Question
\(Co(H_2O)_6^{2+}(aq)+4Cl^−(aq)\)⇄\(CoCl_4^{2−}(aq)+6H_2O(l)\) ΔH>0
pink blue
A student poured 10mL of \(CoCl_2\)(aq) into a test tube and added a few drops of concentrated \(HCl\) , which resulted in a deep-blue solution. The reaction that occurred is represented by the chemical equation shown above. Then, the student placed the test tube inside a beaker that contained ice and water for about five minutes. Which of the following describes what the student most likely observed next, and why?
A The color of the solution changed from blue to pink, because lowering the temperature increased the collision frequency between \(CoCl_4^{2−}\) and \(H_2O\) .
B The color of the solution changed from blue to pink, because cooling caused the equilibrium to shift to form the pink-colored \(Co(H_2O)_6^{2+}\) .
C The color of the solution did not change, because more water was not added to the solution.
D The color of the solution did not change, because heat is not released from this reaction.
▶️Answer/Explanation
Ans:B
The forward reaction is endothermic; thus, cooling the test tube and its contents shifts the equilibrium toward the formation of the pink-colored \(Co(H_2O)_6^{2+}\).
Question
\(Fe^{3+}(aq) + SCN^−(aq)\)⇄\(FeSCN^{2+}\)(aq)
colorless colorless red-brown
The formation of \(FeSCN^{2+}\) in an aqueous solution is represented by the chemical equilibrium shown above. A light red-brown solution is prepared combining 12.50mL of 0.5M \(Fe(NO_3)_3\) , 0.5mL of 0.002M \(KSCN\), and 37.0mL of water that had been slightly acidified. If an additional 1.0mL of 0.5M \(Fe(NO_3)_3\) is added, which of the following predicts and explains correctly whether the darkness of the color of the solution will change?
A The color of the solution will lighten because most of the volume added is solvent.
B The color of the solution will darken because the equilibrium will favor the dissociation of \(FeSCN^{2+}\).
C The color of the solution will darken because the equilibrium will favor the formation of more \(FeSCN^{2+}\).
D The color of the solution will not change because the solution already contains \(Fe^{3+}\) ions.
▶️Answer/Explanation
Ans:C
Increasing the amount of a reactant in a solution at equilibrium will favor the formation of products. In this case, the addition of \(Fe^{3+}\) ions disrupts the equilibrium. As a result, the reaction favors the formation of \(FeSCN^{2+}\), and the solution will darken (will turn a darker shade of red-brown).
Question
A chemist carried out the reaction at 573K, starting with 0.100mol of each reactant in a 1.00L container with variable volume. The reaction mixture quickly reached equilibrium, as indicated in the graph above. After 20 seconds, the chemist reduces the volume of the equilibrium system by half while keeping the temperature constant. Which of the following predictions about the yield of \(NOCl(g)\) is best, and why?
A By halving the volume, the pressure doubles. The system will respond to the increase in pressure by decreasing the total number of moles of gas in the system. Thus, the yield will increase because the reaction will shift toward more product.
B By halving the volume, the pressure doubles. The system will respond to the increase in pressure by increasing the total number of moles of gas in the system. Thus, the yield will decrease because the reaction will shift toward more reactants.
C By halving the volume, the pressure is reduced by half. The system will respond to the decrease in pressure by increasing the total number of moles of gas in the system. Thus, the yield will decrease because the reaction will shift toward more reactants.
D By halving the volume, the pressure is reduced by half. The system will respond to the decrease in pressure by decreasing the total number of moles of gas in the system. Thus, the yield will increase because the reaction will shift toward more products.
▶️Answer/Explanation
Ans:A
In the balanced equation for the reaction, there are more moles of reactants than moles of products. As the reaction proceeds, the total pressure decreases. By halving the volume as temperature remains constant, the pressure increases. According to Le Chatelier’s principle, the forward reaction will then be favored, reducing the number of moles of gaseous particles, and thereby reducing the pressure. Given that the forward reaction will be favored, the yield of \(NOCl(g)\) will increase.
Question
\(2BaO_{2}(s)\rightleftharpoons 2BaO(s)+O_{2}(g)\) \(\Delta H^{\circ}=162kj/mol_{rxn}\)
A sealed rigid vessel contains \(BaO_₂\)(s) in equilibrium with Ba(s) and O(g) as represented by the equation above. Which of the following changes will increase the amount of BaOy(s) in the vessel?
(A) Removing a small amount of O(g)
(B) Removing a small amount of BaCs
(C) Adding He gas to the vessel
(D) Lowering the temperature
▶️Answer/Explanation
Ans:D