Chemistry Reaction Rates and Activation Energy

Study Notes

Reaction rates are determined by the formation of transition states
Reaction transition states are high energy intermediates between reactants and products
Transition states are highly unstable and rapidly break down, either back to the reactants or to the products
The rate of reaction is thus determined by how quickly the transition state forms
Concentration of reactants involved in transition state formation affects reaction rate (law of mass action)
For reactions with more than one reactant, the probability of collisions between reactant molecules affects the rate
A collision is necessary, but also requires a minimum necessary energy (activation energy, E_a)
The Arrhenius equation describes the relationship between the rate constant (k) and the temperature

Minimum energy needed for a reaction to occur
Not all collisions result in a reaction; collisions must possess sufficient energy to overcome the activation energy barrier
Increasing temperature increases the proportion of molecules with energy greater than the activation energy, therefore increasing the reaction rate
The Arrhenius equation shows the relationship between rate constant and temperature
The Arrhenius equation is non-linear; it shows an exponential increase in rate constant, with an increase in temperature

Catalysts increase reaction rates by providing an alternative lower energy pathway
Catalysts are unchanged after the reaction
Heterogeneous catalysts are in a phase separate to the reactants, and often offer a surface on which the reaction takes place
Homogeneous catalysts are in the same phase as the reactants, and are often chemicals that react with one reactant, but are regenerated when the product is formed

Reactions proceed in both forward and reverse directions, and at equilibrium, the rates of both reactions are equal
When the concentration of products and reactants are constant and no more net change occurs.
As products accumulate, the reverse reaction speed up and forward slows down, eventually leading to equilibrium