Which of these redox reactions is spontaneous as written?

ni + zn2+ → ni2+ + zn

mg + li+ → mg2+ + li

cu + au3+ → cu2+ + au

zn + ca2+ → zn2+ + ca

Explanation: when = +ve, reaction is spontaneous

= -ve, reaction is non spontaneous

= 0, reaction is in equilibrium

1.

Here Ni undergoes oxidation by loss of electrons, thus act as anode. zinc undergoes reduction by gain of electrons and thus act as cathode.

Where both are standard reduction potentials.

As the standard emf comes to be negative, the reaction is non spontaneous.

2)

Here Mg undergoes oxidation by loss of electrons, thus act as anode. Lithium undergoes reduction by gain of electrons and thus act as cathode.

As the standard emf comes to be negative, the reaction is non spontaneous.

3)

Here Cu undergoes oxidation by loss of electrons, thus act as anode. Gold undergoes reduction by gain of electrons and thus act as cathode.

As the standard emf comes to be positive, the reaction is spontaneous.

4)  

Here Zn undergoes oxidation by loss of electrons, thus act as anode. Calcium undergoes reduction by gain of electrons and thus act as cathode.

As the standard emf comes to be negative, the reaction is non spontaneous.