Topic 10, Thermodynamics & Kinetics
Topic 10, Thermodynamics and Kinetics
(Key concepts)

Potential Energy Diagrams
 By plotting potential energy along the y axis and the reaction coordinate along the x axis, one can illustrate the pathway of endothermic and exothermic reactions.

An endothermic reaction:

(1) represents the PE of the reactants
(2) represents the activation energy
(3) represents the heat of reaction (delta H)
(4) represents the PE of the products
This is the characteristic shape of a potential energy diagram for an endothermic reaction.  The products contain more energy than the reactants; the difference is absorbed. 

An exothermic reaction:

This is the characteristic shape of the potential energy diagram for an exothermic reaction.  The reactants contain more stored energy than the products; the difference is given off. 

The addition of a catalyst, lowers the activation energy, increasing the rate of reaction.

Rate of Reaction

Rate of reaction is measured by the amount of reactants turned into products in a unit of time. 

In order for a reaction to take place, reacting particles (atoms, molecules, ions, etc.) must collide with proper orientation and sufficient energy.  Therefore, anything that changes the frequency and or efficiency of collisions also changes the rate of reaction.

There are five important factors or conditions that influence rate of reaction:

1.  Nature of reactants: Ionic(aq) react faster than covalent; ions already separated (dissociated); fewer bonds to break; more collisions between reactant particles
2.  Concentration, [  ]: The greater the concentration of reactant particles, the faster the rate of reaction; more collisions 
3.  Temperature, T: The higher the temperature, the faster the rate of reaction; the particles collide with greater frequency and with greater effectiveness
4.  Surface area, SA: The greater the surface area, the faster the rate of reaction; powdered zinc has more surface area than the same amount of lump zinc; increased surface area increases collision frequency
5.  The addition of a catalyst: Catalysts speed up the rate of reaction by lowering the activation energy needed to initiate the reaction; catalysts remain unchanged by the reaction; may be used over and over again