find the actual instantaneous rate of change

  17. Calculus Application First Derivative = slope of tangent line to curve at t = 2 First Derivative = Velocity

  18. Let’s Practice p. 600 #14.21 • (a) Calculate averages between intervals of time. • (b) Calculate average rate over entire time interval. • (c) Use LoggerPro to graph data. Select natural exponent function. • ANSWERS FOUND ON p. A-18 at back of book. AVERAGE = OVER SPECIFIC TIME INTERVAL INSTANTANEOUS = @ SPECIFIC TIME VALUE

  19. -[C4H9Cl] t Rate = = [C4H9OH] t Reaction Rates and Stoichiometry C4H9Cl(aq) + H2O(l) C4H9OH(aq) + HCl(aq) • In this reaction, the ratio of C4H9Cl to C4H9OH is 1:1. • Thus, the rate of disappearance of C4H9Cl is the same as the rate of appearance ofC4H9OH.

  20. 1 2 [HI] t Rate = − = [I2] t Reaction Rates and Stoichiometry What if the ratio is not 1:1? 2 HI(g)  H2(g) + I2(g)

  21. aA + bB cC + dD = − = = Rate = − 1 a 1 b 1 c 1 d [C] t [D] t [A] t [B] t Reaction Rates and Stoichiometry • To generalize, then, for the reaction

  22. Sample Exercise 14.3 p. 563

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