Theories of Reaction Rates

6.3

Factors that Affect Reaction Rate

Why does the rate of a reaction increase with• Increased concentration of reactants• Increased temperature• Increased surface area

Collision Theory In order for a reaction to occur,

reacting particles (atoms, molecule, or ions) must collide with one another.

If collision is necessary for a reaction to occur, then it makes sense that the rate of the reaction will increase if there are more collisions per unit time.

Collision Theory and Surface Area

Increasing surface area of a solid-phase reactant speeds up a reaction.

With greater surface area, more collisions can occur. • Starting a fire: small twigs rather than logs.

Beyond Collision Theory Not every collision

between reactants results in a reaction.

Collision must be EFFECTIVE:• One that results in the

formation of products.

Orientation of Reactants Collision geometry: the correct

orientation of reactants relative to one another.

Activation Energy(1) Reactants must collide with energy that

is sufficient to begin to break the bonds in the reactants and to begin to form the bonds in the products.

In most reactions, only a small fraction of collisions have sufficient energy for a reaction to occur.

ACTIVATION ENERGY, Ea: the minimum collision energy that is required for a successful reaction.

Activation Energy(2)

Graph shows the distribution of kinetic energy in a sample of reacting gases at two different temperatures, T1 and T2, where T2>T1.

Important Observations1) At both temperatures, a relatively small fraction

of collisions have sufficient kinetic energy – the activation energy – to result in a reaction.

2) As the temperature of a sample increases, the fraction of collisions with sufficient energy to cause a reaction increases significantly.

Transition State Theory Examines the transition, or change, from

reactants to products. Kinetic energy of reactants is transferred

to potential energy as the reactants collide: law of conservation of energy. • Basketball analogy: kinetic energy of ball

converted to potential energy, which is stored in the deformed ball as it hits the floor. The potential energy is converted to kinetic energy as the ball bounces away.

Potential Energy Diagrams Diagram that charts the potential

energy of a reaction against the progress of the reaction.

The ‘hill’: Ea barrier. Notice the difference between exothermic and

endothermic reactions. There is no way to predict the activation energy

of a reaction from its enthalpy change: a highly exothermic reaction may be very slow because of a high activation energy.

Activation energy is determined by analyzing the reaction rate.

Activation Energies of Reversible Reactions

For an exothermic reaction, the activation energy of the reverse reaction, Ea(rev) equals Ea(fwd) + ∆H.

For an endothermic reaction, Ea(rev) equals Ea(fwd) - ∆H.

Transition State `Change-over`point Activated Complex: chemical species that

exists at the transition state• neither product nor reactant• Partial bonds highly unstable.

Activated complex: can either break down to form products or decompose to re-form the reactants. • Like a rock teetering on top of a mountain can

fall either way.

Tracing a reaction with a Potential Energy Diagram

Sample Problem – Drawing a Potential Energy Diagram

Seatwork/Homework PPs, page 294, #13-16

Read page 295 and make brief notes

Section Review, page 296, #1- 8.