Chemical Reactions Factors Affecting Rates of Reaction January 9, 2015

WARM-UP: Identify the type of reaction below: a.2 Na + Cl 2  2 NaCl b.Zn + 2 HCl  ZnCl 2 + H 2 c.PbO 2  Pb + O 2 d.2 H 3 PO Ca(OH) 2  Ca 3 (PO 4 ) 2 + H 2 O a.CaCl 2 + 2AgNO 3  Ca(NO 3 ) 2 + 2AgCl a.CH O 2  CO H 2 0 SYNTHESIS S.R. DECOMP D.R. COMB.

Collision Theory Molecules must… Collide with each other hard enough to react Be in a favorable orientation In a successful collision, existing bonds are broken and new bonds formed while reactants become products

Collision Theory Con’t REACTION RATE: Informally, reaction rate is the speed at which a chemical reaction takes place Formally, the reaction rate is the change in concentration of reactants or products over time

Factors that Affect Reaction Rates 1. TEMPERATURE: Increasing temperature means the particles move faster Particles collide more frequently and with more energy Reactions occur faster

Factors that Affect Reaction Rates 2. CONCENTRATION: Increasing concentration means the molecules are closer together Particles collide more frequently Reactions occur faster

Factors that Affect Reaction Rates 3. PARTICLE SIZE: Molecules can only collide on the surface Smaller particles have a larger surface area Reactions occur faster Dissolving speeds up a reaction Smallest particles are molecules or ions

Factors that Affect Reaction Rates 4. CATALYSTS: A catalyst is a substance that speeds up a reaction without being used up Example: enzymes Increases the number of collisions Reduces the activation energy The minimum energy needed for a reaction to occur

Activation Energy: the minimum amount of energy required to make a reaction proceed forward Catalysts speed up reactions by lowering activation energy Activation Energy

Check for Understanding 1.According to “Collision Theory”, what two conditions must be fulfilled for a reaction to occur? 2.How does increasing temp cause the reaction to occur faster? 3.Which has a larger surface area, a single stick of butter or butter cubed into small pieces? Which would cause a reaction proceed faster? 4.What is the activation energy?

Check for Understanding ANSWERS 1.The molecules must collide with enough strength to react and be in a favorable orientation 2.It speeds up the molecules and they collide more frequently 3.Cubed butter because it is smaller in size; cubed butter will react faster 4.Minimum amount of energy required for a reaction to occur

Reaction rate can be stated as: 1. The decrease in the concentration of reactants with time or 2. The increase in the concentration of products with time Reaction Rate

In the reaction: A --> B 1. A is the reactant and B is the product 2. Notice that the concentration of A decreases over time and B increases REACTANTS: Minimum # moles at 60 min Maximum # moles at 0 min PRODUCTS: Minimum # moles at 0 min Maximum # moles at 60 min EQUILIBRIUM: Approximately 24 min (both reactants and products have 0.5 moles) REACTION RATE

Energy Reaction coordinate Reactants Products

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

Energy Reaction coordinate Reactants Products Activated Complex or Transition State

Energy Reaction coordinate Reactants Products Overall energy change

An Endothermic Reaction (Energy Absorbed) Energy ReactantsProducts  CaCO 3 (s) CaO (s) + CO 2 (g) +176 kJ

An Exothermic Reaction (Energy Released ) Energy ReactantsProducts  C + O 2 CO kJ

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

Energy Reaction coordinate Reactants Products Activation Energy - Minimum energy to make the reaction happen Activated Complex or Transition State PE of the Reactants

Energy Reaction coordinate Reactants Products Activation Energy - Minimum energy to make the reaction happen Activated Complex or Transition State PE of the Reactants PE of the Activated Complex

Energy Reaction coordinate Reactants Products Activation Energy - Minimum energy to make the reaction happen Activated Complex or Transition State PE of the Reactants PE of the Activated Complex PE of Products

Energy Reaction coordinate Reactants Products Activation Energy - Minimum energy to make the reaction happen Activated Complex or Transition State PE of the Reactants PE of the Activated Complex PE of Products Heat of Reaction (∆H)

Energy Reaction coordinate Reactants Products Activation Energy Activated Complex or Transition State PE of the Reactants PE of the Activated Complex PE of Products Heat of Reaction (∆H) PE of the Reverse Reaction