Notes 14-1 Obj 14.1, 14.2

14.1 Factors That Affect Reaction Rates A.) Studies the rate at which a chemical process occurs. B.) Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs).

C.) Outline: Kinetics Reaction Rates How we measure rates. Rate Laws How the rate depends on amounts of reactants. Integrated Rate Laws How to calculate amount left or time to reach a given amount. Half-life How long it takes to react 50% of reactants to react. Arrhenius Equation How rate constant changes with T. Mechanisms Link between rate and molecular scale processes.

D.) Factors That Affect Reaction Rates 1.) Concentration of Reactants As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. 2.) Temperature At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. 3.) The Physical State of the Reactants The more readily reactants collide with each other, the more rapidly they react. 4.) Catalysts Increase the speed of the reaction by changing the mechanism.

Notes 14.2 Reaction Rates A.) Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time.  [A] vs  t 1.) if average rate of disappearance is used, then you must use a negative in front of the rate to display a positive quantity (rates are always expressed as positive quantities).

Calculate the average rate at which A disappears over the time interval from 20s to 40s Calculate the average rate of the appearance of B over the time interval from 0 to 40s

In this reaction, the concentration of butyl chloride, C 4 H 9 Cl, was measured at various times, t. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) [C 4 H 9 Cl] M

B.) The average rate of the reaction over each interval is the change in concentration divided by the change in time: C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq) Average Rate, M/s

1.) Note that the average rate decreases as the reaction proceeds. 2.) This is because as the reaction goes forward, there are fewer collisions between reactant molecules. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

3.) A plot of concentration vs. time for this reaction yields a curve like this. 4.) The slope of a line tangent to the curve at any point is the instantaneous rate at that time. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

Calculate the initial disappearance of C 4 H 9 Cl at t = 0 (the initial rate) Determine the instantaneous rate of disappearance of C 4 H 9 Cl at t = 300s

5.) The reaction slows down with time because the concentration of the reactants decreases. C 4 H 9 Cl (aq) + H 2 O (l)  C 4 H 9 OH (aq) + HCl (aq)

C.) Reaction Rates and Stoichiometry What if the ratio is not 1:1? H 2 (g) + I 2 (g)  2 HI (g) 2 HI is made for each H 2 used.

To generalize, for the reaction aA + bBcC + dD Reactants (decrease) Products (increase)

a.) How is the rate at which ozone disappears related to the rate at which oxygen appears in the reaction 2O 3 (g)  3O 2 (g)? b.) If the rate at which O 2 appears, ∆ [O 2 ]/∆t, is 6.0 X M/s at a particular instant, at what rate is O 3 disappearing at this same time, -∆ [O 3 ]/∆t The decomposition of N 2 O 5 proceeds according to the following equation: 2N 2 O 5 (g)  4NO 2 (g) + O 2 (g) If the rate of decomposition of N 2 O 5 at a particular instant in a reaction vessel is 4.2 X M/s, what is the rate of appearance of a.) NO 2 b.) O 2