Q.(a)

(a) Oxygen has wide uses from respiration to steelmaking and rocketry, and bleaching powder is made by passing chlorine over slaked lime; (b) Werner's theory explains coordination compounds using primary and secondary (spatially directed) valencies.

(a)(i) Uses of oxygen:

1. Essential for respiration in living organisms and for combustion of fuels.
2. Used (mixed with acetylene or hydrogen) in oxy-acetylene/oxy-hydrogen flames for welding and cutting metals, because it supports a very high-temperature flame.
3. Used in large quantities in the manufacture of steel, where oxygen is blown through molten pig iron (basic-oxygen/Bessemer-type converter process) to burn off excess carbon and other impurities.
4. Used as an oxidiser in rocket propellants (liquid oxygen, LOX).
5. Used in hospitals for oxygen therapy and in anaesthesia.
6. Used in water and sewage treatment (e.g. ozonation) and in the manufacture of various chemicals.

(a)(ii) Preparation of bleaching powder:

Bleaching powder (calcium oxychloride, $CaOCl_2$) is manufactured industrially by passing chlorine gas over dry slaked lime ($Ca(OH)_2$) spread in thin layers (traditionally in a Backmann's plant or similar chamber):

$Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O$

$CaOCl_2$ is technically a mixed salt of calcium chloride and calcium hypochlorite, $Ca(OCl)Cl$, and it releases chlorine (and hence has oxidising/bleaching action) when treated with dilute acids.

(b) Postulates of Werner's theory (of coordination compounds):

1. Metals exhibit two types of valency: primary valency, which is ionisable and is satisfied by negative ions (this corresponds to the modern idea of oxidation state), and secondary valency, which is non-ionisable and is satisfied by either neutral molecules or negative ions (this corresponds to the modern idea of coordination number).
2. Every metal atom/ion tends to satisfy both its primary and secondary valencies; if the primary valency is not fully satisfied by negative ions, neutral molecules may help satisfy it (as constituents of the outer sphere). …