Chemistry 14BL Fall 2004 Report Guidelines for the Kinetics Study of Red F D C 3 with Bleach Please follow the instructions in the report section of the experiment handed out in class A copy of the experiment begins on the following page Fall 2004 1 THE REACTION OF RED FOOD COLOR WITH BLEACH TECHNICAL AND THEORETICAL SKILLS In this assignment you will monitor the rate of a chemical reaction prepare a volumetric solution determine the extinction coefficient of a compound use a spreadsheet to analyze data calculate the order and rate constant of a reaction SAFETY FD C Red 3 or Erythrocin B sodium the food coloring dye that you will use in this experiment is intensely colored Always wear safety glasses or goggles and a protective lab coat or apron Handle the dye carefully avoiding spilling on the balance or bench top Sodium hypochlorite household bleach is a bronchial irritant Keep solutions covered or in the hood avoid breathing the vapors Immediately wipe any spills of either reagent and wash any residues with large quantities of water INTRODUCTION In this assignment you will study rate of the reaction of FD C Red 3 Red 3 with sodium hypochlorite Since this reaction is very visible you will use a spectrophotometer to quantitatively follow the rate of disappearance of the colored reagent Your rate data will allow you to determine the rate law and to propose a possible mechanism for the reaction I I NaO O O I C I COONa FD C 3 Because of the extended conjugation within the molecule the absorption occurs in the visible region of the spectrum at 500 nm When the dye reacts with hypochlorite the color disappears A possible explanation of this reaction is that the bleach oxidizes the central methylene carbon to an alcohol The resulting triphenylmethanol derivative with the sp3 methane carbon would no longer have the extended conjugation system and the absorption of the less conjugated product would occurs at a lower wavelength outside of the visible region of the spectrum Schematically we can represent this reaction as shown on the next page Fall 2004 2 This stoichiometry however does not indicate how the reaction occurs or the mechanism of the reaction A study of how the concentrations of the reactants affect the rate of the reaction gives insight into the mechanism and whether this simple explanation is correct O NaO O OClI I I C O NaO O I I I I I HO COONa COONa Red Extended Conjugation Colorless Isolated Areas of Conjugation The Rate Law The rate of a reaction can be represented either by the disappearance of reactants for the appearance of products Since Red 3 is the only colored species in the reaction we can monitor the rate of the reaction shown above by recording the decrease in the color of the solution with time That is Rate d Red 3 b a k Red 3 OCl dt 1 where the exponents a and b indicate the molecularity or order of the reaction with respect to each reagent and k is the overall rate constant for the reaction at room temperature The objective of this experiment is to determine the values of the exponents a and b and the value of k at room temperature If the concentration of the bleach is held constant throughout a reaction by having a large excess present then the rate law simplifies to Rate d Red 3 a Red 3 dt 2 b where OCl has been absorbed in the pseudo rate constant Rearranging this rate expression to d Red 3 dt 3 a Red 3 gives a form that can be integrated depending on the value of a If a 1 the integrated expression becomes ln Red 3 t and a plot of ln Red 3 vs time will give a straight line with a slope of Fall 2004 3 4 Fall 2004 4 If however a 2 the integrated expression becomes 1 t Red 3 5 and a straight line occurs with a plot of 1 Red 3 vs time Thus if one can experimentally determine the concentration of the dye at various times during a reaction the relationship of that concentration with time that gives a linear fit allows an experimenter to establish the molecularity of the reaction with respect to the dye that is a value for a A second set of rate data collected for reactions where the concentration of the dye is held constant and the initial excess concentration of the bleach is changed in a simple ratio between trials can then allow a determination of a value for b b For example since the pseudo rate constant k OCl if OCl is doubled between trials and the also doubles then b must 1 On the other hand if OCl is doubled and the observed rate increases by a factor of 4 then b 2 However if OCl is doubled between trials and there is no change in the observed rate of the reaction then b 0 and it can be concluded that the bleach is not involved in the rate determining steps of the mechanism PRE LAB ASSIGNMENT 1 Calculate the molarity of a 6 15 w v sodium hypochlorite NaOCl solution 2 Calculate the molarity of a solution of Erythrocin B MW 879 9 g mol if a 0 5028 g sample is diluted to 100 mL in a volumetric flask and then two serial dilutions are carried out in which 5 mL aliquots are diluted to 100 mL using volumetric equipment 3 The half life of a substance that is being consumed by a first order rate reaction is the time A required for the concentration to fall to one half of the initial value Show that for At o 2 the equation 0 693 A ln o k t t o becomes t 1 2 At k where Ao is the amount at time 0 At is the amount at time t and t1 2 is the time required for the concentration to fall to one half the initial value 4 In a few brief sentences explain why it is necessary to mix the solution very rapidly when the bleach is added the purpose of the blank solution how the shapes of the plots dye vs time ln dye vs time and 1 dye vs time are used to determine the order of the reaction PROCEDURE The TA will assign you into a group with 2 other students For each rate reaction trial one person will be the timer one person will be the data recorder and one person will read out the absorbances as indicated by the timer You should rotate roles between rate runs Before you Fall 2004 5 begin work agree upon who will be responsible for each part of the work and the report Record this in your notebooks Sign each of the notebooks indicating the agreement You will turn in one report with all the group member names on it and the allocation of responsibility Solution Preparation You will need to prepare several solutions to carry out the necessary rate runs It is most …
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