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Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
165
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 16
7. A laboratory chemist places a flask in an ice bath to prevent the
reaction in the flask from foaming over.
8. Molecules of unburned fuel in a car exhaust are oxidized as
they pass over powdered palladium.
16.3 Reaction Rate Laws
As reactants are used up, their concentrations decrease, the number
of collisions decreases, and the reaction rate slows. A rate law is a
mathematical expression that relates reaction rate to concentrations
of reactants. The reaction order mathematically defines the extent
to which reaction rate depends on the concentrations of reactants.
Because most chemical reactions involve two or more reactants, the
rate law often includes the concentrations of all reactants.
Determining Rate Laws Consider the following reaction and its
experimentally determined rate law.
2NO 2 (g) ϩ F 2 (g) 0 2NO 2 F(g)
Rate ϭ k[NO 2 ][F 2 ]
This rate equation shows that the rate depends on the concentrations
of both NO 2 and F 2 , each to the first power. In other words, the
reaction is first order in NO 2 and first order in F 2 . So, if [NO 2 ] is
doubled while [F 2 ] remains the same, the rate doubles. Also, if [F 2 ]
is doubled while [NO 2 ] remains the same, the reaction rate doubles.
Now, examine the following reaction and its rate law.
2NO(g) ϩ O 2 (g) 0 2NO 2 (g)
Rate ϭ k[NO] 2 [O 2 ]
Because the rate depends on the square of the concentration of NO,
doubling [NO] while leaving [O 2 ] the same will increase the reaction rate by a factor of 2 2 , or 4.
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