Hartley — A Study in Thermo- Chemistry. 39 



The Smelting of Tin Ores. — Reduction of Stannic Oxide : — 



SnOa + 2 CO = Sn + 2 CO2 + (69 X 2 - 135-8) heat-units. 

 Heat evolved = 2-2 units. 



These are all exothermic reactions, and for the most part easily 

 realized ; the last, however, evolves but little heat, and so requires 

 the highest temperature to be applied in order to begin the 

 reaction. 



(2.) Eeduction by Carbon. 



Zinc Smelting. — In the smelting of zinc, the ore which is usually 

 calamine, or zinc carbonate, is mixed with powdered anthracite, 

 and strongly heated in fire-clay retorts ; the carbonate is first con- 

 verted into oxide, and this is reduced ; the metal, being volatile, is 

 condensed as the vapour escapes. 



The reaction is as follows : — 



ZnO + C - Zn + CO + (28 - 86-2) heat-units. 

 Heat absorbed = 58'2 units. 



Here it will be seen that the reaction is strongly endothermic, 

 and it probably does not take place in the zinc-smelting furnace. 

 It necessitates a very high temperature in order that 58*2 heat- 

 units may be supplied to each molecule of zinc oxide and charcoal. 

 It is, however, much more than probable that the CO2 driven off 

 the calamine became CO by the action of excess of carbon, and 

 the reaction is then as follows : — 



ZnO + CO = Zn + CO2 + (69 - 86-2) heat-units. 



Heat absorbed = 17'2 units. 



But 



CO2 + C = 2 CO + (56 - 97) heat-units. 



Heat absorbed = 41 units. 



The result as to heat disturbance is therefore the same, viz. 

 absorption of 58*2 units. 



