18.3 Groups V-A & VI-A: Nitrogen, Phosphorus, Oxygen & Sulphur Compounds

Groups V-A & VI-A: Nitrogen, Phosphorus, Oxygen & Sulphur Compounds

Section 18.2 covered Groups I-A to IV-A and the halogens. This section covers the two groups between them: Group V-A (N, P, As, Sb, Bi; valence configuration $ns^2np^3$) and Group VI-A (O, S, Se, Te, Po; $ns^2np^4$). Industrial processes and the oxyacids of these elements are the main sources of questions.

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1. Group V-A: General Trends

• Metallic character increases down the group: N and P are non-metals, As and Sb are metalloids, and Bi is a metal.
• Common oxidation states are $-3$, $+3$ and $+5$; the stability of $+5$ decreases down the group (inert pair effect), so bismuth is mostly $+3$.
• Nitrogen is a gas made of $\text{N}_2$ molecules with a very strong triple bond (about $946\text{ kJ/mol}$), which makes it unreactive at room temperature. Phosphorus exists as $\text{P}_4$ tetrahedra with single bonds.
• Hydrides: $\text{NH}_3 > \text{PH}_3 > \text{AsH}_3$ in basic strength and thermal stability; ammonia's high boiling point comes from hydrogen bonding.

Oxides of Nitrogen

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2. Nitric Acid

Manufacture by the Ostwald Process

1. Catalytic oxidation of ammonia: ammonia and air pass over a platinum (or platinum–rhodium) gauze at about $800$–$900^\circ\text{C}$: $4\text{NH}_3 + 5\text{O}_2 \xrightarrow{\text{Pt}} 4\text{NO} + 6\text{H}_2\text{O}$
2. Oxidation of nitric oxide: on cooling, $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$.
3. Absorption in water: $4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3$.

The ammonia itself comes from the Haber process ($\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3$, iron catalyst), discussed with equilibrium in Section 17.2.

Reactions of Nitric Acid

Nitric acid is a strong acid and a powerful oxidizing agent. With most metals the products depend on concentration, and hydrogen gas is usually not released because it is oxidized:

$3\text{Cu} + 8\text{HNO}_3(\text{dilute}) \rightarrow 3\text{Cu(NO}_3)_2 + 2\text{NO} + 4\text{H}_2\text{O}$

$\text{Cu} + 4\text{HNO}_3(\text{conc.}) \rightarrow \text{Cu(NO}_3)_2 + 2\text{NO}_2 + 2\text{H}_2\text{O}$

• Very dilute nitric acid gives hydrogen only with very reactive metals such as magnesium and manganese.
• Iron, aluminium and chromium become passive in concentrated nitric acid, because a thin oxide film forms on the surface.
• Aqua regia (3 parts concentrated HCl to 1 part concentrated $\text{HNO}_3$ by volume) dissolves gold and platinum.
• Non-metals are oxidized: $\text{S} + 6\text{HNO}_3 \rightarrow \text{H}_2\text{SO}_4 + 6\text{NO}_2 + 2\text{H}_2\text{O}$.
• Nitric acid is used to make fertilizers (ammonium nitrate), explosives and dyes, and to nitrate benzene (Section 19.3).

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3. Phosphorus and Its Compounds

Allotropes

Oxides and Oxyacids

• $\text{P}_4\text{O}_6$ (phosphorus trioxide) forms when phosphorus burns in limited air; with cold water it gives phosphorous acid $\text{H}_3\text{PO}_3$.
• $\text{P}_4\text{O}_{10}$ (phosphorus pentoxide) forms in excess air; it is a powerful dehydrating agent and with water gives orthophosphoric acid $\text{H}_3\text{PO}_4$.

Only hydrogen atoms attached to oxygen are acidic, which is why $\text{H}_3\text{PO}_3$ forms only two series of salts. Phosphorus compounds are important in fertilizers (superphosphates) and detergents.

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4. Group VI-A: Oxygen and Ozone

• Oxygen is a colourless, paramagnetic gas (explained by molecular orbital theory, Section 16.3) and the most abundant element in the Earth's crust.
• Ozone ($\text{O}_3$) is an allotrope of oxygen formed by passing a silent electric discharge through oxygen: $3\text{O}_2 \rightleftharpoons 2\text{O}_3$. It is a pale blue gas with a characteristic smell and a bent molecule.
• Ozone is a stronger oxidizing agent than oxygen ($\text{O}_3 \rightarrow \text{O}_2 + [\text{O}]$). It liberates iodine from potassium iodide, bleaches, and is used to sterilize water.
• In the stratosphere the ozone layer absorbs harmful ultraviolet radiation (see Section 20.2 for its depletion).

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5. Sulphur and Its Compounds

Allotropes of Sulphur

The temperature $95.6^\circ\text{C}$ is the transition temperature between the two crystalline forms (see polymorphism in Section 16.5).

Sulphur Dioxide

$\text{SO}_2$ is produced by burning sulphur or roasting sulphide ores. It is an acidic oxide ($\text{SO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{SO}_3$), acts mostly as a reducing agent (it decolourises acidified potassium permanganate), and bleaches by reduction. It is a major air pollutant and a cause of acid rain.

Sulphuric Acid: The Contact Process

1. Burn sulphur or roast pyrites to obtain $\text{SO}_2$; purify and dry the gas so it does not poison the catalyst.
2. Oxidize it in the converter: $2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3 \qquad \Delta H < 0$ using vanadium pentoxide ($\text{V}_2\text{O}_5$) at about $400$–$450^\circ\text{C}$ and slightly above atmospheric pressure. A moderate temperature is a compromise: a low temperature favours more $\text{SO}_3$ at equilibrium, but the rate is too slow.
3. Absorb $\text{SO}_3$ in concentrated (98%) sulphuric acid to form oleum ($\text{H}_2\text{S}_2\text{O}_7$), then dilute it with water: $\text{H}_2\text{S}_2\text{O}_7 + \text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4$.

$\text{SO}_3$ is not dissolved directly in water because the reaction is so exothermic that it forms a fine mist of acid that is hard to condense.

Properties of Sulphuric Acid

When diluting, always add the acid slowly to water, never water to acid, because the heat released can make the water boil and spit out acid.

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6. Halogen Oxyacids and Noble Gas Uses

The oxyacids of chlorine become stronger acids, but weaker oxidizing agents, as the oxidation state of chlorine increases:

Acid strength: $\text{HClO} < \text{HClO}_2 < \text{HClO}_3 < \text{HClO}_4$. More oxygen atoms pull electron density away from the O–H bond and spread the negative charge of the anion, stabilizing it.

Uses of the noble gases: helium fills balloons and is used in breathing mixtures for divers and in cooling superconducting magnets; neon gives the red glow in advertising signs; argon provides an inert atmosphere for welding and filling light bulbs; krypton and xenon are used in high-intensity lamps and photographic flash tubes; radon is radioactive and has been used in radiotherapy.