Investigate the rate of reaction of luminol in various factors. The objective was to study the effects of chemiluminescence in several factors: concentration of luminol solution, concentration of hydrogen peroxide and temperature. Rate equation can be fou

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Investigate the rate of reaction of luminol in various factors

Objective

        The objective was to study the effects of chemiluminescence in several factors: concentration of luminol solution, concentration of hydrogen peroxide and temperature. Rate equation can be found by analysing the data.

Background theory

        Luminol can emit light by reacting with hydrogen peroxide and metal catalyst, which H2O2 acts as an oxidising agent. This is an oxidation reaction, but it is not a redox reaction, because only luminol is gaining 2 O atoms. H2O2 oxidises luminol to a dianion, which is very unstable, and the electrons can move along O=CN--N-C=O. The dianion then reacts with O2, substitute the N from it, and release N2. The dianion is at excited state, releasing energy and fall to the ground state. The energy released can be calculated by e=hv, where h is the Planck’s constant and v is the frequency of the colour observed.

 

        After colourless hydrogen peroxide is added to the light blue luminol solution, the mixture glowed in blue, and then it is decayed to a yellowish brown solution. Bubbles are evolved in the process.

        In the experiment, the reactants are only hydrogen peroxide and luminol solution, although the luminol solution contains more than one chemical(Sodium carbonate Na2CO3, Sodium hydrogen carbonate NaHCO3, Ammonium carbonate (NH4)2CO3 and Copper(II) sulphate CuSO4  5H2O are in the luminol solution). Moreover, the products did not involve in the rate of reaction. The word and chemical equations of the reaction are concluded as follow(3):

Luminol solution + Hydrogen peroxide                 ground state dianion + nitrogen + hv

        Luminol (aq) + H2O2 (aq)                ground state dianion(aq) + N2 (g) + hv

                   + 3H2O2 (aq)                                         + 4H2O (l) + N2 (g) + hv

        The rate of reaction measures how fast the reactants are all reacted. Certain factors would affect the rate of reaction, they are temperature and concentration of different solution, catalysts and surface area.

        Concentration would affect the reaction rate, which can be explained by collision theory(1), increasing the concentration, there are more reactant molecules present in the same volume of solution, and collisions of reactant occur more frequently, therefore the reaction rate increase.

        Temperature would also affect the reaction rate, as the temperature higher, there is more energy in each reactant molecules, they have faster speed for reaction, and as a result, more collisions occur. This can be explained by the Maxwell-Boltzmann distribution(2).

        Catalysts are substances help to lower the activation enthalpy, using an alternative route. They are also not used up after the reaction. For luminol reactions, the catalysts used can only be metal catalysts, e.g. CuSO4  5H2O and K4[Fe(CN)6](4).

        The smaller the substance, the larger the surface area, which means it has more site for reaction to take place. However, this factor is not applicable in this investigation, it is because I am using solution throughout the investigation.

        

        The rate equation indicates each the effects of changing each reactant, for a general reaction        A + B                    Product, the rate equation is given by:

Rate = k [A]m[B]n

                where [A] and [B] are the concentration of the reactants respectively; k is the rate constant for the reaction, which varies with temperature; m and n are the order of reaction with respect to A and B respectively, and they usually are 0, 1 or 2.

        In this case, A was represented by luminol solution and B was hydrogen peroxide. The rate equation would be:

Rate = k [Luminol]m[H2O2]n

                where m and n were to be found.

        The experiment will be investigated further, and splitting into 3 parts, with varying 3 factors: concentration of luminol, concentration of hydrogen peroxide and temperature. The experiment is generally measuring the chemiluminescence of luminol, i.e. how long does it glow in these conditions? The dependent variable is time and the independent variables are concentration of luminol, concentration of hydrogen peroxide and temperature respectively. To develop fair tests, when investigating 1 of the independent variables, the remaining 2 have to remain constant. For example, when investigating the concentration of hydrogen peroxide, the concentration of luminol solution and the temperature must remain the same throughout the investigation.

        A sample has made to determine the end point of the reaction, and to give a approximation of time to the reaction. The time is measured at the instant when the hydrogen peroxide is added, and stopped until the mixture decayed to the same colour as the sample.

        There are 2 methods to find the order of reaction with respect to each reactant: using concentration-time graph and using rate-concentration graph. In this case, the easiest way to find the order is using concentration-time graph, and I would also perform the method using rate-concentration graph, to verify my result.

  1. Using concentration-time graph

        Plot concentration-time graph, i.e. concentration in the x-axis and time in the y-        axis. Draw a line of best fit. If the concentration-time graph gives a straight line,         that reactant has zero order; if the graph gives a curve with constant half-life, that         reactant has first order; if the graph gives a curve but not constant half-life, the         reactant has second order.

  1. 2, Using rate-concentration graph

        Draw tangent for each of the concentration in the concentration-time graph, the         gradient of each tangent represents the initial rate of each concentration. Plot rate-        concentration graph, i.e. rate in the x-axis and concentration in the y-axis. Draw a         line of best fit. If the graph gives a horizontal line, the reactant has zero order; if         the graph gives a straight line, decreasing from one end, the reactant has first         order; if the graph gives a curve, the reactant has second order.

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Variables and predictions

        For Investigation A, the rate of reaction on varying the concentration of H2O2, I am going to use a range of concentration, from 0.03M to 0.3M, total 10 sets of results. The 2 dependent variables, which will stay constant, are temperature and the concentration of luminol solution. I am going to use 100% luminol solution throughout this investigation; and the temperature will be room temperature, although it is varying everyday, it is still assumed constant. Since more H2O2 molecules are in 0.3M, more luminol molecules will be ...

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