Lab # 9

Rate Law Determination of the Crystal Violet Reaction

Name : Candice Joseph

Date : May 8 , 2007

1 . Purpose

The purpose of this experiment was to determine the of reaction with respect to crystal violet (CV ) for the reaction between crystal violet (CV ) and sodium hydroxide (NaOH

2 . Introduction

Crystal violet ion (CV ) is a purple colored dye commonly used in super market labeling . When this ion is made to react with hydroxide ion it forms a colorless compound , which can be written as CVOH . The reaction between crystal violet and

hydroxide ion is presented below The rate expression for this reaction is rate k[CV ]m[OH-]n (2

Where k rate constant , m is the of reaction with respect to CV and n is the of reaction with OH- . To experimentally determine of reaction (1 , with respect to one reactant (here CV concentration of the other reactant (here OH- ) is intentionally kept much larger than that of the other reactant . In this experiment concentration of OH- was kept 1000 times that of CV . Under this condition the rate equation (2 ) reduces to k[CV ]m (3

If this reaction is zero with respect to CV i .e . m 0 , then equation (3 ) reduces to k0 (4

If this reaction is 1st with respect to CV i .e . m 0 , then equation (3 ) reduces to k0[CV ] (5

If this reaction is 2nd with respect to CV i .e . m 0 , then equation (3 ) reduces to k0[CV ]2 (6

Using calculus , the rate equations (4 ) through (6 ) can be can be converted into linear forms . The linear form of the rate equations are presented in table 1 , below

Table 1 : Linear relationship between concentration of CV and time for reactions of different s (9 Here the subscript 0 and t for [CV ] refers to initial and instantaneous concentrations of CV respectively

All these rate equations from (7 ) through (9 ) are of y mx c form

If reaction is zero then plot of [CV ]t vs . t will be linear and slope of this line will be -k0

If reaction is 1st then plot of ln[CV ]t vs . t will be linear and slope of this line will be -k1

If reaction is 2nd then plot of 1 /[CV ]t vs . t will be linear and slope of this line will be k2

Beer 's Law states that absorbance of a solution is directly proportional to the concentration . Therefore , in equations (7 ) through (9 absorbance can be taken as proxy for concentration and if

Absorbance vs time plot is linear then reaction is zero with ko being negative slope of line

ln (Absorbance ) vs t plot is linear then reaction is 1st with k1 being negative slope of line

1 (Absorbance ) vs t plot is linear then reaction is 2nd with k2 being slope of line

In this experiment above three plots were made from the experimentally observed values of [CV ]t...