The Entropies of n -Butane and Isobutane, with Some Heat Capacity Data for Isobutane

The entropies of n-butane (S298° = 75.8 e.u.) and isobutane (S298° = 70.0 e.u.) have been calculated from thermal data by the third law of thermodynamics. In this connection, heat capacity data for isobutane over the temperature range 79°—261°K have been obtained by the Nernst method and also, as a partial basis for these calculations, the mode of extrapolating entropies below liquid air temperatures has been critically examined. The discrepancies between these new results and the corresponding entropies obtained by statistical methods appear too large to be accounted for by errors in the thermal data or extrapolations involved. The thermodynamics of the isomerization reaction is briefly considered.