Group 15 trends, nitrogen and ammonia

A gas that is everywhere and almost useless

Take a breath. Nearly four out of every five molecules you breathe in are nitrogen, $\text{N}_{2}$. You breathe them out again unchanged. Plants stand in the same air and cannot use it. A farmer must still add fertiliser to the soil. Why is a gas so common and so hard to use?

The answer is in the bond. The two nitrogen atoms in $\text{N}_{2}$ are held by a triple bond, $\text{N}\equiv \text{N}$. It takes about 945 kJ/mol to break it. Textbooks print values between 941 and 946 kJ/mol. The molecule is also small and has no dipole. Few reagents can attack it at room temperature, so nitrogen has to be forced to react. In the Haber process, an industrial plant does it with heat, pressure and a catalyst.

The family of group 15

Group 15 has five elements: nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb) and bismuth (Bi). Each has the outer electron arrangement $ns^2np^3$: two electrons in the $s$ orbital and three in the $p$ orbitals. Moving down the group, the atoms get larger and hold the outer electrons more loosely.

• N: a non-metal. Covalent radius 70 pm, first ionisation enthalpy 1402 kJ/mol, electronegativity 3.0.
• P: a non-metal. Radius 110 pm, ionisation enthalpy 1012 kJ/mol, electronegativity 2.1.
• As: a metalloid. Radius 121 pm, ionisation enthalpy 947 kJ/mol, electronegativity 2.0.
• Sb: a metalloid. Radius 141 pm, ionisation enthalpy 834 kJ/mol, electronegativity 1.9.
• Bi: a metal. Radius 148 pm, ionisation enthalpy 703 kJ/mol, electronegativity 1.9.

Read the list from top to bottom. The radius grows. The ionisation enthalpy falls, and the drop from N to P is the biggest, 390 kJ/mol. The character changes from non-metal to metal. Nitrogen and phosphorus are non-metals. Arsenic and antimony are metalloids, which have some properties of both kinds. Bismuth is a typical metal.

Oxidation states. The group shows $-3$, $+3$ and $+5$. A nitrogen atom takes three electrons to become $-3$, as in $\text{NH}_{3}$. The $+3$ and $+5$ states use the three $p$ electrons or all five outer electrons. Going down the group, the $+5$ state becomes less stable and the $+3$ state more stable. This is the inert pair effect: the two $s$ electrons are held tightly and do not take part in bonding. So $\text{Bi}(\text{V})$ compounds are strong oxidising agents, while $\text{Bi}(\text{III})$ is the common state of bismuth.

Why nitrogen is different from the rest

Nitrogen differs from phosphorus more than phosphorus differs from arsenic. There are four reasons.

1. Small size and high electronegativity. Nitrogen forms strong bonds and hydrogen bonds.
2. No d orbitals in the second shell. The highest covalency of nitrogen is 4, as in the ammonium ion $\text{NH}_{4}^{+}$. It forms $\text{NF}_{3}$ and $\text{NCl}_{3}$ but no $\text{NCl}_{5}$. Phosphorus has the third shell and forms $\text{PCl}_{5}$. The Indian syllabus explains $\text{PCl}_{5}$ by using vacant 3d orbitals. Many modern books explain it without d orbitals, with bonding spread over several atoms. The result is the same: phosphorus can hold more than eight electrons and nitrogen cannot.
3. The ability to form multiple bonds. Nitrogen makes multiple bonds by the sideways overlap of $p$ orbitals, with itself and with carbon and oxygen: $\text{N}\equiv \text{N}$, $\text{C}\equiv \text{N}$, $\text{N}=\text{O}$. The bigger atoms of the family overlap sideways poorly, so they prefer single bonds. Phosphorus exists as $\text{P}_{4}$ with single bonds only.
4. Weak N-N single bond. The lone pairs on two close nitrogen atoms repel one another. The N-N single bond is weaker than the P-P bond, so nitrogen does not form long chains, but phosphorus forms chains and rings.

The hydrides

All five elements make hydrides of formula $\text{MH}_{3}$: $\text{NH}_{3}$, $\text{PH}_{3}$, $\text{AsH}_{3}$, $\text{SbH}_{3}$ and $\text{BiH}_{3}$. Their shape is a pyramid with a lone pair on the central atom. The properties change in a regular way.

• Bond angle. The angle falls from 107.8° in $\text{NH}_{3}$ to 93.6° in $\text{PH}_{3}$ and then hardly changes. As the central atom gets larger and less electronegative, the bonding pairs sit farther from it and repel less. The lone pair then takes up more room, and the angle closes up towards 90°. Some books print 107° and 93.5°.
• Boiling point. Ammonia (240 K) boils far higher than phosphine (186 K), because ammonia molecules hold one another by hydrogen bonds. After $\text{PH}_{3}$ the boiling point rises with the size of the molecule.
• Basic character falls from $\text{NH}_{3}$ to $\text{BiH}_{3}$. The lone pair is spread over a larger atom and is harder to donate.
• Thermal stability falls, and the reducing power rises, from $\text{NH}_{3}$ to $\text{BiH}_{3}$.

Nitrogen the element

Nitrogen is obtained industrially by the liquefaction and fractional distillation of air. In the laboratory, a warm solution of ammonium chloride and sodium nitrite gives it:

$\text{NH}_{4}\text{Cl}\ +\ \text{NaNO}_{2}\ \rightarrow \ \text{N}_{2}\ +\ 2\text{H}_{2}\text{O}\ +\ \text{NaCl}$

Very pure nitrogen comes from heating barium azide, $\text{Ba}(\text{N}_{3})_{2}$, which gives barium and nitrogen. Nitrogen is inert at room temperature. At high temperature it reacts with metals and with hydrogen. Lithium is the one metal that reacts with it at room temperature: $6\text{Li}\ +\ \text{N}_{2}\ \rightarrow \ 2\text{Li}_{3}\text{N}$. Magnesium burns in nitrogen to give $\text{Mg}_{3}\text{N}_{2}$. With oxygen, it needs the temperature of lightning or an engine, above 2000 K, to give $\text{NO}$.

Ammonia and the Haber process

Ammonia is made on a huge scale from nitrogen and hydrogen, by the Haber process:

$\text{N}_{2}\ +\ 3\text{H}_{2}\ \rightleftharpoons \ 2\text{NH}_{3}$

The enthalpy change is $\Delta H^\circ = -92.2$ kJ for the equation as written, which is $-46.1$ kJ for each mole of ammonia. Look at the equation with the eyes of Le Chatelier. Four moles of gas become two. So high pressure pushes the equilibrium to ammonia. The reaction gives out heat. So a low temperature pushes it to ammonia too. But at a low temperature the reaction is far too slow, even with a catalyst. The factory chooses a compromise: about 200 bar, about 700 K (other books print 450 °C to 500 °C), and a catalyst of iron with small amounts of $\text{K}_{2}\text{O}$ and $\text{Al}_{2}\text{O}_{3}$ as promoters. Some books name molybdenum as the promoter. The ammonia is cooled and condensed to a liquid, and the unused gases are sent round again.

Start at 700 K and 200 bar. About 41 percent of the mixture is ammonia at equilibrium in this model. Now lower the temperature to 500 K. The yield doubles, but the rate would be very low. Now drop the pressure to 1 bar at 700 K. The yield falls to less than 1 percent. The model uses ideal gases and constant $\Delta H$ and $\Delta S$. A real plant has a lower yield in each pass, because the real gases and the speed of the reaction matter.

Practise with new numbers

Ammonia in the laboratory

In the laboratory, ammonia is made by heating an ammonium salt with a strong base:

$2\text{NH}_{4}\text{Cl}\ +\ \text{Ca}(\text{OH})_{2}\ \rightarrow \ 2\text{NH}_{3}\ +\ \text{CaCl}_{2}\ +\ 2\text{H}_{2}\text{O}$

The gas is dried over quicklime, $\text{CaO}$. Concentrated sulphuric acid and phosphorus pentoxide cannot be used, because ammonia is a base and reacts with them. Calcium chloride cannot be used either, because it forms the compound $\text{CaCl}_{2}\cdot 8\text{NH}_{3}$.

Properties. Ammonia is a colourless gas with a sharp smell. It is very soluble in water, and the solution is weakly basic:

$\text{NH}_{3}\ +\ \text{H}_{2}\text{O}\ \rightleftharpoons \ \text{NH}_{4}^{+}\ +\ \text{OH}^{-}$

• It gives white fumes of ammonium chloride with hydrogen chloride.
• Its solution precipitates the hydroxides of metals. Iron(III) chloride gives a brown precipitate of $\text{Fe}(\text{OH})_{3}$. Zinc sulphate gives white $\text{Zn}(\text{OH})_{2}$.
• It forms complexes. A little ammonia with $\text{Cu}^{2+}$ gives a pale blue solid. An excess gives the deep blue ion $[\text{Cu}(\text{NH}_{3})_{4}]^{2+}$. Silver chloride dissolves in ammonia as $[\text{Ag}(\text{NH}_{3})_{2}]^{+}$.
• Hot copper(II) oxide is reduced by ammonia: $3\text{CuO}\ +\ 2\text{NH}_{3}\ \rightarrow \ 3\text{Cu}\ +\ \text{N}_{2}\ +\ 3\text{H}_{2}\text{O}$.
• Nessler's reagent, an alkaline solution of $\text{K}_{2}[\text{HgI}_{4}]$, gives a brown precipitate or colour with ammonia. It is a test for ammonia and ammonium ions.

Ammonia is used to make fertilisers such as urea and ammonium sulphate, to make nitric acid, and as a refrigerant.

Try it in Python

The program uses the closed formula for the equilibrium of the Haber reaction. It prints the percentage of ammonia at four temperatures and then at a low pressure.

A second program: the trend in the group

The program works out how much the first ionisation enthalpy falls from one element to the next.

Common mistakes

• Saying that the Haber process needs a low temperature. The equilibrium likes a low temperature, but the rate does not. The factory uses a compromise.
• Drying ammonia with concentrated sulphuric acid. The gas is a base and reacts with the acid.
• Giving nitrogen a covalency of 5. The highest covalency of nitrogen is 4, in $\text{NH}_{4}^{+}$. The oxidation state +5 does not need five bonds.
• Forgetting hydrogen bonds in ammonia. They explain its high boiling point and its large solubility.

Remember these

• Group 15 is $ns^2np^3$ and goes from non-metal (N, P) to metalloid (As, Sb) to metal (Bi).
• Oxidation states are $-3$, $+3$ and $+5$. The $+3$ state is more stable down the group.
• Nitrogen: small, no d orbitals, forms multiple bonds, hydrogen bonds in $\text{NH}_{3}$.
• Haber process: $\text{N}_{2}\ +\ 3\text{H}_{2}\ \rightleftharpoons \ 2\text{NH}_{3}$, about 200 bar, 700 K, iron catalyst with promoters.
• Ammonia is dried over $\text{CaO}$, gives a deep blue complex with $\text{Cu}^{2+}$ and is tested with Nessler's reagent.

Check yourself

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

1. Phosphorus makes $\text{PCl}_{5}$. Why does nitrogen not make $\text{NCl}_{5}$?

2. Ammonia boils at 240 K and phosphine at 186 K. Explain.

3. Why is anhydrous calcium chloride not used to dry ammonia?

4. At equilibrium, the yield of ammonia is greater at 400 K than at 700 K. Why does the factory use 700 K?

References

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements, on group 15, nitrogen and ammonia.
• OpenStax, Chemistry 2e, Chapter 18, on nitrogen and its compounds, and Appendix G for the standard enthalpy and entropy values used in the model.