ELIiCTRO-MOTlVE SERIES OF THE METALS. 247 



each atom a number expressing its tendency to stick to its electric 

 charge when in the ionic form, or its tendency to take up an 

 electric charge if in the atomic form. Since the conversion of 

 metal into salt consists in the change metal to ion, we have then 

 that the position of a metal in the Electro-motive Series really 

 gives us its relative tendency to oxidation, to salt formation, and 

 to displace hydrogen from acids. 



According to this view, the liberation of hydrogen from an 

 acid b}' ^1 metal is exactly the same process as the deposition of 

 copper from a solution of copper sulphate by metallic zinc, since 

 this reaction mav be expressed bv the equation, 



+ ++-}-++■ 

 Zn + Cu + S04=: Zn -4- Cu + SO^. 

 Metal ions ions ions metal ions. 



+ + -h + 



i.c, Zn -j- Cu = Cu + Zn. 

 A'letal ions metal ions. 



Such a deposition of one metal by another is governed by 

 the relative affinity of the metal atom to go into the ionic state, 

 i.e., by its position in tlie Electro-motive Series. Now hydrogen 

 behaves in all electrolytic processes as a metal, and so has a 

 definite position in the Electro-motive Series. Hence the metals 

 above it should displace hydrogen from a solution of its ions, i.e.,. 

 from an acid solution. On the other hand, no hydrogen will, as 

 a rule, be evolved by the action of acids on the metals which are 

 below hydrogen. This important method of viewing the action 

 of acids on metals is capable of extension iji many ways. Applying" 

 the law of mass action to the reaction 



]\Ietal -\- Hydrogen ions := Metal ions 4- Hydrogen gas, 

 i.e., an acid. i.e., a salt. 



we see thiat the tendency of the reaction to proceed frooi left to 

 right depends not only on the affinity of the metal for an electric 

 charge (which affinity finds its expression in the mass action 

 constant), but also on the concentration of the hydrogen ions, 

 i.e., on what we often refer to as the " strength " of the acid and 

 lalso on its concentration. Thus it is that some metals will cause 

 the evohition of .hydrogen froin weak acids like acetic and which 

 contain relatively few hydrogen ions, whereas other metals lower 

 down in the Electro-motive Series are unafi'ected. Further, the 

 affinity of potassium, sodium and calcium for an electric charge 

 or their ionisation tendency is so great that they will displace 

 hydrogen from solutions containing even very small amounts of 

 hydrogen ions, e.g., from pure water. It is, however, clear that 

 only the metals at the top of the series will .possess this capability. 



Further, we know that hot concentrated sulphuric acid will 

 attack copper and silver, forming sulphates and liiberating S02. 



A student occasionally asks if the Hberation of vSOsis due to 

 the reduction of the H% SOi by nascent hydrogen or by copper. 

 According to its position in the Electro-motive Series copper 

 cannot displace hydrogen from sulphuric acid. It can only go 



