Activation energy

Activation energy

• minimum energy that a collision must possess for a reaction to occur (E) When the energy of a collision is equal to or greater than the activation energy, a reaction can occur.

energy profile diagram

• Ea is represented on an energy profile diagram

• represents the enthalpy (chemical potential energy) of the reactants and the products over the course of the reaction.

endothermic vs exothermic (activation energy barrier)

• have a peak that represents the activation energy.

• represents minimum energy that must be absorbed to break the bonds of reactants so that a chemical reaction can progress. The activation energy is measured from the enthalpy of the reactants to the top of the peak.

explain endo/exo using this graph

• exothermic reaction releases more heat energy during the reaction than it absorbs.

• An endothermic reaction absorbs more heat energy during the reaction than it releases.

• This is represented on the diagram as AH and is the difference in enthalpy between the reactants and the products.

transition state (+ draw/label and example graph)

• When the activation energy is absorbed, a new arrangement of the atoms occurs.

• occurs at the stage of maximum potential energy in the reaction: the activation energy

• Bond breaking/ bond forming both occurring at this stage = arrangement of atoms is unstable.

• The atoms rearrange into the products as the reaction progresses.

activation energy and reaction rate

• magnitude of activation energy = ability for reaction to occur = what proportion of collisions results in a successful reaction.

• the reaction rate is dependent upon the activation energy.

does collison always mean a chemical change?

• existence of an activation energy for a reaction means that collisions between reactants do not always result in a chemical change.

• e.g., N and O molecules collide frequently in the air around us at room temperature.

• only when energy is provided by a spark, such as in car engines or during a lightning strike = energy of collisions is increased enough to overcome the activation energy barrier.

• allows nitrogen monoxide to be produced. The nitrogen monoxide formed in this process can then react to form brown nitrogen dioxide (NO)

importance of collision orientation?

• Reacting molecules must also collide with each other in the correct orientation in such a way that particular bonds in the reactants are broken and new bonds are formed in the products.

• In the decomposition of HI gas into H gas and I gas, two HI molecules must collide with H and I atoms orientated towards each other for a reaction to possibly occur.

• orientation is incorrect= particles simply bounce off each other, and no reaction occurs.