A student prepares a deep-red solution by mixing aqueous solutions of 0.0020 M Fe3+…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
A student prepares a deep-red solution by mixing aqueous solutions of 0.0020 M Fe3+ and 0.0020 M SCN− so that the equilibrium
Fe3+(aq) + SCN−(aq) ⇌ FeSCN2+(aq)
is established at 25 °C. After equilibrium is reached, the student slowly adds 100 mL of distilled water to the 100 mL sample while keeping the temperature constant and preventing loss of material to the surroundings.
(a) Write the expression for the equilibrium constant, K, for the reaction.
(b) Compare the total number of dissolved particles on the left side of the balanced equilibrium equation with the total number on the right. State the numerical count for each side.
(c) Using your answer to part (b), explain the direction in which the equilibrium will shift immediately after the water is added. Clearly reference changes in concentration and the need for the system to re-establish equilibrium.
(d) Predict whether the intensity of the red color due to FeSCN2+ will increase, decrease, or remain unchanged after the new equilibrium is reached. Justify your prediction in terms of the concentration of FeSCN2+.
(e) Identify one additional qualitative observation (other than color intensity) that could be monitored experimentally to confirm the direction of the shift you predicted in part (c), and explain how that observation is linked to concentration changes in the solution.