Calculate the standard enthalpy difference between the cis and trans isomers of 2-butene. Specify which stereoisomer is more stable. The heats of formation are, for the cis isomer, 27.40 kJ mol^–1, and for the trans isomer, 211.6 kJ mol^–1 (21.77 and 22.72 kcal mol^–1, respectively).