METALS: EXTRACTION of SODIUM Sodium, the fourth most abundant metal in the Earth's crust (2.4%), is usually found as the chloride (e.g., in the ore rock salt or dissolved in sea water). Sodium has a low melting point (98°C) and a low density (0.97 g cm-³), which are characteristics of Group 1 metals, and forms (usually) white or colourless compounds in only one oxidation state (I), which is typical of main group metals. [.. K > Ca > Na > Mg > Al > Zn > Fe > Sn > Pb > (H) > Cu > Hg > Ag ..]
1. Sodium is extracted by the electrolytic reduction of purified, molten rock salt; the ionic equations for the reactions occurring at the electrodes are:
(a) Label this diagram of a Downs electrolytic cell with: Carbon anode; Chlorine gas; Insulator; Molten electrolyte; Molten sodium; and, Steel cathode.
(b) To lower the melting point of sodium chloride from 774°C to about 600°C, and so reduce the energy costs, calcium chloride is added to the rock salt. As a result of this added impurity, a mixture of molten sodium and solid calcium is obtained. Write an ionic equation for the reduction of calcium ions at the cathode. _______________________________________________________________________  (c) Water must not be present in the Downs cell, partly because a different cathodic reaction occurs. Thus, hydrogen gas is evolved at the cathode from the electrolysis of an aqueous solution containing Na1+(aq), Cl1-(aq), H1+(aq), and OH1-(aq) ions. Write an ionic equation for the reduction of hydrogen ions at the cathode. _______________________________________________________________________  (d) After manufacture, sodium is stored under oil because it is rapidly oxidized by atmospheric oxygen. Furthermore, sodium reacts explosively with many other non-metallic elements and compounds. Construct the symbol equation for the reaction of sodium with: Difluorine ____________________________________________________________ Dioxygen ______________________________________________________________ Water _________________________________________________________________ 
Sodium's high reactivity has necessarily restricted its use to a few applications. Thus, aside from its incorporation into specialized alloys (e.g., Na-Hg amalgam), it is used in street vapour lamps, as a coolant in nuclear reactors, and as a chemical reducing agent.
2. Sodium's use as a coolant in nuclear reactors depends on three of the metal's properties. First, it is a liquid at temperatures between 98°C and 883°C, so the metal flows easily around the hot reactor core (which operates at a temperature of about 660°C). Second, it is a good thermal conductor (because of its free moving delocalized electrons), so excess heat generated in the core is conducted away efficiently. And third, it has a relatively high thermal capacity, so large amounts of heat energy are absorbed for any given temperature rise.
[Q = n × z × F and Q = I × t, where: Q, measured in coulombs (C), is the quantity of electricity; n is the number of moles of substance evolved at the electrode; z is the charge on the ion; F is a constant, with a value of 96500 C mol-¹; I, measured in amps (A), is the current; and t, measured in seconds (s), is the time.]
(a) A fast-breeder reactor under construction required 690 tonnes of sodium. A typical Downs electrolytic cell operates continuously at a current of 25 kA. Determine the time (t) it would take to obtain the required mass of sodium - as follows. Convert this mass (m) of sodium from tonnes to grammes (where 1 tonne = 1000 kg). ___________________________________________________ Calculate the number of moles (n) in this mass (m) of sodium. _________ _______________________________________________________________________ Calculate the quantity of electricity (Q) required to deposit this number of moles (n) of sodium at the cathode. _________________________ _______________________________________________________________________ And finally, calculate the time (t) taken for this quantity of electricity (Q) to be used. ___________________________________________ _______________________________________________________________________  (b) The volume (V1) of one mole of any gas at room temperature (25°C = 298 K; T1) and pressure (0.1 MPa; P1) is 24 dm³; furthermore, the following relationship holds true for gases: P1 × V1 P2 × V2 ¾¾¾¾¾ = ¾¾¾¾¾ T1 T2 Determine the volume (V2) of chlorine gas obtained, at room temperature and high pressure (20 MPa; P2), during the production of this mass of mass of sodium - as follows. State the number of moles (n) of chlorine atoms, Cl(g), formed at the anode. ________________________________________________________________ State the number of moles of chlorine gas, Cl2(g), evolved at the anode. ________________________________________________________________ Calculate the volume of gas (V1) obtained at room temperature (T1) and pressure (P1). ________________________________________________________ Then, using the above relationship, calculate the volume (V2) of gas at the increased pressure (P2). _______________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________  3. Sodium metal reduces liquid ammonia to sodium amide, NaNH2(s), a sparingly used fertilizer. Construct the symbol equation for this redox reaction, which is executed under anhydrous and anaerobic conditions. _______________________________________________________________________ 
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