Activation Energy

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Activation energy is the energy that must be overcome in order for a chemical reaction to occur. It is also the minimum energy required to start a chemical reaction and is stated in kilojoules per mole. Another way to think of it is the energy barrier separating two minima of potential energy (the reactants and products). For a chemical reaction to proceed at a reasonable rate, there should exist an appreciable number of molecules with energy equal to or greater than the activation energy.

Occasionally, the rates of reaction decrease when the temperature increases. Following an approximately exponential relationship so the rate constant can still be fit to an Arrhenius expression, this results in a negative value for the activation energy. Elementary reactions exhibiting these negative activation energies are typically barrierless reactions, in which the reaction proceeding relies on the capture of the molecules in a potential well. Increasing the temperature leads to a reduced probability of the colliding molecules capturing one another, expressed as a reaction cross section that decreases with increasing temperature.

The Arrhenius equation gives the quantitative basis of the relationship between the activation energy and the rate at which a reaction proceeds. This equation suggests that the activation energy is dependent on temperature, but this effect is canceled by the temperature dependence of the reaction rate coefficient.

A substance that modifies the transition state to lower the activation energy is termed a catalyst. A biological catalyst is an enzyme. It is important to note that a catalyst increases the rate of reaction, but isn’t consumed by it nor does it change the energies of the original reactants nor products. Instead, the reactant energy and the product energy remain the same while the activation energy is lowered.

We have written many articles about activation energy for Universe Today. Here’s a link to the Bad Astronomy and Universe Today forum about Nuclear Fusion Energy, and here’s an article about ions.

If you’d like more info on the Activation Energy, check out an article about the activation energy of chemical reactions, and here’s a link to Wise Geek’s article about Activation Energy.

We’ve also recorded an entire episode of Astronomy Cast all about the Atom. Listen here, Episode 164: Inside the Atom.

Sources:
Wikipedia
Purdue University

Tagged as: activation energy, Arrhenius equation, chemical reaction