The temperature-dependence of elementary reaction rates: beyond Arrhenius

Ian W M Smith1

  • 1University Chemical Laboratory, Lensfield Road, Cambridge, UK CB2 1EW.

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Temperature Dependence on Reaction Rate02:55

Temperature Dependence on Reaction Rate

The Collision Theory
Atoms, molecules, or ions must collide before they can react with each other. Atoms must be close together to form chemical bonds. This premise is the basis for a theory that explains many observations regarding chemical kinetics, including factors affecting reaction rates.
The collision theory is based on the postulates that (i) the reaction rate is proportional to the rate of reactant collisions, (ii) the reacting species collide in an orientation allowing contact between...
Effect of Temperature Change on Reaction Rate02:28

Effect of Temperature Change on Reaction Rate

The Arrhenius equation,
Arrhenius Plots02:34

Arrhenius Plots

The Arrhenius equation relates the activation energy and the rate constant, k, for chemical reactions. In the Arrhenius equation, k = Ae−Ea/RT, R is the ideal gas constant, which has a value of 8.314 J/mol·K, T is the temperature on the kelvin scale, Ea is the activation energy in J/mole, e is the constant 2.7183, and A is a constant called the frequency factor, which is related to the frequency of collisions and the orientation of the reacting molecules.
The Arrhenius equation can be used to...
Rate Laws and Equilibrium Constants for Elementary Reactions01:29

Rate Laws and Equilibrium Constants for Elementary Reactions

Reactions proceed through multi-step mechanisms, where each elementary step is a single process and intermediates appear only between successive steps.Elementary reactions are categorized by molecularity which is the number of molecules reacting in one step.For example, unimolecular reactions involve one molecule, bimolecular reactions involve two, and termolecular reactions involve three; higher molecularity reactions are rarer because simultaneous multi-molecule collisions are unlikely.The...
Le Chatelier's Principle: Changing Temperature02:19

Le Chatelier's Principle: Changing Temperature

Consistent with the law of mass action, an equilibrium stressed by a change in concentration will shift to re-establish equilibrium without any change in the value of the equilibrium constant, K. When an equilibrium shifts in response to a temperature change, however, it is re-established with a different relative composition that exhibits a different value for the equilibrium constant.
To understand this phenomenon, consider the elementary reaction:
Rate-Determining Steps03:08

Rate-Determining Steps

Relating Reaction Mechanisms
In a multistep reaction mechanism, one of the elementary steps progresses significantly slower than the others. This slowest step is called the rate-limiting step (or rate-determining step). A reaction cannot proceed faster than its slowest step, and hence, the rate-determining step limits the overall reaction rate.
The concept of rate-determining step can be understood from the analogy of a 4-lane freeway with a short-stretch of traffic-bottleneck caused due to...