Recently, a two-dimensional model that leads to …rst order phase transitions has been proposed. This model also leads to phase diagrams which present the characteristics observed in phase diagrams of the substances : three thermodynamic phases, three coexistence curves, a triple point and a critical point. In the present study, the calculation of the latent heats of the di¤erent …rst order phase transitions as well as the experimental data concerning the phase diagram of a pure substance, here carbon dioxyde, are introduced into the two-dimensional model. By assuming that the pressure does not modify the chemical bond between two nearest-neighbors atoms but modi…es that betwen two next nearest-neighbors atoms we can calculated the phase diagram of the compound in (bar; Kelvin) plane and the Joule values of the di¤erent latent heats of transition. Moreover, the energy value of the chemical bond between two nearest-atoms is obtained.