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Equilibrium of NO2/N2O4

Last updated

Aug 15, 2020
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- Equilibrium of Cobalt complexes
- Equilibrium—Iron thiocyanate

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves

Equipment	Chemicals
Sealed 250 mL bulbs of Nitrogen Dioxide
800 mL Beakers
Utility Clamps
Band Clamps to fit Beakers

Procedure:

Four glass tubes are sealed with NO₂ gas

The first tube is placed in a hot water bath
The second in ice-water
The third in flask with dry-ice/acetone
The fourth in liquid nitrogen

NO₂ is brown and N₂O₄ is colorless. The intensity of the brown color decreases as the temperature decreases. Therefore, a decrease in temperature yields an increase in N₂O₄. Equilibrium is shifted to the N₂O₄ side upon a decrease in temperature.

Discussion:

Brown 2 NO₂ (g) ⇌ N₂O₄ (g) Colorless

The standard enthalpy (∆H_rxn = -57.2 kJ) and the entropy (∆S_rxn = -175.83 J) of reaction can be calculated from the following standard-state enthalpies of formation and standard-state entropies:

Compound	∆H_f (kJ/mol)	∆S (J/mol•K)
NO₂	33.18	240.06
N₂O₄	9.16	304.29

We can then use the assumption that the temperature dependence of ∆H for this reaction is small to estimate the equilibrium constant at various temperatures.

Temperature	∆G (kJ)	K_p
100 ⁰C	8.4	0.066
0 ⁰C	-9.2	58
-78 ⁰C	-22.9	1.3x10⁶
-196 ⁰C	-43.6	3.7x10²⁹

Hazards:

Nitrogen Dioxide is a corrosive and poisonous gas; use caution when handling the glass tubes.

SOP:

Acutely Toxic – Nitrogen Dioxide

Corrosives – Nitrogen Dioxide

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