KINETICS OF REACTIONS 

Learning Objectives

Upon completion of this experiment, students will have experienced:

1. 1. 1. A kinetic study of concentration, temperature and a catalyst on reaction rates.

      2. The derivation of rate law expressions.

      3. An investigation of a reaction mechanism.

 Discussion

Determination of the Rate Law and the effect of concentration on the Rate of reaction:

The rate law of a reaction provides the relationship of the rate the reaction to its dependence on the concentration of the reactants.   If a table of initial concentrations of each reactant and the resulting rate of the reaction is created, then the relationship between how the change in concentration affects the rate of the reaction can help determine the dependence of each reactant in the mechanism of the reaction, which leads to a rate law expression giving the orders or coefficient of the reactants in the rate determining step of the reaction.  

 For a simple reaction in which 2 A + B = 3 C, in general, the rate law can be written as  Rate = k [reactant A]n [reactant B]m, where n and m represent the stoichiometric coefficient of each reactant needed for the reaction to proceed in the rate determining step of the reaction mechanism.  These numbers are NOT the same as the coefficients in the stoichiometrically balanced chemical reaction but are related to the mechanism by which the reaction will occur.  

To determine these values we will explore the effect the concentration of each reactant has on the rate of the reaction.  If the reactant does not change the rate, then it is not necessary within the mechanism for the reaction to proceed and is said to be zero order.  Any value raised to the zero power is always equal to 1.  If the rate is directly proportional to the change in the concentration, then the reactant is first order and its stoichiometric amount within the mechanism is 1, meaning only 1 molecule of the reactant is needed.   For example in a first order reaction, the rate doubles when the concentration doubles.  If the rate increases more than the concentration then the order of the reaction is greater than 1.  For example, rate quadruples when the concentration doubles; therefore, 2n = 4 and n must be 2. 

The determination of the rate law with changing concentration is determined by the "Method of Initial Rates" which sets up a table initial concentration versus initial rate.  

Determination of reaction mechanism:

The reaction we will explore in this lab is the reaction of iodide ion with persulfate ion:

                       2 I1-   + S2O82- =  2 SO42-  +   I2

According to this reaction, stoichiometrically two iodide ions collide simultaneously with one peroxydisulfate ion for the reaction to proceed.  According to collision theory, a three body collision is improbable.  A two body collision is more probable to occur which means this reaction most likely occurs in a series of steps called the mechanism.  

When determining the mechanism of a reaction, several rules are always followed. 

1. 1. The mechanism is usually made of simpler reactions that when combined together are represented by the balanced chemical reaction,

   2. one of the steps of the reaction is slower than all of the other steps, therefore it is called the "rate determining step".  This step is so slow that the rate of the overall reaction is dependent only upon this one step and, therefore,

   3. the rate law is directly related to the stoichiometric quantities of each reactant present in the “rate determining step”.  

The rate expression then for any step of a reaction will be proportional to the product of the concentrations of the reacting species each raised to a power given by the coefficient of the species in the reaction step. 

Temperature Affects: 

Temperature will increase the rate of reactions.  Remember the kinetic molecular theory which states that as temperature increases, the kinetic energy and velocity of the particles increase proportionately, which means the reactants moving faster will tend to collide more often and will have greater effectiveness, resulting in the reaction occurring more often in a shorter period of time.  However, a second consideration to the kinetic molecular theory must also be considered.  Two species colliding will not always result in the correct product or even a change, the correct reactants must collide in the correct orientation, with sufficient energy to allow for bond breakage.  The third is quantified by the activation energy of the process, the second is called the frequency factor, A.  The activation energy can be determined from the Arrhenius equation and a study of the reaction rate constant as a function of temperature.

Catalyst Affects: 

Catalysts are substances that markedly increase the reaction rate by providing alternate or lower energy pathways to the products.  Catalysts participate in the reaction but are always returned to the reaction mixture in the same form.

Pre laboratory Writing Assignment

Complete the pre laboratory writing assignment in your laboratory notebook.   The prewriting must be completed before the laboratory session begins.    Use the procedure document below to explain what has to be done to collect the necessary data of the lab.   The writing should include a purpose/hypothesis of the experiment, a discussion or theory, a detailed step by step summary of the procedure explaining what data will be collected and how you will collect it.   Data Tables or space to collect the data should be included within the procedure discussion.   Look up any SDS on chemicals that you are not familiar.    

We will use class data sheet to explore real data collected in previous semesters to see the effect of concentration, temperature and catalysts on the rate of the reaction.  

Part I. Initial rates and a study of the effect of concentration on rate.

Calculate the concentration and average rate for all seven trials to determine the rate law of the reaction.

Using the provided data, determine the affect of a catalyst on the reaction

Part II. Activation Energy and Temperature effects on reaction rates.

Using the class data, determine the activation energy of this reaction.

Method 2: Concentration of Iodine by Spectrometry   

We will use class data sheet to explore real data collected in previous semesters to see the effect of concentration, temperature and catalysts on the rate of the reaction.  

Part I. Initial rates and a study of the effect of concentration on rate.

Calculate the concentration and average rate for all seven trials to determine the rate law of the reaction.

Using the provided data, determine the affect of a catalyst on the reaction

Part II. Activation Energy and Temperature effects on reaction rates.

Using the class data, determine the activation energy of this reaction.