Thermochemistry - calorimetric determination of the dissociation energy of hydrogen peroxide.

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THERMOCHEMISTRY:  CALORIMETRIC DETERMINATION OF

THE DISSOCIATION ENERGY OF HYDROGEN PEROXIDE

 

 

 

PRELAB ASSIGNMENT

 

PROBLEMS:

 

1.        What is the heat of formation ΔHf of O2(g) at 1 atm?

 

2.        Using Hess’ Law and the heats of reaction given in Table 1, determine the ΔH for the following reaction:

 

        O2(g) + H2(g)    H2O(g)        

 

3.        Using Hess’ Law and the heats of reaction given in Table 1, determine the HO–OH         bond energy.


I.        PURPOSE AND SUMMARY

        In this experiment, you will determine the molar heat of dissociation ΔHdis of the HO-OH bond in hydrogen peroxide.

 

        H2O2(g)    2OH(g)          ΔHdis  =  D (HO–OH)        (1)

 

For this reaction the heat of dissociation is equal to the HO–OH bond energy.  Since it is difficult to measure the heat of this reaction directly, you will use Hess’ Law to determine ΔHdis.  Hess’ Law states that since the enthalpy is a function of state, it does not matter how the final state is obtained.  In other words, the final state is independent of the path taken.  Hess’ Law is exceedingly useful for it allows us to calculate enthalpy changes which are difficult to measure directly by combining results of experiments which are performed easily.  You will measure the molar heat of decomposition of H2O2 and use the heats of reaction given in Table 1 to determine the HO–OH bond energy.

 

Table 1.  Thermodynamic Quantities

 

 

II.        INTRODUCTION

        Adiabatic calorimetry will be used in this experiment to determine the molar heat of decomposition ΔHdec of hydrogen peroxide.  Adiabatic means that there is no heat flow out of the system.

 

        H2O2(aq)    H2O(l) + O2(g)         ΔHdec        (2)

 

In adiabatic calorimetry, the heat change that is caused by the reaction of interest is confined to a reaction vessel called a calorimeter.  The calorimeter is well insulated so that it cannot exchange heat with the surroundings.  To measure ΔHdec occurring at a given temperature T0, it is convenient to use a path composed of two steps:

 

        H2O2(aq, T0) + Sys(T0)    H2O(l, T1) + O2(g, T1) + Sys(T1)                            ΔHA        

                (3)

        H2O(l, T1) + O2(g, T1) + Sys(T1)    H2O(l, T0) + O2(g, T0) + Sys(T0)          ΔHB        

 

Sys represents those parts of the system that are always at the same temperature as the reactants or products because of the experimental design, for example, the inside wall of the calorimeter, the stirrer, the thermometer, and the solvent.  Since ΔH is a function of state and independent of the path taken:

 

        ΔHdec  =  ΔHA + ΔHB        (4)

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The first step is carried out adiabatically so by definition ΔHA = 0 and therefore

 

        ΔHdec  =  ΔHB        (5)

 

        If we assume that Cp is temperature independent, then ΔH for the system is given by

 

        ΔHB  =  ΔHdec  =  Cp(T1 – T0)  =  CpΔT        (6)

 

ΔHB can be calculated as

 

        ΔHB  =  Cp(Sys)ΔT + Cp(H2O)ΔT + Cp(O2)ΔT        (7)

 

The contributions to the change in heat from the products of the reaction (H2O and O2) are small compared with the contribution due to the calorimeter.  The last two terms can be ignored and

 

        ΔHB  =  Cp(Sys)ΔT        (8)

 

 

By measuring ...

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