Chemistry: Rate Equation Essay

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Kinetics of the Crystal Violet-Hydroxide Reaction[1]
Frederick Wight
Manatee Community College

Factors Influencing the Reaction Rate

Several factors can influence the rate at which a reaction occurs. Two of these are the concentration of the reactants and the temperature at which the reaction is performed. Most often an increase in the concentration of the reactants also increases the rate of reaction. Likewise an increase in temperature will increase the rate of the reaction in accordance with the activation energy (Arrhenius Equation). In this lab we will examine both factors to determine the order of a reaction and the activation energy.

The Rate Law and Order of Reaction

Since the reaction rate is often dependent on the concentrations of the reactants the rate law is generally written in terms of these quantities.

For the reaction:
A + B → C

the rate law is:
Rate = k [A]x[B]y

Where: k is the rate constant,
[A] is the concentration of A x is the order with respect to A
[B] is the concentration of B y is the order with respect to B

Determining the order of a reaction requires measuring the concentration of one of the reactants or products as a function of time. We will use 2 different methods to find the rate law: the method of initial rates and the graphical method.

Determining the Order of the Reaction by the Method of Initial Rates

One method of determining the order of reaction is to perform two experiments in which the initial reaction rate is measured at different concentrations of one reactant. If only the concentration of A is changed between these two experiments the change in rate was caused only by the change in A and the order of the reaction with respect to A can be determined. experiment #1: Rate1 = k1 [A]1x [B]1y

experiment #2: Rate2 = k2 [A]2x [B]2y

Since only [A] is changed k1 = k2 and [B]1 = [B]2. Taking the ratio of the two equations we get:

Rate1 = [A]1x
Rate2 [A]2x

Comparing the two rates and two concentrations allows us to calculate x.

Determining the Order of the Reaction by the Graphical Method

A second method of determining the order is by measuring the concentration of one reactant during the course of a single experiment in which all other factors are kept as constant as possible. This is often accomplished by using a large excess of the other reactants. During the course of the reaction only a very small portion of the excess reactant is consumed and its concentration remains relatively constant.
The order of the reaction with respect to the other reactant can be determined by plotting one of the following: [A] vs. time, ln [A] vs. time or [A]-1 vs. time. A reaction which is zero order with respect to A will give a linear plot if [A] is plotted vs. time. A plot of the ln [A] vs. time yields a straight line if the reaction is first order with respect to A. Second order with respect to A is indicated by a linear plot of [A]-1 vs. time. See Table below.

|Reaction Order |Linear plot |
|0 |[Conc.] vs. time |
|1st |ln([Conc.]) vs. time |
|2nd |1/[Conc.] vs. time |

Description of Experiment
We will study the rate of the reaction between hydroxide ion and the dye crystal violet (CV). CV is purple in color while the other reactants and products are colorless. Therefore, if we measure the rate of disappearance of the purple color, we know the rate of disappearance of the CV.

[pic] Crystal Violet, CV (purple) (Colorless)

You will determine the rate law for this reaction:

Rate = k[CV]x[OH]y

You will measure the absorbance of the reaction mixture as a function of time using a spectrophotometer which measures the intensity of the purple color. A spectrophotometer measures the amount of light absorbed by a solution as a…