Purpose

To show the effect of temperature on the equilibrium of NO2 and N2O4

Procedure

Follow the Flinn Procedure.

Reaction

• NO2 Equilibrium:
  2NO2(g)⇌N2O4(g)+N2O3(s)+NO(g)
• Colors: Brown (NO2) ↔ Colorless (N2O4, NO3)

Thermodynamic Data

• Standard Enthalpy (Δ H°): -57.2 kJ
• Standard Entropy (Δ S°): -175.83 J/mol-K

Standard-State Enthalpies of Formation and Entropies

Compound	(ΔH°f) (kJ/mol)	(ΔS°) (J/mol-K)
NO2	33.18	240.06
N2O4	9.16	304.29

Gibbs Free Energy and Equilibrium Constant

Temperature (°C)	(ΔG°) (kJ)	(Kp)
100	8.4	0.066
0	-9.2	58
-78	-22.9	1.3 x 10^6
-196	-43.6	3.7 x 10^29

Approximation

The temperature dependence of (ΔH°) is small, allowing for estimation of the equilibrium constant at various temperatures.

Cobalt Chloride Equilibrium

Overview

The equilibrium between [Co(H2O)6]²⁺ and [CoCl4]²⁻ in aqueous solution demonstrates the effect of concentration and temperature on chemical equilibrium.

Reaction Equation

[Co(H₂O)₆]²⁺ (aq) + 4 Cl⁻ (aq) ⇌ [CoCl₄]²⁻ (aq) + 6 H₂O (l)

Color Changes

• Pink: [Co(H₂O)₆]²⁺
• Blue: [CoCl₄]²⁻

Discussion

This demonstration visually illustrates reversible reactions and the effects of concentration and temperature on equilibrium, reinforcing Le Chatelier’s principle.

Apparatus and Chemicals

• Apparatus:
• Eye protection
• 6 Boiling tubes
• 1 or 2 Racks for boiling tubes
• 100 cm³ Measuring cylinder
• 3 Beakers (250 cm³)
• 2 Dropping pipettes
• Access to a top-pan balance
• Chemicals:
• Cobalt(II) chloride-6-water (CoCl2·6H2O) – 4.0 g (Toxic, Dangerous for the environment)
• Concentrated hydrochloric acid – 100 cm³ (Corrosive)
• Crushed ice – ~200 cm³

Safety Notes

• Refer to CLEAPSS Hazcards for CoCl2·6H2O and concentrated HCl.
• Avoid skin contact with cobalt(II) chloride and wash hands after use.

Procedure

Preparation

1. Boil water in a beaker and prepare crushed ice in a separate beaker.
2. Dissolve 4 g of CoCl2·6H2O in 40 mL of water to form a reddish-pink solution (label as TOXIC).

Demonstration Steps

1. Mixing Solution:

• Add 60 mL concentrated HCl to the pink solution to form a violet solution.
• Adjust with HCl or water to create an ‘in-between’ violet solution for six boiling tubes.

1. Effect of Concentration:

• Use one tube as a control.
• Add water to the second tube (changes to pink) and HCl to the third (changes to blue).
• Demonstrate reversibility by adjusting HCl and water.

1. Effect of Temperature:

• Use three tubes of violet solution, with one as control.
• Place one tube in hot water (turns blue) and another in ice/water mixture (turns pink).
• Show reversibility by swapping tubes.

Teaching Notes

• The color change of cobalt complexes forms the basis for cobalt chloride indicator papers and self-indicating silica gel desiccants.
• The reaction is endothermic; raising temperature shifts equilibrium right (more blue complex).
• Adding HCl increases chloride ion concentration, shifting equilibrium right; adding water decreases it, shifting left.
• An extension activity could involve using sodium chloride to observe equilibrium shifts.

Thionin Equilibrium

Purpose

To demonstrate the effect of light on an equilibrium.

Procedure

1. Follow the Flinn Procedure.
2. Cooling Setup:
   • Prepare a dry ice/alcohol slurry bath.
   • Cool each tube in the slurry, ensuring one tube is left in the bath longer than the other.
3. Warming:
   • After cooling, allow the tubes to warm back up to room temperature.