Rate laws

A + B  C R = k[A][B] 2A + 3B  C R = k[A]2[B]3 You can get the rate law from looking at the coefficients of the reactants. This is the worst method, only use this on the slow step in a reaction mechanism

NO2 (g) + CO (g)  NO (g) + CO2 (g) Write the rate law for this equation

Based on what you have learned so far, you should have written R =k [NO2] [CO]

But that’s not right. Let me give you some more info about the reaction pathway. Step 1 NO2 + NO2  NO3 + NO slow Step 2 NO3 + CO  NO2 + CO2 fast Now write the rate law.

R = k[NO2] [NO2] R = k [NO2]2 Always use the best information that you have available to write the rate law. Sometimes you will have experimental data, which is the best information.

The rate of a reaction involving L, M, and N doubles when the concentration of L is doubled, increases 8 times if the concentration of M is doubled, and quadruples if the concentration of N is doubled. Write the rate law for this reaction.

R = k [L] [M]3 [N]2 First order for L Second order for N Third order for M Sixth order overall

At temperatures below 498K the following reaction takes place At temperatures below 498K the following reaction takes place NO2 (g) + CO (g)  NO (g) + CO2 (g) Doubling the concentration of NO2 quadruples the rate of CO2 being formed. Doubling the concentration CO has no effect on the rate of CO2 formation. Write the rate law.

R = k [NO2]2 This was a different way to ask the same question. Remember, if the concentration of a reactant changes, but the reaction rate stays the same, then you do not need that reactant in your rate law.

Initial rate of formation of C Experiment number Initial [A] Initial [B] Initial rate of formation of C 1 0.20M 2.0 x10-4 M/min 2 0.40M 8.0 x 10-4 M/min 3 1.6 x 10-3 M/min

R = k[A][B]2 Compare experiment 1 to experiment 2 and then 2 to 3. But you do not have to go in order. You can compare any 2 experiments. Take the new rate or new concentration and divide it by the old to find out what the change was.

The rate of a reaction involving X, Y, and Z quadruples when the concentration of X is doubled, increases 16 times if the concentration of Y is quadrupled, and increases by a factor of 8 if the concentration of Z is doubled. Write the rate law for this reaction.

R = k[X]2[Y]2[Z]3

Experiment number Initial [A] Initial [B] Initial [C] rate of formation of D 1 0.20M 0.6M 2.0 x10-4 M/min 2 0.40M 8.0 x 10-4 M/min 3 1.6 x 10-3 M/min 4 2.4M 1.28 x 10-2 M/Min

R =k[A][B]2[C]2 You can see from the chart that when B doubles, the rate increases by a factor of 4. When A doubles, the rate doubles. Then by comparing experiment 2 and experiment 4, the rate increases by a factor of 16 when C quadruples.

Additional Info If the concentration changes on the chart, but the rate does not, you do not need that reactant in your rate law. Remember, you can compare any 2 experiments.