An aqueous cobalt(II) complex exists in equilibrium according to the reaction…
CHEMISTRY · Multi-part · A Level · Chemistry · Chemical equilibria (Le Chatelier’s principle and Kc) · Chemical equilibria (Le Chatelier’s principle and Kc)
Problem
An aqueous cobalt(II) complex exists in equilibrium according to the reaction
[Co(H2O)6]2+(aq)+4Cl−(aq)⇌[CoCl4]2−(aq)+6H2O(l)ΔHfwd=+50kJmol−1
At 25 °C the equilibrium constant is K25=4.0×10−3. A chemist prepares 100.0 mL of a solution that is 0.10 M in [Co(H2O)6]2+ and 0.10 M in Cl−. Water is the solvent and its concentration is effectively constant.
The solution undergoes the following sequence:
1. Heating to 40 °C, where K40=8.0×10−3.
2. Addition of 5.00 mL of 12.0 M HCl (which increases [Cl−] but negligibly changes the total volume).
Answer the questions below. Assume ideal behavior and that activities equal concentrations.
(a) Calculate the equilibrium concentrations of [Co(H2O)6]2+ and [CoCl4]2− at 25 °C before any stresses are applied.
(b) After heating to 40 °C but before any chloride is added, determine the equilibrium concentrations of the two cobalt species at the new temperature.
(c) Immediately after the HCl addition (before the system re-equilibrates), write the numerical value of the reaction quotient Q. Then calculate the final equilibrium concentrations of both cobalt species at 40 °C.
(d) Compute the ratio [[CoCl4]2−]initial[[CoCl4]2−]final and identify which of the two stresses—temperature change or chloride addition—had the larger impact on product formation. Justify your answer quantitatively.