Molal Volumes of the Isotopic Homologs of Ethylene

Abstract

The molal volumes of ethylene (C₂H₄), trans‐dideuteroethylene (C₂H₂D₂), and tetradeuteroethylene (C₂D₄) have been measured over the temperature range 105–175°K with a precision 10⁻²% (± 0.005cm³). The order of the molal volumes is $V_{C_2{H}_4}{\mathrm{\hspace{0.17em}>\hspace{0.17em}V}}_{C_2{H}_2{D}_2}{\mathrm{\hspace{0.17em}>\hspace{0.17em}V}}_{C_2{D}_4}$ . The difference in molal volume between ethylene and each of its deutero derivatives is approximately 0.05 cm³ mol⁻¹ per deuterium atom and decreases with temperature. The difference in molal volumes is compared with two limiting models. The “size effect” model, which is a consequence of the difference in lengths of the C–H and C–D bonds leads to the correct magnitude of the difference in molal volume, but predicts $\mathrm{\delta V\hspace{0.17em}/\hspace{0.17em}V}$ to be temperature independent for ethylene, contrary to the experimental findings. Calculations based on an anharmonic intermolecular potential show that the hindered rotation in liquid ethylene makes a significant contribution to the molal volume difference. Finally a correlation is made between the observed molal volume differences and the measured vapor‐pressure differences of the deuteroethylenes.