NCING OXIDATION-REDUCTION EQUATIONS ELECTRON METHOD)

In this method of balancing the redox equations, only those reactants and products are balanced that contain the elements undergoing a change in oxidation state, That is t say, only those reactants and products are balanced which are actually oxidized or reduced. For this purpose it is important to transform molecular equation into ionic equation. Following key points must be kept in mind while writing down an ionic equation: 

(a) Tonic substances are written in the ionic form only if the ions are separated from each other in the reaction medium. For example solid NaCl, Na’ and 

Cl are not written because these ions are held together in its crystal. When present in solution, however, NaCl would be indicated by Na’ and CI’, or either of these ions alone if only the sodium or the chlorine undergoes a 

change in oxidation state. Insoluble salts such as BaSO, are always written in the neutral form. 

(b) Partially ionized substances are written in the ionic form only if the extent of ionization is appreciable. Thus water which is very little ionized, is written as H,O. Strong acids such as HCI, HNO,, are written in the ionized form but weak acids such as nitrous acid, sulphurous acid (HNO, H,SO,) are always written in the molecular form. Strong bases such as NaOH, KOH may be written in the ionic form as Na* and OH’ or K* and OH-. A weak base like 

NH, is written as NH,. 

(c) Complex ions such as ferrocyanide [Fe(CN),]* or ferricyanide, [Fe(CN),J* 

which are stable and are written as such and never as separate ions like Fe** and CN-, 

Rules for Balancing by Ion-Electron Method : 

1. Write a skeleton equation which includes those reactants and products that contain the elements undergoing a change in oxidation state. 

2. Transform the molecular skeleton equation into ionic form. If the given equation is already in the ionic form, then this step is omittted.