The thermodynamic parameters of monomer and dimer molecules of cerium and praseodymium tribromides

The method of high-temperature mass spectrometry is used for studying the composition of saturated vapor over cerium and praseodymium tribromides. Monomer and dimer molecules are found in the temperature ranges of $789$–$994$ K and $804$–$957$ K for cerium and praseodymium, respectively. The partial pressures of vapor components are determined, $p$(Pa), the temperature dependences of which are approximated by the equations
$$\begin{gathered} \log p(\text{CeBr}_3 ) = ( - 14.63 \pm 0.08) \times 10^3 /T + (14.54 \pm 0.09),\quad T = 789 {-}994\text{ K};\\ \log p(\text{Ce}_2\text{Br}_6 ) = ( - 19.72 \pm 0.61) \times 10^3 /T + (17.60 \pm 0.64),\quad T = 918{-}980\text{ K};\\ \log p(\text{PrBr}_3 ) = ( - 14.13 \pm 0.12) \times 10^3 /T + (14.09 \pm 0.14),\quad T = 804{-}957\text{ K};\\ \log p(\text{Pr}_2\text{Br}_6 ) = ( - 18.90 \pm 0.50) \times 10^3 /T + (17.15 \pm 0.53),\quad T = 903{-}955\text{ K}. \end{gathered} $$
The values of pressure of vapor components are used along with literature data for the calculation of enthalpies of sublimation in the form of monomer and dimer molecules by the procedures of the second and third laws of thermodynamics. Based on analysis of the results, thermodynamic parameters of monomer and dimer molecules (in kJ mol$^{-1}$) are recommended,
$$ \begin{gathered} \Delta _s H^0 (\text{CeBr}_3 , 298.15) = 305 \pm 5,\quad \Delta _s H^0 (\text{PrBr}_3 , 298.15) = 293 \pm 5,\\ \Delta _s H^0 (\text{Ce}_2\text{Br}_6 , 298.15) = 410 \pm 28,\quad \Delta _s H^0 (\text{Pr}_2\text{Br}_6 , 298.15) = 403 \pm 28,\\ \Delta _f H^0 (\text{CeBr}_3 , \text{gas}, 298.15) = - 587 \pm 6,\quad \Delta _f H^0 (\text{PrBr}_3 , \text{gas}, 298.15) = - 597 \pm 7,\\ \Delta _f H^0 (\text{Ce}_2\text{Br}_6, \text{gas}, 298.15) = - 1372 \pm 28,\quad \Delta _f H^0 (\text{Pr}_2\text{Br}_6, \text{gas}, 298.15) = - 1378 \pm 28. \end{gathered} $$