Factors Effecting Reaction Rate

Activation Energy A certain amount of energy must be available to “propel” the molecules from one chemical state to another This energy barrier is called the Activation Energy (Ea)‏ Arrangement of atoms that has this maximum energy is called the activated complex

Energy Profile

Collision Theory In order to react molecules and atoms must touch each other. They must hit each other hard enough to react. Anything that increase these things will make the reaction faster.

Energy Reactants Products Reaction coordinate

Activation Energy - Minimum energy to make the reaction happen Reactants Products Reaction coordinate

Activated Complex or Transition State Energy Reactants Products Reaction coordinate

Energy Reactants Overall energy change Products Reaction coordinate

Things that Effect Rate Temperature - Higher temperature faster particles. - More and harder collisions. - Faster Reactions. Concentration - More concentrated closer together the molecules. - Collide more often. - Faster reaction.

Things that Effect Rate Particle size - Molecules can only collide at the surface. - Smaller particles bigger surface area per volume. - Smaller particles faster reaction.

Orientation Colliding alone is not enough to guarantee reaction Relative orientations of the molecules may determine if the energy gets to the right place or if they are suitably oriented to form the new bonds As temp increases, the number of collisions increases as does the fraction that are oriented correctly

Things that Effect Rate Catalysts- substances that speed up a reaction without being used up.(enzyme). - Speeds up reaction by giving the reaction a new path. - The new path has a lower activation energy. - More molecules have this energy. - The reaction goes faster. Inhibitor- a substance that blocks a catalyst.

Energy Reactants Products Reaction coordinate

Homogenous Catalyst Catalyst present in the same phase as the components of the reaction Example: NO acts as a catalyst for the decomposition of ozone NO provides another mechanism for the reaction which has a lower Ea

Heterogeneous Catalyst Exists in a phase different from reactants Catalyzed reactions of gases often occur at the surface of metals Places where reacting molecules are absorbed on the metal surface are called active sites

Catalysts Hydrogen bonds to surface of metal. Break H-H bonds Pt surface

Catalysts H H C C H H H H H H Pt surface

Catalysts C C The double bond breaks and bonds to the catalyst. Pt surface

Catalysts C C The hydrogen atoms bond with the carbon Pt surface H H H

Catalysts H H C C H H H H H H Pt surface

Example A reaction occurs in a two step mechanism as shown A + C → B + D (slow) B + A → C + F (fast) What substance acts as a catalyst? What substance is an intermediate? Write the overall reaction equation. Write the rate law for the overall reaction.