Chapter 12 Chemical Kinetics Speed Steps (The study of how reaction rates change under varying conditions and what molecular events occur during an overall reaction) Speed Steps (how fast or slow a reaction proceeds) (a reaction mechanism) ←We’ll look at this today

Data for a reaction: 2 N2O5(g) → 4 NO2(g) + O2(g) [N2O5] Time (min) ←This stands for the concentration of dinitrogen pentoxide 0.166 0 0.124 10 0.093 20 0.069 30 0.048 40 0.034 50 0.024 60 0.018 70 0.014 80 0.012 90 0.011 100 The following data shows how the concentration changes as the reaction proceeds.

= = -1.42 x 10-5 M/s = -1.71x 10-5 M/s Here is the Rate of Change for Rate = D[N2O5] 0.166 M – 0.048 M 0.240 On the graph paper given, create the scales that will be shown for each axis. 0.220 = 0.200 D time 0 s – 2400 s Rate = -4.92 x10-5 M/s 0.180 Called Average Rates (over a time interval) …for 2400 → 4800 s Rate = 0.048 M – 0.014 M 0.160 2400 s – 4800 s [N2O5] = -1.42 x 10-5 M/s 0.100 Find the points shown in black placed on that red line and mark them on yours, then find the slope of this line 0.080 0.060 Now, draw the best tangent line on your graph where the red dot appeared Instantaneous Rate Rate = 0.060 M – 0.012 M 0.040 Now, plot each point from your data! 1400 s – 4200 s 0.020 = -1.71x 10-5 M/s 1000 2000 6000 7000 8000 Time (sec)

rate constant = k [N2O5]x [N2O5] Rate = -D[N2O5] Dtime Time (sec) 0.240 As the reaction proceeds the average reaction rate goes down (look at the calculated values!) The rate at a given moment can be found by finding the slope of the line tangent to the curve at that point The rate of the reaction then depends on the concentration of the reactant that remains Therefore: 0.220 0.200 0.180 0.160 [N2O5] Called the order rate constant 0.100 Rate = -D[N2O5] = k [N2O5]x 0.080 Dtime Negative because the reactant is decreasing and we want a positive value for the rate Rate Expression Rate Law 0.060 …which means that the concentration raised to some power times the constant of proportionality, k, also equals the rate But since the rate is also directly related to the concentration of the reactant remaining, we could also say… 0.040 0.020 1000 2000 6000 7000 8000 Time (sec)