AP Chemistry Section 20.3
Continued
DEVANGI
LAURA
Phosphorous Oxides and Oxyacids
 phosphorous reacts with oxygen to form oxides
 In these it has oxidation states of either +3 or +5
 The oxide P4O6 is formed when elemental
phosphorous is burned in limited oxygen and P4O6
is made when oxygen is in excess
 You can think of these as merely adding oxygens to
the basic P4 structure
 Intermediate states include: P4O7, P4O8, and P4O9
More Fun Stuff
 P4O10 Tetraphosphorous decoxide has large affinity for
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water and so is powerful dehydrating agent
Can be used to convert HNO3 and H2SO4 to their parent
oxides (N2O5 and SO3)
P4O10 dissolves in water to form phosphoric acid
(H3PO4) aka orthophosphoric acid
The reaction is P4O10 (s) + 6H2O (l)  4H3PO4 (aq)
pure phosphoric acid is white solid melting at 42
degrees Celsius
Aqueous phosphoric acid is much weaker and a poor
oxidizing agent
The Fun Continues
 Phosphate minerals are the main sources of phosphoric
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acid
They are only found in nature in a combined state, such
as Ca3(PO4)2 and Ca5(PO4)3F
Fluorapatite can be converted to phosphoric acid by
crushing the phosphate rock and forming a slurry with
sulfuric acid
Reaction: Ca5(PO4)3F (s) +5H2SO4 (aq) + 10H2O (l) 
HF (aq) + 5CaSO4 + 2H20 (s) + 3H3PO4 (aq)
Product gypsum is used to make wallboard for buildings,
and process is called wet process, producing impure
phosphoric acid
 Producing white phosphorous:
 12Ca(PO4)3F + 43SiO2 + 90C  9P4 + 90CO +
20(3CaO . 2SiO2) + 3SiF4
 White phosphorous from above is burned in air to
make tetraphosphorous decoxide, which is then
combined with water to form phosphoric acid
 Phosphoric acid undergoes condensation reactions,
meaning a water molecule is eliminated in joining of
two acid molecules (picture from book)
 Product H4P2O7 is called pyrophosphoric acid
 Further heating produces polymers like
tripolyphosphoric acid (H5P3O10)
 Sodium salt of tripolymphosophirc acid is widely
used in detergents because P3O10 5- anion can form
complexes with metal ions such as Mg2+ and Ca2+
which otherwise would interfere with detergent’s
action
 Molecular structure and hybridization of central
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atom of phosphoric acid molecule?
Hydrogen atoms are attached to oxygens
Phosphorous atom is surrounded by four effective
pairs, which are arranged tetrahedrally
Atoms is sp3 hybridized
(picture in book)
 When P4O6 is put in water, phosphorous acid
(H3PO3) is formed. Phosphorous acid is a diprotic
acid (NOT TRIPROTIC because the hydrogen atom
bonded directly to the phosphorous atom is not
acidic in aqueous solution. Only those bonded to
oxygen atoms can be given up as protons)
 Third oxyacid of phosphorous is hypohosphrous acid
(monoprotic)
Phosphorous in Fertilizers
 Helps in plant growth
 In large quantities in soil, but often present as
insoluble minerals, so hard for plants to absorb it
 BUT, soluble phosphate fertilizers can be made by
treating phosphate rock with sulfuric acid to make
superphosphate of lime, which is a mixture of CaSO4
. 2 H20 and Ca(H2PO4)2 . H2O
 Reaction between ammonia and phosphoric acid
gives ammonium dihydrogen phosphate (super
effective since in gives both nitrogen and
phosphorous)
Phosphorous Halides
 Phosphorous forms all possible halides in general
forms Px3 and Px5 (only exception is PI5)
 The PX3 molecule has pyramidal structure
 Under normal conditions of temperature and
pressure, PF3 is colorless gas, PCl3 is liquid, PBr3 is
liquid, and PI3 is unstable red solid
 These PX compounds all react with water to produce
phosphorous acid (reaction: PX3 + 3H2O (l) 
H3PO3 (aq) + 3HX (aq)
Phosphorous Halides continued
 In gaseous and liquid states, PX3 compounds are in
trigonal bipyramidal structure
 But, PCl5 and PBr5 form ionic solids
 Solid PCl5 contains mixture of octahedral PCl6 –
ions and tetrahedral PCL4 – ions and solid PBr5 has
PBr4 + ions and Br- ions
 PX5 compounds react with water to form phosphoric
acid: (reaction: PX5 + 4H2O (l)  H3PO4 (aq) +
5HX (aq)