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