universal gas constant
What is the source of the rule? The rule probably began with a back-of-the-envelope calculation based on the Arrhenius equation, first published in 1889. Arrhenius explained the variation of rate constants with temperature for several elementary reactions using the relationship
k = A exp(-Ea/RT)where the rate constant k is the total frequency of collisions between reaction molecules A times the fraction of collisions exp(-Ea/RT) that have an energy that exceeds a threshold "activation energy" Ea at a temperature of T (in kelvins). R is the universal gas constant.
To see what temperature rise is required to change the rate constant from k1 (at T1) to k2 (at T2), take the ratio of the Arrhenius equations for each of the two temperatures:
If the activation energy has a particular value, and if the temperature change occurs in the right range, and if the reaction is an elementary one that obeys the Arrhenius equation, then a 10°C rise might double the reaction rate. It's a very iffy generalization.
Can we at least say that reaction rate increases with temperature? For reactions that occur in a single step involving a molecular collision of some kind, the answer is yes. Only molecules with sufficient energy are able to react. Heat increases the average energy of the molecules and so it would be expected that reaction rate would always increase with increasing temperature.
But many reactions aren't that simple, and their rates may actually decrease with increasing temperature.
What are some examples of reactions that slow down when temperature rises? Nearly all biochemical reactions, for example, are catalyzed by rather delicate protein molecules called enzymes. Warming a biochemical reaction increases its rate as expected - up to a certain temperature. Heating beyond that point actually decreases the reaction rate and further heating can stop the reaction completely. Heat causes the enzyme to unravel or unfold, and the enzyme's shape is critical to its ability to accelerate the reaction.
Even some inorganic reactions can slow down when things heat up. Consider this reaction with intermediate B:
|A B||(fast, exothermic)|
When can relying on the rule be dangerous? Consider the formation reaction for gaseous hydrogen chloride,
Heating the H2/Cl2 mixture will at first elevate the reaction rate only slightly. Once the propagation steps begin to occur at significant rates, further heating causes a sudden, tremendous acceleration in reaction rate. Instead of a doubling of rate that might otherwise have been expected, the rate may suddenly increase by orders of magnitude- resulting in a violent explosion!
Author: Fred Senese firstname.lastname@example.org
Copyright © 1997-2010 by Fred Senese
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Last Revised 02/15/10.URL: http://antoine.frostburg.edu/chem/senese/101/kinetics/faq/print-temperature-and-reaction-rate.shtml