CHEMISTRY

TOPIC: SODIUM AND ITS COMPOUNDS – PART 3

By Kingsley Idiagbor, B.Sc. (Hon), PGDCs, NCE, MNSM

 

 

Sodium Trioxocarbonate (IV)

hen carbon (IV) oxide is absorbed by sodium hydroxide (or caustic soda), the resultant compound is Sodium hydrogen trioxocarbonate (IV), NaHCO3.  This, in turn is filtered off, washed and heated to dryness to finally obtain Sodium trioxocarbonate (IV).  The process is summarized in these chemical equations:

2NaOH_(aq)  +  CO_2(g) ------------> Na₂CO_3(aq)  +   H_2(g)

Na₂CO_3(s)  + H₂O _(l)  + CO_{2 (g)}----->2NaHCO₃

2NaHCO₃--------------------> Na₂CO_3(s)  + H₂O _(l)  + CO_{2 (g)}

Solvay Process

This is the industrial process for the manufacture of sodium trioxocarbonate (IV) on a large scale. The process is named after the Solvay tower that is used. This is a cylindrical and tall tower that consists of baffle plates which slows down the flow of liquid and any gas and also to provide large surface area for the reactions taking place in the tower.

 

Solvay process involves the dissolution of ammonia gas in concentrated brine to form what is known as ammoniacal brine. This ammoniacal brine solution is made to trickle down from a shower at the peak of the Solvay tower through the dome-shaped baffle plates.  At the same time, carbon (IV) oxide gas is forced upwards from the bottom of the tower.  The following reactions will take place:

·        The ammoniacal brine reacts with the carbon (IV) oxide gas to form ammonium hydrogen trioxocarbonate (IV);

NH_3(g) + H₂O_(l) + CO_2(g) ------------------->NH₄HCO_{3 (aq)}

·        In the presence of sodium chloride, sodium hydrogen trioxocarbonate (IV) is formed

NH₄HCO_3(aq)  + NaCl_(aq) ------------------->NaHCO_3(s)+ NH₄Cl_(aq)

·        The sodium hydrogen trioxocarbonate (IV) is filtered, washed and heated to dryness to obtain sodium trioxocarbonate (IV) while steam and carbon (IV) oxide which are also formed may be recycled in the Solvay tower.

2 NaHCO_{3 (s)}----------------------> Na₂CO_3(s)  + H₂O _(l)  + CO_{2 (g)}

·        The sodium trioxocarbonate (IV) obtained at this stage is in the form called soda ash and may then be dissolved further in water and recrystallized to produce washing soda which is a decahydrate.

Na₂CO_3(s) + 10 H₂O _(l) ------------------> Na₂CO₃,10H₂O_(s)

 

The Economy of the Solvay Process

It is a continuous process and so very economical. Many of the by-products of the process may be recycled often times in the tower.  Furthermore, the chief raw materials are readily obtainable and cheap. The carbon (IV) oxide used, for example, may be procured by heating limestone. CaCO_3(s)  --------------->CaO_(s)  +  CO_2(g)

By treating the ammonium chloride formed with the calcium oxide got from the decomposition of limestone, ammonia can readily be obtained to be used again and again in the tower.  Perhaps, the only by-product that may not be so useful is the calcium chloride.

CaO_(s)   +   2NH₄Cl_(aq) -------------> CaCl_2(aq)  + H₂O_(l)  +  2NH_3(g)

 

Properties

·        In the form of soda ash, sodium trioxocarbonate (IV) is a very fine white powder but washing soda is crystalline and a little bit translucent (i.e. can allow light to pass through);

·        Soda ash is sparingly soluble in water but the washing soda, i.e. the decahydrate, can dissolve in water to form an alkaline solution (by hydrolysis) and this solution can turn red litmus paper to blue.

·        If exposed to the atmosphere, Na₂CO₃, 10H₂O i.e. the decahydrate loses some of its water of crystallization to form the monohydrate.  Thus:  Na₂CO₃, 10H₂O_(s)  --------------> Na₂CO₃, H₂O_(s)   + 9H₂O_(l)

 

This process is called efflorescence.

·        Sodium trioxocarbonate, Na₂CO₃ unlike most other trioxocarbonates is heat-stable.  By this we mean that it is not affected by heat.  It is not decomposed by heat.  Sodium trioxocarbonate (IV) shares this property with potassium trioxocarbonate (IV). 

·        However, Na₂CO₃ forms carbon (IV) oxide gas when it reacts with dilute acids.

Na₂CO_{3(aq)  +}  HCl _(aq)---------------->NaCl_(aq)    +   H₂O_(l)    +   CO_2(g)

 

Uses of  Na₂CO₃

·        It is extensively used in the making of glass;

·        Sodium trioxocarbonate (IV) is also used in the manufacture of sodium hydroxide, borax and soap;

·        It is a useful reagent in the qualitative analysis of metal cations

·        Sodium trioxocarbonate, like sodium hydroxide,  is used to standardize dilute acids especially in acid-base titrations.

 

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