chemistry collision theory applcations (part 1 )

Collision theory chemistry model :
The collision theory chemistry model of the theory of chemical reactions can be used to explain the observed rates for the two laws in one stage and multi-step reactions. Collision theory chemistry model assumes that the rate of any stage of a reaction depends on the frequency of collisions between the particles involved in this step.

The figure below provides a basis for understanding the implications of the collision theory chemistry of simple collision model, a stage of reactions, such as the following

ClNO2(g) + NO(g) <----> NO2(g) + ClNO(g)

The kinetic molecular theory assumes (related theory to collision theory chemistry) that the number of collisions per second in a gas depends on the number of particles per liter. The rate of NO ₂ and ClNO which are formed in this reaction should be directly proportional to the concentration of NO and two ClNO₂

Rate = k(ClNO₂)(NO)

The collision theory chemistry model suggests that the rate of one stage of reaction is proportional to the concentration of reagents used in this step. The rate of duty for a reaction time must therefore agree with the stoichiometry of the reaction .

Example on collision theory chemistry model :

The following reaction, for example, occurs in a single step.

CH3Br(aq) + OH-(aq) <----> CH3OH(aq) + Br-(aq)

When these molecules collide (as collision theory chemistry stated) in the right direction, a pair of nonbonding electrons on the OH-ion can be donated to the carbon atom at the center of the CH3Br molecule, as shown below

When this occurs, a carbon-oxygen forms together with the carbon-bromine bond is broken. The net result of this reaction is the substitution of an OH-ion for a Br - ion. Because the reaction occurs in one step, dealing with collisions between the two reagents, the rate of this reaction is proportional to the concentration of reagents

and the rate (as posted in collision theory chemistry) :

Rate = k(CH₃Br)(OH^-)

Not all reactions occur in a single step. The following reaction occurs in three steps, as shown in the figure below.

(CH₃)₃CBr(aq) + OH^-(aq) (CH₃)₃COH(aq) + Br^-(aq)

In the first step, the (CH₃)₃CBr molecule dissociates into a pair of ions.

First step

The positively charged (CH₃)₃C⁺ ion then reacts with water in a second step.

Second step

The product of this reaction then loses a proton to either the OH^- ion or water in the final step.

Third step

The second and third steps in this reaction are very much faster than first.

(CH3)3CBr (CH3)3C+ + Br-		Slow step
(CH3)3C+ + H2O (CH3)3COH2+		Fast step
(CH3)3COH2+ + OH- (CH3)3COH + H3O		Fast step

The rate of the first step is therefore more or less equal to The overall rate of reaction

The first step is also called rate-limiting step in this reaction . Because only one reagent is involved in the rate-limiting step, the overall rate of reaction is proportional to the concentration of only this reagent ; so the rate-limiting step literally limits the rate at which the products of the reaction can be formed .

the summary for collision theory chemistry rate

Rate = k((CH₃)₃CBr)

The stoichiometry of the reaction predicts different rate law for this reaction . Although the reaction consumes both (CH₃)₃CBr and OH^-, the rate of the reaction is only proportional to the concentration of (CH₃)₃CBr.

The following general rules explains The rate laws for chemical reactions in chemistry collision theory :

1. At any step in a reaction The rate is directly proportional to the concentrations of the reagents consumed in that step.
2. The sequence of steps, or the mechanism, by which the reactants are converted into the products of the reaction determining The overall rate law for a reaction .
3. The rate law for the slowest step in the reaction dominates The overall rate law for a reaction .