The heat capacity of crystalline hydrogen peroxide between 12" I<. and the melting point has been determined with a low temperature adiabatic calorimeter. T h e heat of fusion was also measured and found to be 2987 f 3 cal./mole.The two samples of hydrogen peroxide used were 99.97 mole % pure as deduced from behavior on melting and from premelting heat capacities; the triple point was estimated to be 272.74" I<.The only anomaly observed it1 the heat capacity measuren~ents was the absorption of 1.3 cal./mole a t 216.8 f 0.15" K., the lower eutectic temperature of H20-Hg02 solutions. Such an effect is to be expected if the only significant impurity is water. The entropy of hydrogen peroxide as an ideal gas a t 1 atm. pressure and 25" C. computed from the thermal measurements is 55.76 f 0.12 cal./mole deg. Comparison of this datum with the recalculated statistical entropy leads to a value of 3.5 kcal./mole for the height of a hypothetical single barrier hindering internal rotation in the molecule. From these results it is concl~~ded that hydrogen peroxide does not consist of two tauton~eric modifications.
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