Iron(III) ion (Fe3+) reacts with thiocyanate ion (SCN) to form a red complex ion with the formula [FeSCN2+. The net ionic equation describing this reaction is given in Equation 10.
Fe3+ (aq, It. yellow) + SCN-(aq, colorless) = (FeSCN]2+ (aq, red) (Eq. 10)
A student studying this equilibrium begins with an equilibrium mixture that is light pink.
(1) What change will the student observe when a solution containing Fe+ ion is added to this mixture?
The Correct Answer and Explanation is :
When a solution containing Fe³⁺ ions is added to the equilibrium mixture of the reaction
[
Fe^{3+} (aq, \text{yellow}) + SCN^- (aq, \text{colorless}) \rightleftharpoons [FeSCN]^{2+} (aq, \text{red}),
]
the student will observe a shift toward the formation of more red [FeSCN]²⁺ complex. This occurs due to a principle known as Le Chatelier’s Principle, which states that if a system at equilibrium is disturbed by adding more of one of the reactants, the system will adjust to counteract that disturbance by shifting the equilibrium position to favor the side that consumes the added component.
In this case, the student starts with a light pink solution, which indicates the presence of both Fe³⁺ and [FeSCN]²⁺, though in relatively lower concentrations. Upon adding more Fe³⁺, the system will respond by using the added Fe³⁺ to produce more of the red [FeSCN]²⁺ complex. As a result, the solution will become darker red in color as the concentration of the red complex increases.
This shift happens because, according to the equation, Fe³⁺ and SCN⁻ react to form [FeSCN]²⁺. Increasing the concentration of Fe³⁺ increases the rate at which the red complex is formed, thus leading to a deeper red solution. The equilibrium shifts to the right, meaning more [FeSCN]²⁺ is produced until a new equilibrium is reached.
In summary, when Fe³⁺ is added to the equilibrium mixture, the student will see a color change toward a darker red. This happens because the system shifts to produce more [FeSCN]²⁺ in response to the increased concentration of Fe³⁺, according to Le Chatelier’s Principle.
Now, I will generate an image showing the colors of the reactants and products of this equilibrium.
The image above illustrates the equilibrium of the reaction, showing the transition from yellow Fe³⁺ and colorless SCN⁻ ions to the red [FeSCN]²⁺ complex. As more Fe³⁺ ions are added, the solution shifts towards a deeper red color, reflecting the formation of more [FeSCN]²⁺, in line with the explanation I provided earlier.