The reaction you mentioned involves the equilibrium between Fe3+, SCN− (thiocyanate), and the complex ion FeSCN2+, which is responsible for the color changes observed in this system.
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Initial State: At equilibrium, the solution appears orange due to the presence of the FeSCN2+ complex ion, which has a characteristic orange color.
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Effect of Adding Fe3+: When Fe3+ ions are added to the system, the equilibrium will shift to the right (forward direction), increasing the concentration of the FeSCN2+ complex. This might temporarily cause an increase in the intensity of the orange color or enhance the overall orange hue, but it wouldn’t necessarily indicate a definitive shift away from equilibrium.
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Effect of Removing SCN−: Removing SCN− ions from the system will shift the equilibrium to the left (reverse direction) to compensate for the decrease in SCN− concentration. This shift would lead to a reduction in the concentration of the FeSCN2+ complex, resulting in a decrease in the orange color, potentially making the solution appear paler or more yellow, indicating a shift from orange.
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Effect of Removing FeSCN2+: If FeSCN2+ is removed (e.g., through precipitation or reaction with another species), this would also shift the equilibrium to the left, resulting in a similar color change toward pale yellow as the concentration of the FeSCN2+ complex decreases.
In summary, a color change from orange to pale yellow indicates a shift from equilibrium, specifically when SCN− is removed or when the FeSCN2+ complex is removed. This shift suggests that the reaction is favoring the formation of the reactants (Fe3+ and SCN−) at the expense of the product (FeSCN2+).