Typical Graphs

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

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

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

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

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

Practice Example 14.1 p. 560

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.

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].

Instantaneous Rate of Change Instantaneous Rate of Change = slope of tangent line to curve at a point “t” @ t = 0, initial rate

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

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

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

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.

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

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

Let’s Practice p. 600 #14.21 AVERAGE = OVER SPECIFIC TIME INTERVAL (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

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 of C4H9OH. Rate = -[C4H9Cl] t = [C4H9OH]

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

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

Sample Exercise 14.3 p. 563