Equilibrium Under Stress

\(\text{Reactants} + \mathbf{Heat} \rightleftharpoons \text{Products}\)

• • Increase Temp: Shifts Right (\(\rightarrow\)), \(K\) increases
• • Decrease Temp: Shifts Left (\(\leftarrow\)), \(K\) decreases

Exothermic (\(\Delta H < 0\))

Heat as Product

\(\text{Reactants} \rightleftharpoons \text{Products} + \mathbf{Heat}\)

• • Increase Temp: Shifts Left (\(\leftarrow\)), \(K\) decreases
• • Decrease Temp: Shifts Right (\(\rightarrow\)), \(K\) increases

Temperature is the ONLY stress that alters the numerical equilibrium constant \(K_{\text{eq}}\)!

Visualizing the Shift: Cobalt System

Lab Preview

Reversible Endothermic Complexation:

\(\underbrace{[\text{Co(H}_2\text{O)}_6]^{2+}}_{\text{\bf Pink Complex}} + 4\text{Cl}^- + \mathbf{Heat} \rightleftharpoons \underbrace{[\text{CoCl}_4]^{2-}}_{\text{\bf Blue Complex}} + 6\text{H}_2\text{O}\)

Ice Bath (\(0^\circ\text{C}\))

Heat removed → System shifts LEFT

Solution turns vibrant PINK

Hot Bath (\(70^\circ\text{C}\))

Heat added → System shifts RIGHT

Solution turns deep BLUE

Adding concentrated \(\text{HCl}\) (\(\text{Cl}^-\) ions) also shifts the reaction to deep blue!

Lab Readiness Check

Turn & Talk (2 Min)

Predict with Your Lab Partner:

Silver ions (\(\text{Ag}^+\)) react with chloride (\(\text{Cl}^-\)) to form an insoluble white precipitate: \(\text{Ag}^+ + \text{Cl}^- \rightarrow \text{AgCl}_{(s)}\).

Question: If \(\text{AgNO}_3\) is added to a blue tube of cobalt chloride:

1. Which chemical species is effectively removed from equilibrium?
2. Which direction will the system shift (Left or Right)?
3. What color change will you observe?

Put on safety goggles and prepare for the Stress Test Lab Beaker • Pipette • Well-Plate

System B Investigation

Equilibrium Reaction B: \(\underbrace{\text{Fe}^{3+}}_{\text{(pale yellow)}} + \underbrace{\text{SCN}^-}_{\text{(colorless)}} \rightleftharpoons \underbrace{[\text{Fe(SCN)}]^{2+}}_{\text{(deep blood red)}} + \text{Heat}\)

Part 4: Post-Lab Synthesis Questions

1. In Part 2, adding \(\text{AgNO}_3\) caused a cloudy white precipitate (\(\text{AgCl}_{(s)}\)) and a color change from blue back to pink. Explain this shift using Le Chatelier's principle and the reaction quotient \(Q\).

2. In Part 3, heating the \([\text{Fe(SCN)}]^{2+}\) solution in Well 5 made the red color fade significantly. Based on this observation, prove whether the reaction is endothermic or exothermic, and predict what happens to the value of \(K_{eq}\).

3. A student claims: "Adding extra \(\text{KSCN}\) in Well 3 increased the equilibrium constant \(K_{eq}\) because more red product formed." Refute or support this claim with thermodynamic reasoning.

Stress Test Lab Activity • Page 2 of 2 Dynamic Equilibrium • AP / Honors Chemistry

Graphical & Industrial Equilibrium

Concentration vs. Time & Industrial Optimization

Data Interpretation

3 Part 3: Interpreting Concentration vs. Time Curves

The graph below illustrates a closed system containing \(\text{N}_2(g) + 3\text{H}_2(g) \rightleftharpoons 2\text{NH}_3(g)\) that is perturbed at time \(t_1\) and again at time \(t_2\).

Concentration (M) Time (min) t₁ t₂ Equilibrium 1 Equilibrium 2 Equilibrium 3 [H₂] [N₂] [NH₃]

a. Identify the stress imposed at \(t_1\):

b. Why does \([\text{H}_2]\) decrease \(3\times\) faster than \([\text{N}_2]\) after \(t_1\)?

4 Part 4: Quantitative Shift Calculation

Consider the gas-phase decomposition: \(\text{COCl}_2(g) \rightleftharpoons \text{CO}(g) + \text{Cl}_2(g)\) where \(K_c = 0.083\) at \(400^\circ\text{C}\). At equilibrium in a 1.0 L flask, \([\text{COCl}_2] = 0.30\text{ M}\), \([\text{CO}] = 0.158\text{ M}\), and \([\text{Cl}_2] = 0.158\text{ M}\). Suddenly, \(0.10\text{ mol}\) of \(\text{CO}\) is injected. Calculate the new equilibrium concentrations once equilibrium is re-established.

5 Part 5: Chemical Engineering Application (Haber-Bosch Trade-Off)

The synthesis of ammonia is exothermic (\(\Delta H = -92.4\text{ kJ/mol}\)). According to Le Chatelier's principle, what operating temperature maximizes equilibrium yield of ammonia? Why do industrial chemical plants operate at a high temperature (\(450^\circ\text{C}\)) instead?

Predict The Shift Worksheet • Page 2 of 2 Dynamic Equilibrium • AP Chemistry