Collision Theory and Activation

Energy

Reactant molecules MUST collide to produce a

chemical reaction The concentrations of reactants affect the # of

collisions among reactants For reactions occurring in one step—rate of

reaction is proportional to product of reactant concentrations Rate = k[A] [B]

Rate of any reaction step dependent on collision frequency

Collision Theory

1) Collision Rates between reactants

2) % of collisions with reactants arranged in proper orientation to produce reaction.

3) % of collisions with energy energy (activation energy) to produce reaction.

Variables Affecting Reaction Rate

Increase in concentrations of reactants

Temperature increases

WHY?

When do collision rates increase?

Small percentage of collisions actually convert

reactants to products. Why?

1) Molecular Orientation Random orientation Not all collisions have correct orientation

2) Molecular Energy at Collision Molecules have different kinetic energies Collision energy is energy source to get a reaction

started

Most collisions do NOT result in a chemical

reaction!

The amount of collision energy needed to

overcome Ea so the reaction can occur

Amount of energy needed for a chemical reaction to happen, energy needed to convert reactants to products.

Activation Energy (Ea)

Activation Energy--Exothermic

Activation Energy--Endothermic

1) MUST have a collision

2) Collision must happen with the correct molecular orientation to generate a reaction

3) Collision energy ≥ Ea

When will reactions occur?

HOW?

Temperature increases reaction

rate.

Rate constant and reaction rate are

temperature dependent.

Enables the activation energy for a reaction to be determined based on the relationship between reaction rate and temperature.

Arrhenius Equation

lnk = -Ea ( 1/T ) + lnA

R k = rate constant Ea = activation energy (J) R = 8.314 J/molK T = Kelvin Z = proportionality constant, changes based on reaction

Arrhenius Equation

Different form of equation can be used to

observe how temperature changes affect the rate constant (k)

ln (k1/k2) = Ea (1/T2 – 1/T1)

R

Arrhenius Equation

Calculate activation energy (Ea) for HI

decomposition with the following data.

Example 1

Temperature (K) Rate Constant (M/s)

573 2.91 x 10-6

673 8.38 x 10-4

773 7.65 x 10-2