Explore the nitrogen oxides: unexpected equilibrium

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Watch a demonstration of this experiment and download the technician notes from the Education in Chemistry website: rsc.li/3LtKCMF

You can use the syringe of nitrogen dioxide, NO_2, produced using the Avogadro’s law revisited demonstration to explore a counterintuitive example of equilibria chemistry.

Demonstrate this in your 14–16 and post-16 lessons on the nitrogen oxides and equilibrium.

Kit

60 cm³ syringe loaded with nitrogen dioxide
60 cm³ syringe loaded with nitrogen monoxide
Syringe caps
Kettle and container for a hot water bath
Access to freezer or salted ice bath
Approximately 3 cm silicone tubing with a 4 mm internal diameter
50 cm³ stop bath of 0.4 M sodium hydroxide

Health, safety and disposal

Read our standard health and safety guidance and carry out a risk assessment before running a live demonstration.
Wear eye protection.
Work in an efficiently running fume cupboard.
Nitrogen dioxide and nitrogen monoxide are toxic, corrosive and oxidising – avoid skin contact and inhalation. CLEAPSS members should consult HC068B.
0.4 M sodium hydroxide is an irritant to skin and eyes. Dilute the contents of the stop bath and dispose of down a foul-water drain.

Preparation

Make a capped syringe of NO₂ using the technique in Avogadro’s law revisited . You can use a syringe left over from the previous demonstration or make a fresh sample. Eject 10 cm³ (leaving 30 cm³) into a stop bath of 0.4 M sodium hydroxide, NaOH, in a fume cupboard to allow you to move the syringe’s plunger.

In front of the class and teaching goal

The syringe of NO₂ actually contains a mixture of NO₂ (red-brown) with dinitrogen tetroxide, N₂O₄ (colourless), according to the equilibrium in the equation below.

Equation 1: 2NO₂(g) ⇌ N₂O₄(g)

The syringe of NO₂ actually contains a mixture of NO₂ (red-brown) with N₂O₄ (colourless) according to the equilibrium in equation 1.

Equation 1: 2NO₂(g) ⇌ N₂O₄(g)

The equilibrium is exothermic as written (ΔH = -57 kJ) and lies to the right at room temperature with a K_c of 215 at 25°C.

Students should predict that depressing the plunger will cause the equilibrium to shift right, leading to a paler colour. Holding a finger over the syringe cap to secure it, depress the plunger and show that, counterintuitively, the colour darkens briefly before paling once more to a similar colour seen at the start. Show the opposite effect by retracting the plunger. This happens because the NO₂, which causes the red-brown colour, is contained in a smaller volume and it takes a few seconds for the chemistry to catch up.

Make a hot water bath using the kettle (up to 80°C, above which the plastic may weaken) and an ice bath. Placing the syringe in the ice bath causes the gas to contract slightly while getting substantially paler. When left in the hot water, the colour darkens significantly even though the volume increases slightly.

More resources

Watch Avogadro in action and demonstrate hydrogenating an alkene without the need for heating.
Read these CPD articles on reversible reactions and equilibrium constants before using the strategies in your 14–16 lessons.
Discover 5 ways to explain rate and equilibrium to improve your learners’ exam answers.
Display the infographic poster and download the scaffolded storyboard activity for 14–16 students to describe and sequence a dynamic equilibrium.

You can demonstrate a second equilibrium if you have time (equation 2). Working in a fume cupboard, connect the syringe of NO₂ (and N₂O₄) to a syringe of NO and draw 30 cm³ of the colourless gas. Leave the 60 cm³ syringe either in the freezer or in a salted ice bath for 10 minutes. A blue liquid will form.

Equation 2: NO(g) + NO₂(g) ⇌ N₂O₃(l)

Either keep the NO₂syringe when finished or bubble the gaseous contents through a stop bath of 0.4 M NaOH. When I am finished with a syringe, I demonstrate the reaction of NO₂ with water and the neutralisation of the resulting acid. If you add a few drops of universal indicator to the stop bath and draw up 5 cm³ into the syringe, the colour will change as the gas dissolves into and reacts with the NaOH solution, demonstrating the formation and neutralisation of nitric acid.

Declan Fleming