During a chemical reaction, reactant bonds need to be broken so that the atoms can rearrange to form the products.  The kinetic energy of all the particles (the molecules, compounds, elements, atoms and ions) reacting is responsible for the breaking of these bonds.  Remember, the temperature of the particles is a measure of the average kinetic energy of all the particles.

During a chemical reaction, reactants collide with each other, as well as, the walls of the container.  The rate at which a particular reaction occurs is dependent upon 2 criteria:

1)  the frequency of collisions

2)  the fraction of collisions that are effective (collision has sufficient energy and correct orientation in space so that bonds break and a rearrangement of the particles can occur)

-in general, smaller and simpler particles have a higher chance of having an effective collision

This can be expressed mathematically;

Rate  =  frequency of collisions    X    fraction of collisions that are effective

Therefore, increasing either factor will increase the rate.

THINK-PAIR-SHARE:  What category do the factors of concentration, surface area, temperature, nature of reactant and catalyst fall under in the rate equation given above.  Write them under the factor you think they belong to above.

Activation Energy, Ea

The activation energy is the minimum energy required for an effective collision to occur.

Particles could potentially collide with the correct orientation but with an insufficient amount of kinetic energy which results in an ineffective collision.  Generally, the lower the activation energy needed the faster the reaction and the greater the rate of the reaction.

What is the activated complex?

-the unstable chemical species that results from the reactants colliding which consists of partially broken and partially formed bonds.  It represents the maximum potential energy point in the chemical change (hence it is unstable). At this point, the chemical reaction can either proceed to the product side or reverse back to the reactant side.

Draw the potential energy diagram of an endothermic reaction and of an exothermic reaction.  Label the reactants, products, activated complex and the enthalpy change or ΔH.  Briefly describe what is occurring in each diagram.

Read pages 392 to 395 and summarize the information.

Reaction Mechanisms

According to our knowledge of the collision theory, it is logical to assume that

reactions occur in smaller steps.  For example, consider the following example;

WX  +  Y  +  Z   ->  XYZ

The chance of 3 particles (X, Y and Z) simultaneously colliding with each other with sufficient energy and with the correct orientation to break and form new bonds is highly unlikely and this likelihood decreases with an increasing number of particles involved in a reaction.  Yet, not all reactions that involve many particles proceed slowly.

It is reasonable to conclude then, that chemical reactions occur in a sequence of smaller steps called elementary steps. These elementary steps involve only one, two, or three-particle collisions.  The entire sequence of elementary steps is called the reaction mechanism and adds up to a final net reaction (or target equation).  This would be similar to the Hess’ s Law questions in which you calculated enthalpy change from a series of intermediate steps.

Any products which are formed during any elementary step which are not present when the reaction is complete are called reaction intermediates. The slowest step in the mechanism is the rate-determining step.  The rate-determining step determines the overall rate of the reaction.  In other words, the final products of the overall reaction cannot appear faster than the products of the slowest step.  This step will normally have a high EA on a potential energy diagram.

The elementary steps are best guesses at how the particles are arranging themselves, however, when proposing a mechanism, make sure to follow these rules;

1)  each step must be elementary

2)  the elementary steps must add up to the target equation

3)  the slowest step, called the rate-determining step, must be consistent

with the rate equation

(exponents in the rate law = coefficients of reactants)

i.e.  rate = k [WX]2 [Y]1

Therefore, the rate-determining step must be;

2WX  + 1 Y -> products or reaction intermediates

Example of a Reaction Mechanism:

The following reaction is second order with respect to NO2 and zeroth order with respect to CO.

CO  +  NO2 ->  CO2 +  NO

i)  Write the rate law for this reaction.

Rate = k [ NO2 ]2

ii)  Write a rate mechanism for this reaction.

NO2 +  NO2 ->  NO3 +  NO         (slow)

NO3 +  CO  ->  NO2 +  CO2 (fast) 