Rate laws. A + B  C R = k[A][B] 2A + 3B  C R = k[A] 2 [B] 3.

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Presentation transcript:

Rate laws

A + B  C R = k[A][B] 2A + 3B  C R = k[A] 2 [B] 3

NO 2 (g) + CO (g)  NO (g) + CO 2 (g) Write the rate law for this equation

Based on what you have learned so far, you should have written R =k [NO 2 ] [CO]

But that’s not right. Let me give you some more info about the reaction pathway. Step 1 NO 2 + NO 2  NO 3 + NO slow Step 2 NO 3 + CO  NO 2 + CO 2 fast Now write the rate law.

R = k[NO 2 ] [NO 2 ] R = k [NO 2 ] 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 NO 2 (g) + CO (g)  NO (g) + CO 2 (g) Doubling the concentration of NO 2 quadruples the rate of CO2 being formed. Doubling the concentration CO has no effect on the rate of CO 2 formation. Write the rate law.

R = k [NO 2 ] 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.

Experiment numberInitial [A]Initial [B]Initial rate of formation of C 10.20M 2.0 x10 -4 M/min 20.20M0.40M8.0 x M/min 30.40M 1.6 x M/min

R = k[A][B] 2 This one was simple. 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 numberInitial [A]Initial [B]Initial [C]rate of formation of D 10.20M 0.6M2.0 x10 -4 M/min 20.20M0.40M0.6M8.0 x M/min 30.40M 0.6M1.6 x M/min 40.20M0.40M2.4M1.28 x 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.