The Keq expression for the initial equilibrium is:
Keq = [FeSCN²⁺][NO₃⁻] / [Fe³⁺][SCN⁻]
How to write Keq?
The equilibrium reaction between Fe(NO3)3 and KSCN solutions involves the formation of FeSCN²⁺ ions:
Fe³⁺(aq) + SCN⁻(aq) ⇌ FeSCN²⁺(aq) + NO₃⁻(aq)
To write the equilibrium constant (Keq) for this reaction, we follow the general format:
Keq = [Products] / [Reactants] raised to their stoichiometric coefficients.
In this case, the products are FeSCN²⁺ and NO₃⁻, and the reactants are Fe³⁺ and SCN⁻. Their stoichiometric coefficients are all 1. Therefore, the Keq expression for the initial equilibrium is:
Keq = [FeSCN²⁺][NO₃⁻] / [Fe³⁺][SCN⁻]
Keq is a dimensionless constant, so the units of concentration for all the species must be consistent.