Final answer:
The species in the reaction Fe3+ + SCN⇌ FeNCS2+ are iron(III) ions which are yellow to brownish, colorless thiocyanate ions, and the FeNCS2+ complex which is red. Adding more Fe3+ shifts the equilibrium to the right, producing more red FeNCS2+ and darkening the color of the solution.
Step-by-step explanation:
In the chemical reaction Fe3+ + SCN⇌ FeNCS2+, the species involved have the following colors: Iron(III) ions (Fe3+) are typically yellow to brownish in color in solutions, the thiocyanate ion (SCN-) is colorless, and the iron(II) thiocyanate complex (FeSCN2+) has a characteristic red color.
When more Fe3+ is added to the reaction, according to Le Chatelier's Principle, the equilibrium will shift to minimize the stress caused by the increase in Fe3+ concentration. This will result in an increased rate of the forward reaction, shifting the equilibrium to the right and producing more FeNCS2+. Consequently, the solution's color will become a darker red as more of the red FeNCS2+ is formed.