Phosphorus Compounds and Phosphoric Acid – Trends in Chemical Properties of Group 15 Elements and Their Compounds (Senior 4 Chemistry) | Ganzaa

Trends in Chemical Properties of Group 15 Elements and Their Compounds

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Senior 4 · Chemistry

Trends in Chemical Properties of Group 15 Elements and Their Compounds

5 lessons

Lesson 4 of 5

Phosphorus Compounds and Phosphoric Acid

25 min

From rock to root

A bag of DAP fertiliser, one of the three most used fertilisers on Rwandan farms, carries phosphorus that was once locked in phosphate rock. Plants need phosphorus for DNA, for the ATP that carries energy in every cell, and for strong roots, but the calcium phosphate in rock is almost insoluble, so it must be turned into soluble phosphates. The key step is phosphoric acid. This lesson follows phosphorus from the element to its oxides, chlorides and oxoacids, and ends with how phosphoric acid and phosphate fertilisers are made.

Definition

Basicity of an acid: the number of hydrogen ions one molecule of the acid can give. Example: phosphoric(V) acid, $\mathrm{H_3PO_4}$ , is tribasic; it forms three series of salts, $\mathrm{NaH_2PO_4}$ , $\mathrm{Na_2HPO_4}$ and $\mathrm{Na_3PO_4}$ .

Dehydrating agent: a substance that removes water, or the elements of water, from another substance. Example: phosphorus(V) oxide, $\mathrm{P_4O_{10}}$ , which reacts vigorously with water.

Making phosphorus

Phosphorus is made by heating phosphate rock with sand and coke in an electric furnace. Silica takes up the calcium as a slag; carbon removes the oxygen, and phosphorus distils off as $\mathrm{P_4}$ vapour and is condensed under water:

\mathrm{2Ca_3(PO_4)_2 + 6SiO_2 + 10C \rightarrow 6CaSiO_3 + 10CO + P_4}

The oxides

Both oxides are built on the $\mathrm{P_4}$ tetrahedron. In phosphorus(III) oxide, $\mathrm{P_4O_6}$ , an oxygen atom bridges each of the six P–P edges; in phosphorus(V) oxide, $\mathrm{P_4O_{10}}$ , each phosphorus also carries one extra terminal oxygen. Both are acidic oxides that react with water:

\mathrm{P_4O_6(s) + 6H_2O(l) \rightarrow 4H_3PO_3(aq)} \qquad \mathrm{P_4O_{10}(s) + 6H_2O(l) \rightarrow 4H_3PO_4(aq)}

$\mathrm{P_4O_{10}}$ reacts so strongly with water that it is one of the most powerful drying agents; it even removes the elements of water from nitric acid, giving dinitrogen pentoxide: $\mathrm{4HNO_3 + P_4O_{10} \rightarrow 2N_2O_5 + 4HPO_3}$ .

The chlorides

Phosphorus trichloride, $\mathrm{PCl_3}$ , is a colourless fuming liquid of pyramidal molecules. Phosphorus pentachloride, $\mathrm{PCl_5}$ , is a pale yellow solid; in the gas it is a trigonal bipyramidal molecule (phosphorus can use its 3d orbitals to hold five pairs, which nitrogen cannot), and in the solid it is ionic, $\mathrm{[PCl_4]^+[PCl_6]^-}$ . On heating it dissociates: $\mathrm{PCl_5 \rightleftharpoons PCl_3 + Cl_2}$ .

Both chlorides are hydrolysed vigorously by water, giving steamy fumes of hydrogen chloride:

\mathrm{PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl} \qquad \mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}

With a little water, $\mathrm{PCl_5}$ stops at phosphorus oxychloride: $\mathrm{PCl_5 + H_2O \rightarrow POCl_3 + 2HCl}$ . In organic chemistry, white fumes of HCl when $\mathrm{PCl_5}$ is added are a test for an –OH group.

Oxoacids: count the O–H bonds, not the H atoms

The three common oxoacids all have the formula $\mathrm{H_3PO_n}$ , but only hydrogen atoms bonded to oxygen are acidic; hydrogen bonded directly to phosphorus is not (Figure 10.6).

Figure 10.6: Phosphorus oxoacids. Highlighted hydrogen atoms (on oxygen) are acidic.

Phosphoric(V) acid, $\mathrm{H_3PO_4}$

Manufacture. Most phosphoric acid is made by the wet process: phosphate rock is treated with sulphuric acid, and the insoluble calcium sulphate (phosphogypsum) is filtered off:

\mathrm{Ca_3(PO_4)_2(s) + 3H_2SO_4(aq) \rightarrow 3CaSO_4(s) + 2H_3PO_4(aq)}

Purer acid, for food and medicines, is made by burning phosphorus to $\mathrm{P_4O_{10}}$ and dissolving the oxide in water.

Properties.

A colourless solid (melting at 42.4 °C) usually sold as a thick, syrupy 85% solution: its molecules hydrogen-bond strongly to each other and to water.
A weak, tribasic acid: $\mathrm{p}K_{a1} = 2.16$ , $\mathrm{p}K_{a2} = 7.20$ , $\mathrm{p}K_{a3} = 12.32$ . Each proton is harder to remove than the last, because it must leave an ion that is already negative.
Not an oxidising agent (unlike nitric and concentrated sulphuric acid), so it is used to make hydrogen bromide and hydrogen iodide from their salts (Unit 12).
On strong heating it loses water, forming diphosphoric acid, $\mathrm{2H_3PO_4 \rightarrow H_4P_2O_7 + H_2O}$ , and then metaphosphoric acid.

Phosphate fertilisers

Fertiliser	Made from	Main phosphorus compound
superphosphate	phosphate rock + sulphuric acid	$\mathrm{Ca(H_2PO_4)_2}$ mixed with $\mathrm{CaSO_4}$
triple superphosphate	phosphate rock + phosphoric acid	$\mathrm{Ca(H_2PO_4)_2}$
DAP (diammonium phosphate)	phosphoric acid + ammonia	$\mathrm{(NH_4)_2HPO_4}$

Each turns the insoluble $\mathrm{PO_4^{3-}}$ of rock into the soluble dihydrogenphosphate or hydrogenphosphate ions that roots can absorb; DAP also supplies nitrogen.

Worked example 1: what does "18-46-0" mean?

Bags of DAP are labelled 18-46-0: 18% nitrogen, 46% "phosphate" expressed as $\mathrm{P_2O_5}$ , and 0% potash. Calculate the percentages of N and of $\mathrm{P_2O_5}$ -equivalent in pure $\mathrm{(NH_4)_2HPO_4}$ , and suggest why the label values are lower. $A_{r}$ : N 14.01, H 1.01, P 30.97, O 16.00.

Solution. $M = 2(18.05) + 1.01 + 30.97 + 4(16.00) = 132.08\ \mathrm{g\,mol^{-1}} \qquad \%\,\mathrm{N} = \frac{28.02}{132.08} \times 100 = 21.2\%$ $\%\,\mathrm{P} = \frac{30.97}{132.08} \times 100 = 23.4\% \qquad \text{as } \mathrm{P_2O_5}: \ 23.4 \times \frac{141.94}{61.94} = 53.7\%$ Pure DAP would be 21-54-0. Commercial DAP is made from impure wet-process acid and contains some monoammonium phosphate, water and impurities from the rock, so a typical product is guaranteed only at 18-46-0.

Worked example 2: titrating a tribasic acid

25.0 cm³ of 0.100 mol dm⁻³ phosphoric acid is titrated with 0.100 mol dm⁻³ sodium hydroxide. Calculate the volume of alkali needed to reach (a) the first end point, where $\mathrm{NaH_2PO_4}$ has formed, and (b) the second end point, where $\mathrm{Na_2HPO_4}$ has formed.

Solution. $n(\mathrm{H_3PO_4}) = 0.0250 \times 0.100 = 2.50 \times 10^{-3}$ mol.
(a) $\mathrm{H_3PO_4 + NaOH \rightarrow NaH_2PO_4 + H_2O}$ : $2.50 \times 10^{-3}$ mol NaOH, so $V = \frac{2.50 \times 10^{-3}}{0.100} = 25.0\ \mathrm{cm^3}$ .
(b) $\mathrm{H_3PO_4 + 2NaOH \rightarrow Na_2HPO_4 + 2H_2O}$ : $5.00 \times 10^{-3}$ mol, so 50.0 cm³.
The third proton ( $\mathrm{p}K_{a3} = 12.32$ ) is too weak to give a sharp end point in water.

Pale yellow crystals of phosphorus pentachloride sealed in a glass ampoule to keep out moist air, which would hydrolyse them to phosphoric acid and hydrogen chloride. Photo: W. Oelen, CC BY-SA 3.0, via Wikimedia Commons

Warning

Common mistakes. (1) Calling $\mathrm{H_3PO_3}$ tribasic because it has three H atoms: one H is bonded to P, so it is dibasic. (2) Writing $\mathrm{P_2O_5}$ for the molecule: it is $\mathrm{P_4O_{10}}$ ( $\mathrm{P_2O_5}$ is only the empirical formula, kept in fertiliser labels). (3) Saying phosphoric acid is a strong acid because it is tribasic: basicity and strength are different things; it is weak. (4) Forgetting that $\mathrm{PCl_5}$ exists but $\mathrm{NCl_5}$ does not: nitrogen cannot expand its octet.

Safety. Phosphorus chlorides and $\mathrm{P_4O_{10}}$ react violently with water and release hydrogen chloride or great heat: they are handled only in a fume cupboard with goggles and gloves. Concentrated phosphoric acid is corrosive. If any splashes on skin or into eyes, rinse with plenty of running water and get medical help.

Tip

Acidic H = H on O. $\mathrm{H_3PO_4}$ : 3, $\mathrm{H_3PO_3}$ : 2, $\mathrm{H_3PO_2}$ : 1. For fertiliser sums, convert between P and $\mathrm{P_2O_5}$ with the factor $\frac{141.94}{61.94} = 2.29$ .

Science Skool: Reaction of Water with Phosphorus Pentachloride (1 min). Watch how vigorously water attacks PCl₅ and the steamy fumes of hydrogen chloride that it gives off.

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Practice

Write the equation for the manufacture of phosphorus in an electric furnace, and calculate the mass of phosphorus that could be obtained from 1.00 kg of calcium phosphate. $M(\mathrm{Ca_3(PO_4)_2}) = 310.18$ .
Describe the structures of $\mathrm{P_4O_6}$ and $\mathrm{P_4O_{10}}$ , and write equations for their reactions with water.
Write equations for the complete hydrolysis of $\mathrm{PCl_3}$ and of $\mathrm{PCl_5}$ .
Calculate the amount of hydrogen chloride formed when 2.085 g of $\mathrm{PCl_5}$ is completely hydrolysed. $M(\mathrm{PCl_5}) = 208.22$ .
Draw the structure of phosphonic acid, $\mathrm{H_3PO_3}$ , and explain why it is dibasic.
Write the equation for the wet process, and give one environmental problem it causes.
Explain why phosphoric acid, not concentrated sulphuric acid, is used to make hydrogen iodide from potassium iodide.
A farmer needs to supply 23 kg of nitrogen to a field. What mass of DAP labelled 18-46-0 is needed, and how much $\mathrm{P_2O_5}$ -equivalent does it add?

Show answersHide answers

$\mathrm{2Ca_3(PO_4)_2 + 6SiO_2 + 10C \rightarrow 6CaSiO_3 + 10CO + P_4}$ . $n = \frac{1000}{310.18} = 3.224$ mol of $\mathrm{Ca_3(PO_4)_2}$ , containing $6.448$ mol P; mass $= 6.448 \times 30.97 = 200\ \mathrm{g}$ .
Both: $\mathrm{P_4}$ tetrahedron with an O bridging each of the six edges; $\mathrm{P_4O_{10}}$ also has a terminal O on each P. $\mathrm{P_4O_6 + 6H_2O \rightarrow 4H_3PO_3}$ ; $\mathrm{P_4O_{10} + 6H_2O \rightarrow 4H_3PO_4}$ .
$\mathrm{PCl_3 + 3H_2O \rightarrow H_3PO_3 + 3HCl}$ ; $\mathrm{PCl_5 + 4H_2O \rightarrow H_3PO_4 + 5HCl}$ .
$n(\mathrm{PCl_5}) = \frac{2.085}{208.22} = 0.0100$ mol; HCl $= 5 \times 0.0100 = 0.0500$ mol.
P with one =O, two –OH and one H bonded directly to P. Only the two H atoms on oxygen are released as $\mathrm{H^+}$ ; the P–H bond is not polar enough to ionise.
$\mathrm{Ca_3(PO_4)_2 + 3H_2SO_4 \rightarrow 3CaSO_4 + 2H_3PO_4}$ . It produces large heaps of phosphogypsum, which can contain traces of radioactive elements (and fluoride), so it is hard to use or dispose of.
Concentrated sulphuric acid is an oxidising agent and would oxidise the iodide (or HI) to iodine; phosphoric acid is not an oxidising agent, so HI is obtained.
Mass of DAP $= \frac{23}{0.18} = 128\ \mathrm{kg}$ (about 130 kg); $\mathrm{P_2O_5}$ -equivalent $= 0.46 \times 128 = 59\ \mathrm{kg}$ .

Quick check

Phosphonic acid, H₃PO₃, has three hydrogen atoms but is only dibasic. Why?

Key points

Phosphorus is made from phosphate rock, sand and coke in an electric furnace. $\mathrm{P_4O_6}$ and $\mathrm{P_4O_{10}}$ are acidic oxides giving $\mathrm{H_3PO_3}$ and $\mathrm{H_3PO_4}$ ; $\mathrm{P_4O_{10}}$ is a powerful drying agent.
$\mathrm{PCl_3}$ and $\mathrm{PCl_5}$ are hydrolysed to the oxoacids and HCl; $\mathrm{PCl_5}$ exists because P can expand its octet (N cannot).
Only H bonded to O is acidic: $\mathrm{H_3PO_4}$ tribasic, $\mathrm{H_3PO_3}$ dibasic, $\mathrm{H_3PO_2}$ monobasic.
Phosphoric acid (wet process: rock + $\mathrm{H_2SO_4}$ ) is weak, tribasic and non-oxidising; it makes phosphate fertilisers such as superphosphate and DAP.

Sources and credits

Image: Phosphorite (GeoDIL number - 301).jpg by Mike Davis, via Wikimedia Commons, CC0.
Image: DAP (Diammonium Phosphate) Granules (1).jpg by Suyash Dwivedi, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Image: Phosphorus pentachloride ampoule.jpg by W. Oelen, via Wikimedia Commons, licensed under CC BY-SA 3.0.
DAP among Rwanda's main fertilisers: How Rwanda's Fertiliser Subsidy Scheme Works, The New Times (Kigali), 14 July 2026, via allAfrica.
$\mathrm{p}K_a$ values, melting point and the wet process from Wikipedia, Phosphoric acid; phosphate rock from Phosphorus; both CC BY-SA 4.0, adapted by Ganzaa.
First aid for chemical splashes: NIOSH Pocket Guide, first aid, CDC.
Video: Reaction of Water with Phosphorus Pentachloride, Science Skool, embedded from YouTube.
The text, structure diagram, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1From rock to root

2Making phosphorus

3The oxides

4The chlorides

5Oxoacids: count the O–H bonds, not the H atoms

6Phosphoric(V) acid, H3PO4 \mathrm{H_3PO_4} H3​PO4​

7Phosphate fertilisers