Oxidizing Power of Fuming Nitric Acid

Introduction

This post serves the purpose of showing how interesting and frightening Fuming Nitric Acid can be. Fuming Nitric Acid was made in a previously post: click here.

Reagents

  • Fuming Nitric Acid;
  • Latex Gloves (not a reagent per say);
  • Copper sample;
  • Water (tap water works just fine).

Dangers

The first demonstration is particularly dangerous. It involves burning Latex so general fire safety is a must. The fumes released by burning Latex and the Nitrogen Oxides released are toxic so the demonstrations here described must be done in a very well ventilated area.

About the dangers of Fuming Nitric Acid (copied from its production post):

Fuming Nitric Acid is very corrosive and is a powerful oxidizer. It is infamous by its capability of setting latex gloves on fire. It is also permeable in most types of gloves. It releases toxic and corrosive Nitrogen Oxide gases. It can react violently with a lot of organic matter, specially solvents. A big spill of Fuming Nitric Acid may end monstrously.

Gloves are not recommended while working with Fuming Nitric Acid.

PS: Copper is especially dangerous when electrons are speeding through it.

Demo 1

In the first demonstration Fuming Nitric Acid is poured into a Latex glove. The Latex glove quickly catches fire.

Note 1: Approximately 2 mL of Fuming Nitric Acid are used in each take.

Note 2: This area is an old area used for testing dangerous procedures. My apologies for choosing not the best looking place.

 

The mechanism behind this reaction eludes me a bit. Fuming Nitric Acid starts reacting with the Latex. This reaction is much likely a set of nitration reactions that quickly get into a run away stage (the large quantities of Nitrogen oxides released seem to support this theory). The heat generated by the run away is enough to put the glove on fire.

Demo 2

In this demonstration Fuming Nitric Acid is  poured into Copper.

1- Initially, 2mL of Fuming Nitric Acid are added to a Copper sample. Nothing significant happens.

2- Some time is allowed to pass. Then some water is added. A vigorous reaction takes place releasing copious amounts of Nitrogen oxides.

3- Some more time is allowed to pass and more water is poured in. Reaction stops. A beautiful pale blue solution is revealed.

 

Initially, when 2 mL of Fuming Nitric Acid are added to the Copper sample, nothing significant happens (surprisingly). This is because of an effect called “passivation”. The Nitric Acid is so concentrated that the reaction produces an Oxide layer instead of producing aqueous Copper (II) ions. This Oxide layer doesn’t react with Fuming Nitric Acid so it protects the metal from further reaction.

When water is added, the concentration of the acid gets lower and it can now react to form aqueous Copper (II) ions. The reaction rate increases drastically as no more protective Oxide layer is formed. The reaction happening is:

Cu(s) + HNO3(aq) –> Cu(NO3)2(aq) + 2NO2(g) + 2H2O(l)

Finally, more water is added and a beautiful pale blue solution appears while the reaction stops. The reaction stops because the concentration of the acid is decreased considerably to the point where it can no longer react with Copper. The interesting part is that the initial greenish solution turns blue.

Before the final addition of water, the Nitrate ion concentration was high enough to produce a complex ion with Cu2+ ions. This complex ion is greenish in color.When the final addition of water is made, the concentration of the Nitrate ion diminishes and water takes its place surrounding the Copper ions, thus creating the regular pale blue solution of Cu2+ (aq).

Final Notes

Both demos were quite remarkable as they illustrate two unusual properties of Fuming Nitric Acid. There will be more upcoming posts that use this interesting chemical.

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