Page 203 - Chemistry ICSE Class X

Metallurgy 189
heat
3MnO 2 + 4Al(s) o 3Mn + 2Al O + Heat
3
2
manganese aluminium manganese aluminium oxide
dioxide (reducing agent)
heat
Fe O 3 + 2Al(s) o 2Fe(l) + Al O + Heat
3
2
2
ferric oxide iron
heat
Cr O 3 + 2Al(s) o 2Cr + Al O + Heat
3
2
2
chromium oxide chromium
Thermite is placed in a crucible having a plug-hole in its bottom
(Fig. 7.5). A ribbon of magnesium, called fuse, is inserted into the mixture.
6JG CTVKENG VQ DG YGNFGF KU RNCEGF KP C ſTG ENC[ OQWNF DGNQY VJG RNWI JQNG
When magnesium ribbon fuse is ignited, the following reaction takes place: Fig. 7.5 Thermite welding
2Al + Fe O o Al O (s) + 2Fe(l) + Heat
3
2
2
3
aluminium ferric oxide aluminium iron
(haematite) oxide (molten) ŵŝdžƚƵƌĞ ŽĨ ĨĞƌƌŝĐ ŽdžŝĚĞ ;&Ğ O Ϳ
Ϯ ϯ
ĂŶĚ ĂůƵŵŝŶŝƵŵ ƉŽǁĚĞƌ ; ůͿ ŝƐ ĐĂůůĞĚ
How are the metals of high reactivity extracted thermite͘
The metals which appear high up in the activity series are highly reactive.
These metals cannot be obtained from their compounds by carbon
aluminium reduction method DGECWUG VJG[ JCXG XGT[ JKIJ CHſPKV[ HQT DĞƚĂůƐ͕ ƐƵĐŚ ĂƐ ƐŽĚŝƵŵ͕
oxygen. ƉŽƚĂƐƐŝƵŵ͕ ĐĂůĐŝƵŵ͕ ŵĂŐŶĞƐŝƵŵ
ĂŶĚ ĂůƵŵŝŶŝƵŵ ĂƌĞ ŽďƚĂŝŶĞĚ ďLJ
Such highly reactive (or electropositive) metals are obtained by ĞůĞĐƚƌŽůLJƟĐ ƌĞĚƵĐƟŽŶ ŵĞƚŚŽĚ͘
electrolytic reduction method. In electrolytic reduction method, cation of ͻ
the metal is reduced by passing electricity through molten oxide, hydroxide ^ŽĚŝƵŵ ŵĞƚĂů ĐĂŶŶŽƚ ďĞ ŽďƚĂŝŶĞĚ
or chloride of the metal. Electrolytic reduction method is illustrated through ďLJ ĞůĞĐƚƌŽůLJƐŝŶŐ ĂŶ ĂƋƵĞŽƵƐ
the following examples: ƐŽůƵƟŽŶ ŽĨ ƐŽĚŝƵŵ ĐŚůŽƌŝĚĞ͘ dŚŝƐ ŝƐ
ďĞĐĂƵƐĞ͕ ǁŚĞŶ ĂŶ ĂƋƵĞŽƵƐ ƐŽůƵƟŽŶ
Extraction of sodium by electrolytic reduction method. Sodium metal ŽĨ ƐŽĚŝƵŵ ĐŚůŽƌŝĚĞ ŝƐ ĞůĞĐƚƌŽůLJƐĞĚ
is obtained by passing electricity through molten sodium chloride. ;ƵƐŝŶŐ ƉůĂƟŶƵŵ ĞůĞĐƚƌŽĚĞƐͿ͕ ƚŚĞ
heat
NaCl(s) o Na + + Cl – (Dissociation) ŝŽŶ ǁŚŝĐŚ ŐĞƚƐ ƌĞĚƵĐĞĚ Ăƚ ĐĂƚŚŽĚĞ
н
н
solid sodium chloride in molten sodium chloride ŝƐ , ĂŶĚ ŶŽƚ EĂ ͘ Ɛ Ă ƌĞƐƵůƚ͕ ƚŚĞ
–
+
At cathode: Na + e o Na (Reduction) ƉƌŽĚƵĐƚ ĨŽƌŵĞĚ Ăƚ ƚŚĞ ĐĂƚŚŽĚĞ
sodium ion sodium metal ĚƵƌŝŶŐ ĞůĞĐƚƌŽůLJƐŝƐ ŽĨ ĂŶ ĂƋƵĞŽƵƐ
ƐŽůƵƟŽŶ ŽĨ ƐŽĚŝƵŵ ĐŚůŽƌŝĚĞ ŝƐ
–
At anode: Cl o 1 Cl (g) + e – (Oxidation) hydrogen gas ĂŶĚ ŶŽƚ ƐŽĚŝƵŵ
2 2
chloride ion chlorine gas ŵĞƚĂů͘
The net overall reaction may be written as, ͻ
electricity
NaCl (molten) o Na + 1 Cl (s) ƵƌŝŶŐ ĞůĞĐƚƌŽůLJƟĐ ƌĞĚƵĐƟŽŶ ŽĨ
2 2 ŵŽůƚĞŶ ƐĂůƚƐ͕ ƚŚĞ ĐŽƌƌĞƐƉŽŶĚŝŶŐ
sodium chloride sodium metal chlorine gas ŵĞƚĂů ŝƐ ŽďƚĂŝŶĞĚ Ăƚ ƚŚĞ ĐĂƚŚŽĚĞ
(at cathode) (at anode) ;ƚŚĞ ŶĞŐĂƟǀĞ ĞůĞĐƚƌŽĚĞͿ͘
electricity
or 2NaCl (molten) o 2Na + Cl (g) ͻ
2
Thus, we see that sodium metal is obtained at cathode and chlorine gas >ŝƋƵĂƟŽŶ ŵĞƚŚŽĚ ŝƐ ƵƐĞĚ ĨŽƌ
ƌĞĮŶŝŶŐ ůŽǁͲŵĞůƟŶŐ ŵĞƚĂůƐ͕ ƐƵĐŚ ĂƐ
is evolved at anode. ƟŶ ;^ŶͿ͕ ůĞĂĚ ;WďͿ͕ ďŝƐŵƵƚŚ ; ŝͿ͕ ĞƚĐ͘
Refining of Metals
Metals obtained by reduction processes contain small quantities of
various impurities. The process of removing impurities from the metals
GZVTCEVGF HTQO VJGKT QTGU KU ECNNGF TGſPKPI QH OGVCNU
5QOG EQOOQP OGVJQFU WUGF HQT TGſPKPI OGVCNU CTG FGUETKDGF DGNQY

What is the liquation method for refining metals
In this method, impure metal is placed on the sloping hearth. The
temperature of the sloping hearth is raised slightly above the melting
RQKPV QH VJG OGVCN 6JG OQNVGP RWTG OGVCN ƀQYU FQYP CPF VJG KORWTKVKGU
are left behind on the hearth (Fig. 7.6). Fig. 7.6 The liquation method