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 (98C) and a low density 
(0.97 g cm-), 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 774C to about
600C, 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.
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(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.
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(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 _________________________________________________________________
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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 98C and 883C, so the metal flows easily around the hot reactor core (which operates at a temperature of about 660C). 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-; 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. ___________________________________________ _______________________________________________________________________ [7] (b) The volume (V1) of one mole of any gas at room temperature (25C = 298 K; T1) and pressure (0.1 MPa; P1) is 24 dm; furthermore, the following relationship holds true for gases: P1 V1 P2 V2 = 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). _______________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ [6] 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. _______________________________________________________________________ [2]
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