The structures of the reactants and products of synthesis of 4-nitrophenylarsonic acid were studied by using the B3LYP method of density functional theory. Then the structures were optimized by computational analysis. The changes in reaction enthalpy

Gibbs free energy and equilibrium constant for the primary and side reaction were calculated and analyzed with the basic thermodynamic data obtained from vibration frequency analysis. The simulation results show that the primary reaction process is exothermic and irreversible

the enthalpy change is -15.0 kJ·mol-1

the Gibbs free energy change is 39.7 kJ·mol-1. The side reaction process is endothermic and irreversible

the enthalpy change is 10.0 kJ·mol-1

the Gibbs free energy change is 45.2 kJ·mol-1. The computational studies on thermodynamics of 4-nitrophenylarsonic acid synthesis can provide the theoretical guidance to the reaction process condition control.