What is the importance of half-cell reaction

Half-cell reactions are useful in galvanic and voltaic cells in which the electrons flow from the anode to the cathode through an electrolyte to produce an electromotive force (EMF).

They are useful in galvanic and voltaic cells

Ch4 and ph3 are both covalent since they are composed both of non-metals.    ch4 is non- polar because of  london forces and has a geometry of tetrahedral  ph3 , on the other hand, is umbrella shaped polar and has dipole-dipole intermolecular force (imf) . since ph3 is polar, it has a stronger imf   

The correct answer is "hydrogen".