For preparing a set of standard solutions of FeNCS2+, the equilibrium molar concentration of FeNCS2+ is assumed to equal the initial molar concentration of the (SCN-) in the reaction mixture. why is this assumption valid?
This assumption is valid because the reaction between Fe3+ and SCN- to form FeNCS2+ (also written as Fe(SCN)2+) has a very large equilibrium constant (K). This means that the reaction strongly favors the formation of the FeNCS2+ complex, and nearly all of the SCN- ion will react with Fe3+ to form the complex.
In this case, Fe3+(aq) + 2 SCN-(aq) <=> Fe(SCN)2+(aq)
Since the equilibrium constant is very large, we can assume that the reaction proceeds almost to completion. Therefore, the initial molar concentration of the SCN- ion would be the same as the molar concentration of FeNCS2+ in the equilibrium state. This assumption makes it easier to calculate and prepare the standard solutions required for the experiment, as well as simplifying the equilibrium calculations.
This assumption is valid for preparing a set of standard solutions of FeNCS2+ because the reaction between Fe3+ and SCN- to form FeNCS2+ is a very rapid reaction, and it reaches equilibrium quickly. The equilibrium molar concentration of FeNCS2+ is assumed to equal the initial molar concentration of SCN- because by assuming so, we simplify the calculation. This assumption allows us to prepare standard solutions with known concentrations of FeNCS2+ by simply measuring the initial concentration of SCN- and assuming it remains the same in equilibrium. This assumption is valid as long as the reaction between Fe3+ and SCN- is fast and reaches equilibrium within a short time.
The assumption that the equilibrium molar concentration of FeNCS2+ is equal to the initial molar concentration of SCN- in the reaction mixture is valid because FeNCS2+ is the product of the reaction between Fe3+ and SCN-.
In a standard solution preparation, Fe3+ is usually provided in excess, meaning that the initial concentration of Fe3+ is significantly higher than the initial concentration of SCN-. This ensures that Fe3+ is the limiting reactant in the reaction, and it completely reacts with SCN- to form FeNCS2+.
Once the reaction reaches equilibrium, all the available Fe3+ would have reacted with SCN-, and as a result, the equilibrium molar concentration of FeNCS2+ is equal to the initial molar concentration of SCN-. This is because all the SCN- that was initially present has reacted with Fe3+ to form FeNCS2+.
So, by assuming that the equilibrium molar concentration of FeNCS2+ is equal to the initial molar concentration of SCN-, we are assuming that the reaction has gone to completion, and all the available Fe3+ has reacted with SCN- to form FeNCS2+. This assumption allows us to accurately determine the concentration of FeNCS2+ in the standard solution.