Oxides of nitrogen and nitric acid

Five oxides, five oxidation states

Nitrogen does not burn in air. In a car engine or in lightning, the temperature is so high that a little of it does combine with oxygen. The oxide that forms, $\text{NO}$, is one of five oxides of nitrogen. Their oxidation states are all different. Let us list them.

• $\text{N}_{2}\text{O}$: nitrogen is $+1$. A colourless gas. Neutral.
• $\text{NO}$: nitrogen is $+2$. A colourless gas. Neutral.
• $\text{N}_{2}\text{O}_{3}$: nitrogen is $+3$. A blue solid at low temperature. Acidic.
• $\text{NO}_{2}$: nitrogen is $+4$. A brown gas. Acidic.
• $\text{N}_{2}\text{O}_{4}$: nitrogen is $+4$. A colourless solid or liquid. Acidic.
• $\text{N}_{2}\text{O}_{5}$: nitrogen is $+5$. A colourless solid. Acidic.

An acidic oxide gives an acid when it dissolves in water. A neutral oxide gives neither an acid nor a base. So $\text{N}_{2}\text{O}_{3}$ is the anhydride of nitrous acid, $\text{HNO}_{2}$, and $\text{N}_{2}\text{O}_{5}$ is the anhydride of nitric acid, $\text{HNO}_{3}$.

How they are made

• Dinitrogen oxide (laughing gas). Ammonium nitrate is warmed gently: $\text{NH}_{4}\text{NO}_{3}\ \xrightarrow{\text{heat}}\ \text{N}_{2}\text{O}\ +\ 2\text{H}_{2}\text{O}$.
• Nitrogen monoxide. Copper and dilute nitric acid give $\text{NO}$. Another route is in the laboratory with iron(II) sulphate and nitrite in acid: $2\text{NaNO}_{2}\ +\ 2\text{FeSO}_{4}\ +\ 3\text{H}_{2}\text{SO}_{4}\ \rightarrow \ \text{Fe}_{2}(\text{SO}_{4})_{3}\ +\ 2\text{NaHSO}_{4}\ +\ 2\text{H}_{2}\text{O}\ +\ 2\text{NO}$.
• Nitrogen dioxide. Nitric oxide meets oxygen: $2\text{NO}\ +\ \text{O}_{2}\ \rightarrow \ 2\text{NO}_{2}$. Lead(II) nitrate gives it on heating: $2\text{Pb}(\text{NO}_{3})_{2}\ \rightarrow \ 2\text{PbO}\ +\ 4\text{NO}_{2}\ +\ \text{O}_{2}$.
• Dinitrogen tetraoxide. Nitrogen dioxide is brown and has one unpaired electron. On cooling, two molecules join through an N-N bond to give the colourless $\text{N}_{2}\text{O}_{4}$: $2\text{NO}_{2}\ \rightleftharpoons \ \text{N}_{2}\text{O}_{4}$.
• Dinitrogen pentaoxide. Nitric acid is dehydrated with phosphorus pentoxide.

Structures. $\text{N}_{2}\text{O}$ is a straight molecule with resonance forms $\text{N}\equiv \text{N}^{+}\text{-}\text{O}^{-}$ and $\text{N}^{-}\text{-}\text{N}^{+}\equiv \text{O}$. $\text{NO}$ has an odd number of electrons (11 outer electrons), so it is paramagnetic. $\text{NO}_{2}$ is bent, with an angle of 134°. $\text{N}_{2}\text{O}_{5}$ is two $\text{NO}_{2}$ groups joined through an oxygen atom. In the solid it is ionic, $[\text{NO}_{2}]^{+}[\text{NO}_{3}]^{-}$.

Nitric acid: the Ostwald process

Nitric acid is the most important oxoacid of nitrogen. It is made by the Ostwald process in three steps.

1. Ammonia and air pass over a platinum and rhodium gauze, a fine metal net used as the catalyst, at about 500 K and 9 bar. The ammonia is oxidised: $4\text{NH}_{3}\ +\ 5\text{O}_{2}\ \xrightarrow{\text{Pt}\text{-}\text{Rh},\ \ 500\ \text{K},\ \ 9\ \text{bar}}\ 4\text{NO}\ +\ 6\text{H}_{2}\text{O}$. The heat keeps the gauze hot.
2. The nitrogen monoxide is oxidised further as it cools: $2\text{NO}\ +\ \text{O}_{2}\ \rightarrow \ 2\text{NO}_{2}$.
3. The nitrogen dioxide is absorbed in water: $3\text{NO}_{2}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ 2\text{HNO}_{3}\ +\ \text{NO}$. The NO that forms goes back to step 2.

The acid is about 68 percent by mass. It can be concentrated by distillation up to about 98 percent in the presence of concentrated sulphuric acid, which takes up water. The best way to see the whole process is to add the steps. With the NO recycled, the overall equation is $\text{NH}_{3}\ +\ 2\text{O}_{2}\ \rightarrow \ \text{HNO}_{3}\ +\ \text{H}_{2}\text{O}$. One mole of ammonia gives one mole of nitric acid.

Set the yield to 100 percent and 17 tonnes of ammonia. The acid made is 63 tonnes, the ratio of the molar masses. Then lower the yield to 90 percent. The factor $\dfrac{63}{17}$ is the mass of acid per tonne of ammonia and never changes.

Practise with new numbers

What nitric acid does

Nitric acid is a strong acid and a strong oxidising agent. The nitrogen in it is at +5 and is reduced. The product depends on the metal and on the concentration.

• Copper with concentrated acid: $\text{Cu}\ +\ 4\text{HNO}_{3}\ \rightarrow \ \text{Cu}(\text{NO}_{3})_{2}\ +\ 2\text{NO}_{2}\ +\ 2\text{H}_{2}\text{O}$
• Copper with dilute acid: $3\text{Cu}\ +\ 8\text{HNO}_{3}\ \rightarrow \ 3\text{Cu}(\text{NO}_{3})_{2}\ +\ 2\text{NO}\ +\ 4\text{H}_{2}\text{O}$
• Zinc with concentrated acid: $\text{Zn}\ +\ 4\text{HNO}_{3}\ \rightarrow \ \text{Zn}(\text{NO}_{3})_{2}\ +\ 2\text{NO}_{2}\ +\ 2\text{H}_{2}\text{O}$
• Zinc with dilute acid: $4\text{Zn}\ +\ 10\text{HNO}_{3}\ \rightarrow \ 4\text{Zn}(\text{NO}_{3})_{2}\ +\ \text{N}_{2}\text{O}\ +\ 5\text{H}_{2}\text{O}$
• Zinc with very dilute acid: $4\text{Zn}\ +\ 10\text{HNO}_{3}\ \rightarrow \ 4\text{Zn}(\text{NO}_{3})_{2}\ +\ \text{NH}_{4}\text{NO}_{3}\ +\ 3\text{H}_{2}\text{O}$

The more dilute the acid, the further the nitrogen is reduced: from $\text{NO}_{2}$ (+4) to $\text{NO}$ (+2), to $\text{N}_{2}\text{O}$ (+1), to $\text{NH}_{4}^{+}$ ($-3$). A few metals do not dissolve in concentrated nitric acid. Iron, aluminium and chromium become covered with a thin layer of oxide that protects them. This is passivation. Gold and platinum do not react with nitric acid at all, but a mixture of 3 parts concentrated hydrochloric acid and 1 part concentrated nitric acid, aqua regia, dissolves them. Nitric acid also oxidises non-metals when concentrated and hot:

$\text{C}\ +\ 4\text{HNO}_{3}\ \rightarrow \ \text{CO}_{2}\ +\ 4\text{NO}_{2}\ +\ 2\text{H}_{2}\text{O}$

$\text{S}_{8}\ +\ 48\text{HNO}_{3}\ \rightarrow \ 8\text{H}_{2}\text{SO}_{4}\ +\ 48\text{NO}_{2}\ +\ 16\text{H}_{2}\text{O}$

$\text{I}_{2}\ +\ 10\text{HNO}_{3}\ \rightarrow \ 2\text{HIO}_{3}\ +\ 10\text{NO}_{2}\ +\ 4\text{H}_{2}\text{O}$

$\text{P}_{4}\ +\ 20\text{HNO}_{3}\ \rightarrow \ 4\text{H}_{3}\text{PO}_{4}\ +\ 20\text{NO}_{2}\ +\ 4\text{H}_{2}\text{O}$

Brown ring test for nitrate. To a solution of a nitrate, add a freshly made solution of iron(II) sulphate. Then pour concentrated sulphuric acid carefully down the side of the tube. The acid forms a layer at the bottom. Nitrate is reduced to $\text{NO}$, which joins the iron: $[\text{Fe}(\text{H}_{2}\text{O})_{6}]^{2+}\ +\ \text{NO}\ \rightarrow \ [\text{Fe}(\text{H}_{2}\text{O})_{5}(\text{NO})]^{2+}\ +\ \text{H}_{2}\text{O}$. A brown ring appears where the two liquids meet.

The nitrate ion $\text{NO}_{3}^{-}$ is planar. It has three equal N-O bonds at 120°, an example of resonance.

Try it in Python

The program follows the Ostwald process on paper. It repeats steps 2 and 3, because step 3 gives back some NO each time. It shows that the nitric acid reaches one mole for each mole of ammonia.

Common mistakes

• Saying that copper gives hydrogen with nitric acid. Nitric acid oxidises. The nitrogen is reduced, not the hydrogen. Hydrogen is almost never given off.
• Forgetting that the product depends on the concentration. Copper gives $\text{NO}_{2}$ with concentrated acid and $\text{NO}$ with dilute.
• Calling $\text{NO}$ an acidic oxide. $\text{N}_{2}\text{O}$ and $\text{NO}$ are neutral.
• Writing the overall Ostwald equation without recycling NO. Step 3 gives off NO, which is used again.

Remember these

• Oxides: $\text{N}_{2}\text{O}$ (+1), $\text{NO}$ (+2), $\text{N}_{2}\text{O}_{3}$ (+3), $\text{NO}_{2}$ and $\text{N}_{2}\text{O}_{4}$ (+4), $\text{N}_{2}\text{O}_{5}$ (+5). The first two are neutral.
• Ostwald: $4\text{NH}_{3}\ +\ 5\text{O}_{2}$ over Pt-Rh at about 500 K and 9 bar, then $2\text{NO}\ +\ \text{O}_{2}$, then $3\text{NO}_{2}\ +\ \text{H}_{2}\text{O}$. Overall: one mole of $\text{HNO}_{3}$ for each mole of $\text{NH}_{3}$.
• Nitric acid oxidises metals and non-metals, and iron, aluminium and chromium are passivated by the concentrated acid.
• The brown ring test detects nitrate.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Find the oxidation state of nitrogen in $\text{N}_{2}\text{O}_{5}$ and in $\text{HNO}_{2}$.

2. Why does a piece of iron not dissolve in concentrated nitric acid?

3. The gas that Cu and dilute nitric acid give is colourless, but it turns brown at the mouth of the tube. What is it?

4. How is the brown ring test done, and what is the brown compound?

References

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements, on the oxides of nitrogen and nitric acid.
• OpenStax, Chemistry 2e, Chapter 18, on the compounds of nitrogen.