Chemistry Some important compounds of sodium

Topics to be covered

=> Some important compounds of sodium
=> Sodium carbonate
=> Sodium chloride
=> Sodium hydroxide
=> Sodium hydrogencarbonate
=> Biological importance of sodium and potassium

SOME IMPORTANT COMPOUNDS OF SODIUM

color{green}(★) Industrially important compounds of sodium include sodium carbonate, sodium hydroxide, sodium chloride and sodium bicarbonate.

color{green}(★) The large scale production of these compounds and their uses are described below :

Sodium Carbonate (Washing Soda), Na_2CO_3 .10H_2O

color{green}(★) Sodium carbonate is generally prepared by Solvay Process.

color{green}(•) In this process, advantage is taken of the low solubility of sodium hydrogencarbonate whereby it gets precipitated in the reaction of sodium chloride with ammonium hydrogencarbonate.

color{green}(•) The latter is prepared by passing color{red}(CO_2) to a concentrated solution of sodium chloride saturated with ammonia, where ammonium carbonate followed by ammonium hydrogencarbonate are formed. The equations for the complete process may be written as :

color{red}(2NH_3 +H_2O+ CO_2 → (NH_4)_2 CO_3)

color{red}((NH_4)_2 CO_3 + H_2O + CO_2 → 2 NH_4 HCO_3)

color{red}(NH_4 HCO_3 + NaCl → NH_4 Cl + Na HCO_3)

color{green}(★) Sodium hydrogencarbonate crystal separates. These are heated to give sodium carbonate.

color{red}(2NaHCO_3 → Na_2CO_3 + CO_2 + H_2O)

color{green}(★) In this process color{red}(NH_3) is recovered when the solution containing color{red}(NH_4Cl) is treated with color{red}(Ca(OH)_2). Calcium chloride is obtained as a by-product.

color{red}(2NH_4Cl + Ca (OH)_2 → 2NH_3 + CaCl_2 + H_2O)

color{green}(★) It may be mentioned here that Solvay process cannot be extended to the manufacture of potassium carbonate because potassium hydrogencarbonate is too soluble to be precipitated by the addition of ammonium hydrogencarbonate to a saturated solution of potassium chloride.

Properties

color{green}(★) Sodium carbonate is a white crystalline solid which exists as a decahydrate, color{red}(Na_2CO_3 . 10H_2O).

color{green}(★) This is also called washing soda. It is readily soluble in water.

color{green}(★) On heating, the decahydrate loses its water of crystallisation to form monohydrate.

color{green}(★) Above 373K, the monohydrate becomes completely anhydrous and changes to a white powder called soda ash.

color{red}(Na_2CO_3 . 10 H_2O overset(373K)→ Na_2 CO_3 . H_2O + 9 H_2O)

color{red}(Na_2CO_3 . H_2O overset( > 373 K)→ Na_2 CO_3 + H_2O)

Carbonate part of sodium carbonate gets hydrolysed by water to form an alkaline solution.

color{red}(CO_3^(2-) + H_2O → HCO_3^(-) + OH^(-))

Uses:

(i) It is used in water softening, laundering and cleaning.

(ii) It is used in the manufacture of glass, soap, borax and caustic soda.

(iii) It is used in paper, paints and textile industries.

(iv) It is an important laboratory reagent both in qualitative and quantitative analysis.

Sodium Chloride, NaCl

color{green}(★) The most abundant source of sodium chloride is sea water which contains color{red}(2.7) to color{red}(2.9%) by mass of the salt.

color{green}(★) In tropical countries like India, common salt is generally obtained by evaporation of sea water. Approximately color{red}("50 lakh tons") of salt are produced annually in India by solar evaporation.

color{green}(★) Crude sodium chloride, generally obtained by crystallisation of brine solution, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities.

color{green}(★) Calcium chloride, color{red}(CaCl_2), and magnesium chloride, color{red}(MgCl_2) are impurities because they are deliquescent (absorb moisture easily from the atmosphere).

color{green}(★) To obtain pure sodium chloride, the crude salt is dissolved in minimum amount of water and filtered to remove insoluble impurities. The solution is then saturated with hydrogen chloride gas. Crystals of pure sodium chloride separate out. Calcium and magnesium chloride, being more soluble than sodium chloride, remain in solution.

color{green}(★) Sodium chloride melts at 1081K. It has a solubility of color{red}(36.0 g) in color{red}(100 g) of water at color{red}(273 K). The solubility does not increase appreciably with increase in temperature.

Uses :

(i) It is used as a common salt or table salt for domestic purpose.

(ii) It is used for the preparation of color{red}(Na_2O_2, NaOH) and color{red}(Na_2CO_3.)

Sodium Hydroxide (Caustic Soda), NaOH

color{green}(★) Sodium hydroxide is generally prepared commercially by the electrolysis of sodium chloride in Castner-Kellner cell.

color{green}(★) A brine solution is electrolysed using a mercury cathode and a carbon anode. Sodium metal discharged at the cathode combines with mercury to form sodium amalgam.

color{green}(★) Chlorine gas is evolved at the anode.

color{green}("𝐂𝐚𝐭𝐡𝐨𝐝𝐞 :") color{red}(Na^(+) + e^(-) overset(Hg)→ Na"-amalgam")

color{green}("𝐀𝐧𝐨𝐝𝐞 :") color{red}(Cl^(-) → 1/2 Cl_2 + e^(-))

color{green}(★) The amalgam is treated with water to give sodium hydroxide and hydrogen gas.

color{red}(2Na-"amalgam" + 2H_2O → NaOH+ 2Hg +H_2)

color{green}(★) Sodium hydroxide is a white, translucent solid. It melts at 591 K.

color{green}(★) It is readily soluble in water to give a strong alkaline solution.

color{green}(★) Crystals of sodium hydroxide are deliquescent.

color{green}(★) The sodium hydroxide solution at the surface reacts with the color{red}(CO_2) in the atmosphere to form color{red}(Na_2CO_3).

Uses

color{green}("𝐈𝐭 𝐢𝐬 𝐮𝐬𝐞𝐝 𝐢𝐧 :")

(i) the manufacture of soap, paper, artificial silk and a number of chemicals,

(ii) in petroleum refining,

(iii) in the purification of bauxite,

(iv) in the textile industries for mercerising cotton fabrics,

(v) for the preparation of pure fats and oils, and

(vi) as a laboratory reagent.

Sodium Hydrogencarbonate (Baking Soda), NaHCO_3

color{green}(★) Sodium hydrogen carbonate is known as baking soda because it decomposes on heating to generate bubbles of carbon dioxide (leaving holes in cakes or pastries and making them light and fluffy).

color{green}(★) Sodium hydrogencarbonate is made by saturating a solution of sodium carbonate with carbon dioxide.

color{green}(★) The white crystalline powder of sodium hydrogencarbonate, being less soluble, gets separated out.

color{red}(Na_2CO_3 +H_2O + CO_2 → 2NaHCO_3)

color{green}(★) Sodium hydrogen carbonate is a mild antiseptic for skin infections. It is used in fire extinguishers.

BIOLOGICAL IMPORTANCE OF SODIUM AND POTASSIUM

color{green}(★) A typical color{red}(70 kg) man contains about color{red}(90 g) of color{red}(Na) and color{red}(170 g) of color{red}(K) compared with only color{red}(5 g) of iron and color{red}(0.06 g) of copper.

color{green}(•) Sodium ions are found primarily on the outside of cells, being located in blood plasma and in the interstitial fluid which surrounds the cells.

color{green}(•) These ions participate in the transmission of nerve signals, in regulating the flow of water across cell membranes and in the transport of sugars and amino acids into cells.

color{green}(•) Sodium and potassium, although so similar chemically, differ quantitatively in their ability to penetrate cell membranes, in their transport mechanisms and in their efficiency to activate enzymes.

color{green}(•) Thus, potassium ions are the most abundant cations within cell fluids, where they activate many enzymes, participate in the oxidation of glucose to produce ATP and, with sodium, are responsible for the transmission of nerve signals.

color{green}(•) There is a very considerable variation in the concentration of sodium and potassium ions found on the opposite sides of cell membranes.

color{green}(•) As a typical example, in blood plasma, sodium is present to the extent of color{red}(143 mmolL^(–1)), whereas the potassium level is only color{red}(5 mmolL^(–1)) within the red blood cells. These concentrations change to color{red}(10 mmolL^(–1) (Na^+)) and color{red}(105 mmolL^(–1) (K^+)).

color{green}(•) These ionic gradients demonstrate that a discriminatory mechanism, called the sodium-potassium pump, operates across the cell membranes which consumes more than one-third of the ATP used by a resting animal and about color{red}(15 kg) per color{red}(24 h) in a resting human.