Nitrogen and Phosphorus: Quick Revision

A brief note on Nitrogen and Phosphorus. Read it before a test. Try each question at the end before you open its answer.

The group

• Group 15 is $ns^2np^3$: N, P (non-metals), As, Sb (metalloids), Bi (metal).
• Oxidation states are $-3$, $+3$ and $+5$. The $+3$ state is more stable down the group (inert pair effect).
• Nitrogen: small, no d orbitals, so covalency at most 4 ($\text{NH}_{4}^{+}$ yes, $\text{NCl}_{5}$ no). It forms multiple bonds and hydrogen bonds.
• $\text{MH}_{3}$ hydrides: bond angle falls from 107.8° ($\text{NH}_{3}$) to about 91° ($\text{SbH}_{3}$). Basic strength and stability fall.

Ammonia

• Haber: $\text{N}_{2}\ +\ 3\text{H}_{2}\ \rightleftharpoons \ 2\text{NH}_{3}$, about 200 bar and 700 K, iron with $\text{K}_{2}\text{O}$ and $\text{Al}_{2}\text{O}_{3}$. High pressure and low temperature favour ammonia, and 700 K is a compromise for speed.
• Laboratory: $2\text{NH}_{4}\text{Cl}\ +\ \text{Ca}(\text{OH})_{2}\ \rightarrow \ 2\text{NH}_{3}\ +\ \text{CaCl}_{2}\ +\ 2\text{H}_{2}\text{O}$, dried over $\text{CaO}$.
• Deep blue $[\text{Cu}(\text{NH}_{3})_{4}]^{2+}$ with copper(II) ions. Brown colour with Nessler's reagent.

Nitric acid

• 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 500 K and 9 bar, then $2\text{NO}\ +\ \text{O}_{2}\ \rightarrow \ 2\text{NO}_{2}$, then $3\text{NO}_{2}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ 2\text{HNO}_{3}\ +\ \text{NO}$.
• Copper gives $\text{NO}_{2}$ with the concentrated acid and $\text{NO}$ with the dilute acid. Iron, aluminium and chromium are passivated.

Phosphorus

• White $\text{P}_{4}$ (60° angle, reactive, under water), red (chains), black (layers).
• $\text{PH}_{3}$: pyramidal, 93.6°, weak base, from $\text{P}_{4}$ and NaOH or from $\text{Ca}_{3}\text{P}_{2}$ and water.
• $\text{PCl}_{3}$ (pyramid) and $\text{PCl}_{5}$ (trigonal bipyramid) hydrolyse. $\text{PCl}_{5}\ +\ 4\text{H}_{2}\text{O}\ \rightarrow \ \text{H}_{3}\text{PO}_{4}\ +\ 5\text{HCl}$.
• Basicity is the number of P-OH groups: $\text{H}_{3}\text{PO}_{2}$ 1, $\text{H}_{3}\text{PO}_{3}$ 2, $\text{H}_{3}\text{PO}_{4}$ 3. $\text{H}_{3}\text{PO}_{3}$ disproportionates to $\text{H}_{3}\text{PO}_{4}$ and $\text{PH}_{3}$.

A worked example

Find the mass of nitric acid from $8.5\ \text{t}$ of ammonia at a full yield.

1. Ratio. One mole of ammonia gives one mole of acid.
2. Masses. $\dfrac{63}{17}$ is the mass of acid for each unit mass of ammonia.
3. Answer. $8.5 \times \dfrac{63}{17} = 31.5\ \text{t}$.

Practice with new numbers

A program to try

The program finds the oxidation state of nitrogen from the formula.

Mistakes to avoid

• Giving nitrogen a covalency of 5. The highest covalency is 4.
• Counting every hydrogen of $\text{H}_{3}\text{PO}_{3}$ as acidic. It is dibasic.
• Using sulphuric acid to dry ammonia. The base reacts with the acid.
• Mixing up the products of copper with concentrated and dilute nitric acid.

Check yourself

1. What is the basicity of hypophosphorous acid?

2. Which gas forms when copper reacts with dilute nitric acid, and what do you see at the mouth of the tube?

3. Why is the Haber process run at 700 K, when the yield is higher at a lower temperature?

4. How many moles of NaOH neutralise 2 mol of $\text{H}_{3}\text{PO}_{4}$?

Sources

• NCERT, Chemistry Part I for Class XII, the chapter The p-Block Elements.
• OpenStax, Chemistry 2e, Chapter 18.