THE ELECTRO-MOTIVE SERIES OF THE ^lETALS AS 



AN AID TO THE TEACHING OF INORGANIC 



CHEMISTRY; WITH SPECIAL REFERENCE TO 



THE ACTION OF ACIDS ON METALS. 



By Prof. Ror.ERT Beckett Denjson, D.Sc, Pji.D. 



Perhaps the greatest difficulty in the teaching of Inorganic 

 Chemistry to young students is that they persist in regarding the 

 subject as a collection of facts which have to be learned by heart. 

 When they have grasped the meaning of the Periodic Classifica- 

 tion there does come, it is true, a little law and order among the 

 facts, but there ,are many phenomena with regard to which the 

 Periodic Classification gives but little help. Particularly is this 

 the case when it is a question of salt formation from a metal 

 and an acid, and, in short, wherever the fundamental nature of 

 the process is the formation or disappearance oi ions. Even with 

 a knoiwledge of the Periodic Classification the student generally 

 finds less difficulty with Organic than with Inorganic Chemistry 

 after he has once overcome the initial apparent difficulties O'f the 

 fonmer. This is because, as he puts the matter, you know what 

 is going to happen to an organic substance if you know its con- 

 stitution, whereas in Inorganic Chemistry there is no such guide. 

 Most inorganic substances are electrolytes, and so in aqueous 

 solution are largely in the ionic condition. It is, perhaps, this 

 common behaviour which causes the constitution of an inorganic 

 compound to be of so little importance. What we require to 

 know is the relative tendency of the difi'erent substances to go 

 into the ionic condition. This knowledge is given us, not by the 

 Periodic Classification, but by the Electro-motive Series. 



The Electro-motive, Series is generally regarded as a physi- 

 cal rather than a chemical classification. To the student of 

 Physics it means that if two metals are separated by la suitable 

 electrolyte and joined outside by a wire, an electric current will 

 flow in a certain direction depending on the relative position of 

 the metals in the series. 



To a student of Chemistry, however, it should mean much 

 more than this. Writing down the commoner metals as they 

 occur in the Electro-motive Series, vij;;., 



K, Na. Ca, Mg. Al. Zn. Fe, Sn. Tb, Cu. Hg, Ag, Pt. Au, 



(H) 

 we see at once that the inost eiasily oxidised metals are at the 

 head of the series. This can be demonstrated very strikingly to 

 a class by exhibiting samples of Potassium, Sodium, Calcium, 

 Alagnesium anrl Aluminium. The older the specimens are, the 

 more evident is it that the above order represents the tendency 

 towards oxidation. If now, we take any basic oxide and treat 

 it with an acid, we get, as every student knows, water plus a 

 salt. Personally, I always mention the water first, because it is^ 

 really the formatioii of water whLh causes the reaction to t*--^ 

 place. /IrS^ 



(zIl-lgRARy 





