Reaction Kinetics

Reaction Kinetics Chapter 17

Collision Theory • In order for reactions to occur between substances, their particles must collide • Collision theory: the set of assumptions regarding collisions and reactions • There are three possible outcomes for collisions with only one resulting in a reaction

Three Outcomes • Only the first one results in a reaction

Reasons why a reaction will not occur • The collision is not energetic enough to supply the required energy • The colliding molecules are not oriented in a way that enables them to react with each other

Activation Energy • Activation energy: the minimum energy required to transform the reactants into an activated complex • Activated complex: a transitional structure that results from an effective collision and that persists while old bonds are breaking and new bonds are forming

Reaction Rate • Reaction rate: the change in concentration of reactants per unit time as a reaction proceeds • The rate of reaction depends on the collision frequency and the collision efficiency

Factors that Influence Reaction Rate • Anything that increases the collision rate increases the rate of the reaction • Concentration/pressure • Surface area • Temperature • Presence of a catalyst • Stirring or agitation

Concentration/Pressure • Increase the concentration of the reactants and more collisions will occur • Increasing the pressure for reacting gases is like increasing the concentration

Surface Area • Especially important in a reaction that involves substances in phases that do not mix, like a solid with a liquid, or a gas with a liquid • An increase in the surface area in contact will increase the collision rate

Temperature • Increasing the temperature causes more collisions • more importantly, it increases the proportion of molecules with enough energy to overcome the activation energy • The average energy of the particles is proportional to the temperature in Kelvins

Catalyst • Catalyst: a substance which, when present in relatively small amounts, increases the rate of a chemical reaction, but which is not consumed during the reaction • A catalyst provides a new reaction pathway with a lower activation energy • This means that a greater number of collisions will have the required energy to react

Stirring or Agitation • Stirring or agitation allows more collisions to take place, higher reaction rate

Reaction Kinetics