Equilibrium Calculations

Multiphase Multicomponent Phase Equilibria

The Equilibrium Module is the heart of the HSC program. HSC can calculate multicomponent, multiphase chemical equilibria for systems consisting of pure condensed phases, ideal gases, dilute aqueous solutions, and simple condensed phase solutions. The user specifies system reactants, potential reaction product species, temperature, and pressure. The equilibrium phases and their compositions are then calculated over intervals of temperature or amounts of input species. The results are then presented graphically as a function of the selected independent variable. Results can be plotted with linear or logarithmic scales and definable axes. The numerical data can be exported as text files for use in spreadsheets and other documents.

There are limits to HSC's computational capabilities. Pure condensed phases, ideal gases, ideal solutions, and dilute aqueous solutions are handled easily. More complex solutions can be handled if you know the activity coefficient. Equations of state and complex solution models cannot be utilized easily within HSC.

Here are simple examples of graphs for calculation of NO_x formation and for nickel deposition from the carbonyl process.

Reaction Equations

HSC takes the pain out of the calculation of enthalpy changes and equilibrium constraints associated with a chemical reaction. Simply type in the reaction and specify a temperature range and step. HSC will check the mass balance and calculate the changes in enthalpy, entropy, and Gibbs energy as well as the equilibrium constant at each temperature. A special electrochemical cell equilibrium calculation is also available for calculation of reversible cell potentials and phase compositions in electrochemical systems. For electrochemical reactions in aqueous systems, HSC will calculate the potential versus the standard hydrogen electrode. Results are displayed in a table that can be saved or printed.

Phase Stability Diagrams

In phase stability diagrams equilibria are plotted as a function of two potential variables such as temperature, pressure, or chemical potential. HSC calculates isothermal stability diagrams for 3-component systems in which the partial pressure of two gaseous species are chosen as independent variables. Here are examples for the Ca-O-S and the Pb-Zn-O-Ssystems.

Eh-pH Diagrams

Eh-pH diagrams are a special case of aqueous phase stability diagrams in which the independent variables are electrochemical potential and pH. These diagrams are critical in the understanding of corrosion, dissolution, leaching, and selective precipitation. Different colored phase boundary lines indicate regions of stability of water, solid phases, and predominant ions. The Criss-Cobble model is used for calculations at temperatures other than 25 °C. Black-and-white or color diagrams are produced with the click of a button. Here is an example for the Fe-H-Osystem.