Researchers from the Moscow Institute of Physics and Technology and colleagues in Saudi Arabia are developing an efficient way to get fundamental data on substances in the gaseous state. Their numerical protocol has been used to predict the thermal effect of gas-phase formation of silver compounds and their absolute entropy. Details of the data for more than ninety such compounds can be found in their paper. [Minenkov, Y. et al., Inorg. Chem. (2019); DOI: 10.1021/acs.inorgchem.9b00556] and could point the way to practical applications for such materials in water treatment, wound disinfection, film photography, and perhaps even the seeding of rain clouds for drought-stricken areas.
Knowing the enthalpy and entropy of formation of a material can be used to predict with the material in question will form at a given temperature and pressure from reactants at particular concentrations. It allows reactions to be optimized and in particular show how thin film and pure sample deposition from the gas phase might be carried out efficiently and effectively. Enthalpy and entropy values are usually found in reference books for a limited range of substances or calculated from other experimental values. Of course, you cannot always experimentally measure the heat of some reactions explains MIPT's Yury Minenkov nor rely on published data. "For example, incomplete graphite combustion always yields both carbon monoxide and carbon dioxide. So even by measuring the thermal effect of the reaction, we could not determine the enthalpy of carbon monoxide formation."
Quantum chemistry can come to the rescue though, by treating each constituent molecule of a gas as a system of positively charged nuclei and negatively charged electrons, theoreticians can apply electronic structure calculation methods to solve Schrödinger's equation for the molecule. This gives them the total electronic energy of the molecule, its wave function, and the spatial configuration of nuclei, its three-dimensional geometric structure. Assuming an ideal gas, the entropy and enthalpy values can then be calculated. Atomization, bond making and bond breaking lead to deviation from the ideal and so in the real world, such theoretical calculations do not necessarily yield values that would be corroborated by experiment.
The team has looked at silver sulfide and obtained its enthalpy of formation from the reaction with hydrochloric acid, which yields silver chloride and hydrogen sulfide. The heats of formation for silver chloride, hydrogen sulfide, and hydrochloric acid are known to a high degree of accuracy so and computer modeling can then reveal the thermal effect of the reaction. From these data, one can derive the heat of formation of silver sulfide using Hess' law. Tests with ten compounds with known values showed that their approach worked work and so they extended it to
The team began by checking how well the results of their quantum chemical calculations agreed with the experimental thermodynamic and structural data. Reference books reported the values for ten silver compounds, and they were a good match. Having thus undertaken a proof of principle, the team can be relatively confident of the values they then generated for ninety silver compounds missing from the references books.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase. His popular science book Deceived Wisdom is now available.