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

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Introduction

THERMOCHEMISTRY: CALORIMETRIC DETERMINATION OF THE DISSOCIATION ENERGY OF HYDROGEN PEROXIDE PRELAB ASSIGNMENT PROBLEMS: 1. What is the heat of formation DHf of O2(g) at 1 atm? 2. Using Hess' Law and the heats of reaction given in Table 1, determine the DH 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 DHdis of the HO-OH bond in hydrogen peroxide. H2O2(g) ? 2OH(g) DHdis = 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 DHdis. 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 Process (at 1.0 atm)____ DH (kJ/mol at 298 K) 1. H2O2(aq) ? H2O(l) + O2(g) DHdec 2. H2O2(l) ? H2O2(aq) -3.4 3. H2O2(l) ? ...read more.

Middle

Open the calorimeter and determine the concentration of the dilute sulfuric acid by titrating two 5 mL aliquots (use a pipet !) of the acid with standard 0.5 M NaOH using phenolphthalein as an indicator. 5. The temperature rise due to the addition of the sulfuric acid may be determined by plotting the temperature, T, against the time and extrapolating back to the time when the acid was added. Because of the time required to completely mix the solution some of the first temperature readings may be higher or lower than should be the case and should be disregarded when extrapolating the graph. Only the linear portion of the graph corresponding to a steadily decreasing temperature should be extrapolated back to zero time. The difference between the temperature of the water before the addition of the acid and the extrapolated temperature at t=0 will be the observed temperature change. (See Fig. 1, Expt. 2) 6. The quantity of heat evolved by the reaction of H2SO4 with water may be determined from the measured concentration of acid and the following data on the heat of solution of H2SO4, DHsol(H2SO4), given in terms of Joules per 100 mL of H2O or, in our case, total solution volume. DHsol(H2SO4) J/100 mL H2O Final Molarity of Solution -803 0.109 -1,010 0.139 -1,980 0.276 -3,280 0.459 Plot the data and, by extrapolation, find the heat corresponding to the molarity of the acid in the calorimeter. ...read more.

Conclusion

Enthalpy change = Mass of liquid x specific heat capacity x temperature change Enthalpy change = 50cm� x 4.2 x 8.75�c Enthalpy change = 1837.5 joules H� = - 1837.5 j mol Evaluation The main problem was that my experiment lost a lot of heat into the atmosphere and into the beaker glass. This meant that our calculations would have been very inaccurate as they didn't account for the energy lost as heat and through other methods. As our reactants weren't totally pure, we cannot say whether the masses recorded were correct. For example, when I measured out the calcium carbonate and the calcium oxide, not all of it was pure and I tried to remove some rocks, so I only crushed the reactants I needed. I had no problems with the equipment. There was also a problem with the hydrochloric acid as it may have been contaminated and it might not all have reacted the way it should have. Conclusion From my results I can tell that both reactions were exothermic, but that the reaction with calcium oxide was much more exothermic than the reaction with calcium carbonate. They were different because the bonds that were made in the calcium oxide reaction required less energy to be made than in the calcium carbonate reaction. My Hess' Law cycle can be labelled correctly. I have calculated one enthalpy change, but I would need more data to complete my cycle. I also need to find out how inaccurate my results are. studentcentral.co.uk ...read more.

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