Collision Theory

OUTCOME QUESTION(S): C12-3-06 REACTION RATES Vocabulary & Concepts Formulate an operational definition of reaction rate and solve rate problems. Include: average rate, instantaneous rates Relate the rate of formation of a product to the rate of disappearance of a reactant given experimental rate data and reaction stoichiometry. Use the Collision Theory to explain the rate of chemical reactions. Include: Activation energy Vocabulary & Concepts activated complex/transition state

If the particles do not collide, no reaction occurs. The Collision Theory states: In order for a chemical reaction to occur, the reacting particles must collide with each other. If the particles do not collide, no reaction occurs. Contact is the first step; there are other 2 conditions that must be met…

Just like a key – hold it the wrong way and it can’t open the lock 1. Particles must collide with correct orientation. Just like a key – hold it the wrong way and it can’t open the lock

2. Particles must collide with enough energy. Energy is required to break / form bonds Activation Energy (EA): The minimum energy needed for a reaction to happen. Particles have potential energy stored in bonds Particles have kinetic energy in speed of motion Fast moving particles smash together with enough energy to break old bonds and form new ones If Total Energy (KE + PE) ≠ Activation Energy NO REACTION

Some particles come will low energy, some with high… The frequency of successful collisions determines the rate of a reaction. This is called a “bell curve” and is common when a property is measured in a large population

+ At point of collision: Particles form an unstable intermediate particle called the activated complex or transition state. This transition state is the point of maximum potential energy and exists for a very small time + Potential Energy Kinetic Energy Kinetic Energy Kinetic Energy Kinetic Energy

ΔH = Hproducts – Hreactants Enthalpy (H) is the total energy possessed by the particles in a system. Heat of reaction (ΔH) is the change in enthalpy (energy absorbed/released) during a reaction. ΔH = Hproducts – Hreactants Units of energy are Joules (J) ΔH : products have less enthalpy then reactants. Releases of energy – exothermic +ΔH : products have more enthalpy than reactants. Absorption of energy – endothermic

Reaction coordinate diagram or Potential energy (PE) diagram These diagrams represent the change in energy (as measured by PE) during a reaction

2 H2 + O2 → 2 H2O ΔH + activated complex EA reactants products Potential Energy Kinetic Energy Kinetic Energy Kinetic Energy

CH3CH2Br + OH− → CH3CH2OH + Br− This is a completely labelled diagram (too much)

REQUIRED: be sure to label - both axes, activation energy and enthalpy change

Greater the activation energy – the fewer molecules that can react – the slower the reaction rate.

CAN YOU / HAVE YOU? C12-3-06 REACTION RATES Vocabulary & Concepts Formulate an operational definition of reaction rate and solve rate problems. Include: average rate, instantaneous rates Relate the rate of formation of a product to the rate of disappearance of a reactant given experimental rate data and reaction stoichiometry. Use the Collision Theory to explain the rate of chemical reactions. Include: Activation energy Vocabulary & Concepts activated complex/transition state