Separating Temperature and Chloride Effects in a Cobalt Equilibrium
Students prepare a 0.10 M CoCl2 solution in a water–ethanol mixture. At room temperature the solution is pink. Adding 12 M HCl dropwise turns it blue. A second pink sample placed in a 60 °C water bath also turns blue, and cooling it in ice returns it to pink. A student writes: 'The reaction is exothermic because heating makes it blue. HCl must be a catalyst because it also makes the blue color without being used up.' The teacher has a spectrophotometer, thermostatted cuvette holder, 530 nm and 690 nm filters, and stock solutions of CoCl2, HCl, and NaCl. The relevant equilibrium is Co(H2O)6^2+ (pink) + 4 Cl− ⇌ CoCl4^2− (blue) + 6 H2O. The teacher wants students to separate the effects of temperature and chloride concentration before writing a final explanation.
1. Identify the thermodynamic and mechanistic errors in the student's statement. Use the equilibrium expression and the sign of ΔH.
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The student has the sign of ΔH reversed and mislabels HCl. For Co(H2O)6^2+ + 4Cl− ⇌ CoCl4^2− + 6H2O, heating shifts the system toward the blue CoCl4^2− side. That means the forward reaction absorbs heat, so ΔH > 0 (endothermic). An exothermic forward reaction would be shifted toward reactants by heating, giving more pink. HCl is not a catalyst: it supplies Cl−, a reactant on the left, so adding it shifts the equilibrium right and increases blue CoCl4^2−. A catalyst would change the rate of approach to equilibrium, not the final equilibrium position.
2. Design a controlled experiment using the available equipment to separate the effect of temperature from the effect of chloride concentration. Specify variables held constant and the measurement.
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For the temperature effect, prepare several sealed cuvettes of the same composition, for example 0.10 M CoCl2 with 0.50 M NaCl, and equilibrate them at 5, 25, 45, and 60 °C in the thermostatted holder. Measure absorbance at 690 nm (blue) and 530 nm (pink) after a fixed equilibration time; keep Co total, path length, and wavelength constant. For the chloride effect, prepare samples at 25 °C with the same Co concentration but different Cl− concentrations (0, 0.20, 0.50, 1.0 M added NaCl), then measure the same absorbances. This separates temperature from chloride as variables.
3. How would absorbance data strengthen or refute the student's claim? Include the direction of change expected.
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If the forward reaction is endothermic, raising temperature at fixed Cl− should increase A690 and decrease A530. If HCl acted only as a catalyst, adding a small amount would not change the final absorbance after equilibration; it would only shorten the time needed to reach it. If adding Cl− increases A690 at fixed temperature, that supports a reactant role for chloride. Comparing the two sets of curves lets students assign the temperature and concentration effects separately.