Modern inorganic chemistry

GROUP VI 271

CHEMICAL PROPERTIES

As expected from the enthalpy of formation, water is thermally
very stable but when steam is heated to above 1300 K slight dis-
sociation to the elements does occur. Pure water is almost a non-
conductor of electricity but slight ionic dissociation occurs :

2H 2 O^H 3 O + +OH-.K 298 = l(T^14 mol^2 r^2

Thus water can behave as an acid towards bases stronger than itself

(p. 85), for example

H 2 O + NH 3 ^ NH^ + OH"

H 2 O + COi~ ^ HCOj + OH~

and as a base to acids stronger than itself, for example

H 2 o + HCI ^ H 3 o+ + cr

H 2 O + HNO 3 ^ H 3 O+ 4- NOg

Water can also behave as both an oxidising and a reducing agent :

2H 2 O H- 2e~ -» H 2 (g) 4- 2OH' (aq) : E^ = - 0.83 V

Many metals are oxidised by water. At ordinary temperatures the
more electropositive metals, for example, sodium, calcium (or their
amalgams with mercury), react to give hydrogen, for example :

2Na + 2H 2 O -> 2NaOH 4- H 2 T

The reaction may be visualised as occurring thus :

Na(s) + nH 2 O -* Na + (aq) + <T(aq)

Evidence for the "solvated electron' e~ (aq) can be obtained ; reaction
of sodium vapour with ice in the complete absence of air at 273 K
gives a blue colour (cf. the reaction of sodium with liquid ammonia,
p. 126). Magnesium, zinc and iron react with steam at elevated
temperatures to yield hydrogen, and a few metals, in the presence
of air, form a surface layer of oxide or hydroxide, for example iron,
lead and aluminium. These reactions are more fully considered
under the respective metals. Water is not easily oxidised but fluorine
and chlorine are both capable of liberating oxygen:
2F 2 H- 2H 2 O -> O 2 + 4HF

2C1 2 + 2H 2 O suniight • O 2 H- 4HC1

The reactions are considered in detail in Chapter 12.