According to Le Chatelier's principle, if a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the system will adjust the equilibrium position to counteract the change.In this case, the reaction is:H2 + I2 2HIWhen hydrogen gas H2 is added to the system at constant temperature and pressure, the concentration of H2 increases. To counteract this change and re-establish equilibrium, the system will shift the equilibrium position to the right, in the direction that consumes the added H2. This means that more H2 and I2 will react to form more HI, increasing the concentration of HI in the system.So, the effect on the equilibrium position of the H2 + I2 2HI reaction when hydrogen gas is added to the system at constant temperature and pressure is a shift to the right, resulting in an increased concentration of HI.