18. Enthalpy is the a. heat of formation at constant pressure. b. change in energy of a system at constant pressure. heat of combustion at constant pressure. C. d. energy of a system at constant pressure. 30. According to the postulates of the kinetic theory of gases, the root-mean-square (rms) speed of the molecules of a given gas is proportional to the a. absolute temperature. d. b. reciprocal of the absolute temperature. e. square root of the absolute temperature absolute temperature squared. C. Celsius temperature squared.