0.16 Reaction rates  (Page 7/9)

[link] shows the analogous situation for an exothermic reaction. Again, as thereactants approach one another, the energy rises as the atoms beginto rearrange. At the middle of the collision, the energy maximizes and then falls as the product molecules form. In an exothermicreaction, the product energy is lower than the reactant energy.

[link] thus shows that an energy barrier must be surmounted for the reaction tooccur, regardless of whether the energy of the products is greater than ( [link] ) or less than ( [link] ) the energy of the reactants. This barrier accounts for the temperature dependence ofthe reaction rate. We know from the kinetic molecular theory that as temperature increases the average energy of the molecules in asample increases. Therefore, as temperature increases, the fraction of molecules with sufficient energy to surmount the reactionactivation barrier increases.

Reaction energy

Endothermic reaction

Exothermic reaction

Although we will not show it here, kinetic molecular theory shows that the fraction of molecules with energygreater than $E_a$ at temperature $T$ is proportional to $e^{-\left(\frac{E_a}{RT}\right)}$ . This means that the reaction rate and therefore also the rateconstant must be proportional to $e^{-\left(\frac{E_a}{RT}\right)}$ . Therefore we can write

As a final note on [link] , the constant $A$ must have some physical significant. We have accounted for the probability ofcollision between two molecules and we have accounted for the energetic requirement for a successful reactive collision. We havenot accounted for the probability that a collision will have the appropriate orientation of reactant molecules during the collision.Moreover, not every collision which occurs with proper orientation and sufficient energy will actually result in a reaction. There areother random factors relating to the internal structure of each molecule at the instant of collision. The factor $A$ takes account for all of these factors, and is essentially theprobability that a collision with sufficient energy for reaction will indeed lead to reaction. $A$ is commonly called the frequency factor .

Observation 4: rate laws for more complicated reaction processes

Our collision model in the previous section accounts for the concentration and temperature dependence of thereaction rate, as expressed by the rate law. The concentration dependence arises from calculating the probability of the reactantmolecules being in the same vicinity at the same instant. Therefore, we should be able to predict the rate law for anyreaction by simply multiplying together the concentrations of all reactant molecules in the balanced stoichiometric equation. Theorder of the reaction should therefore be simply related to the stoichiometric coefficients in the reaction. However, [link] shows that this is incorrect for many reactions.

Consider for example the apparently simple reaction