Question

1. Copper makes beautifully colored compounds and polyatomic ions. Answer the questions based on the following equations:

                        Cu(OH₂)₆²⁺(aq)   +   4Br⁻(aq) ⇌ CuBr₄^2-(aq) +   6H₂O(l)

                         blue color                                  bright green

(a) The addition of AgBr(s) to the mixture above causes no change. However, when NaBr(s) is added a noticeable color change occurs. Predict the color change and explain it in terms of LeChatelier’s principle.

(b) Why did the two different solids added in (b) have two different effects?

2. Cave formations like Blanchard Springs Caverns, Arkansas, are very sensitive to atmospheric CO₂ levels. The pertinent reaction is

                                    CaCO₃(s, rock) + CO₂(g) + H₂O(l) ⇌ Ca²⁺(aq) + 2HCO₃⁻(aq) ΔH° = -39.9 kJ

(a) If atmospheric CO₂ levels begin to fall, what effect is this predicted to have on the rock formations in these caves?

(b) Under what conditions, hot or cold, are the rocks presumed to be more soluble?

3. Consult the Figure 14.12 of Tro. p 683.

(a) Sketch a Lewis dot structure for the NO₂ molecule. Refer to your dot structure, and use it to explain why two NO₂ molecules dimerize to make N₂O₄.

(b) Bond formation always releases energy. Is the following equation consistent with this statement? Explain.

2NO₂(g) ⇌ N₂O₄ (g) + heat

(c) When it’s really cold in Little Rock, the sky can look clear and blue. The same sky in July might look brown and gross. In both cases the many cars in the city are producing significant amounts of nitrogen oxides. Describe these observations in terms of LeChatelier’s principle.

Answer 1

1) The reaction taking place is

Cu(H₂O)₆²⁺ (aq, blue color) + 4 Br^- (aq) <=====> CuBr₄^2- (aq, bright green) + 6 H₂O (l)

The equilibrium constant is

K = [CuBr₄^2-]/[Cu(H₂O)₆²⁺][Br^-]⁴ (H₂O is not included in the expression since it is present in large excess).

The equilibrium constant K depends only on the temperature of the system.

(a) When NaBr (s) is added to the above system at equilibrium, the green color of the solution intensifies. This is in accordance with the Le Chatilier principle.

NaBr(s) dissociates in aqueous solution to Na⁺ (aq) and Br^- (aq) ions. Therefore, the concentration of Br^- increases. This increases the denominator in the expression for K above. However, temperature remaining constant, K must remain constant. This is possible only when the numerator in the above expression increases. Therefore, [CuBr₄^2-] increases and hence the green color of the solution turns intense. In other words, the addition of NaBr(s) favors the forward reaction.

NaBr (s) ------> NaBr (aq) ------> Na⁺ (aq) + Br^- (aq)

(b) The addition of AgBr (s) produces no effect on the equilibrium system described above while the addition of NaBr (s) drives the reaction to the product side. This is due to the fact that AgBr is sparingly soluble in water and hence, doesn’t dissociate to produce Br^- ions, unlike NaBr hich completely ionizes in aqueous solution.