According to reference table adv-10, which redox reaction occurs spontaneously? 1.  cu(s) + 2h+ → cu2+ + h2(g) 2.  mg(s) + 2h+ → mg2+ + h2(g) 3.  2ag(s) + 2h+ → 2ag+ + h2(g) 4.  hg(l) + 2h+ → hg2+ + h2(g)

The answers are 1, 3. and 4.
Solution:
The half-cell reactions and potentials of reaction Cu(s) + 2H+ → Cu2+ + H2(g) are
     anode: Cu(s) → Cu2+ + 2e-      -0.34
     cathode: 2H+ + 2e- → H2(g)     0.00
Therefore, the standard electrode potential for the Cu2+/Cu couple is
     E°cell = Eocathode - EoAnode = 0.00V - (-0.34V) = 0.34V
and the reaction will proceed in the forward direction.

The half-cell reactions and potentials of reaction Mg(s) + 2H+ → Mg2+ + H2(g) are
     anode: Mg(s) = Mg2+ + 2e-    +2.37
     cathode: 2H+ + 2e- = H2(g)    0.00
     E°cell = 0.00V - 2.37V = -2.37V
The reaction will not proceed in the forward direction since the standard electrode potential is negative. Hence, the reaction will be spontaneous in the reverse direction.

The half-cell reactions and potentials of reaction 2Ag(s) + 2H+ → 2Ag+ + H2(g) are
     anode: {Ag(s) =Ag+ + e-}x2    -0.80
     cathode: 2H+ + 2e- = H2(g)     0.00
     E°cell = 0.00V - (-0.80V) = 0.80V
The redox reaction is spontaneous since the standard electrode potential is positive.

The half-cell reactions and potentials of reaction Hg(l) + 2H+ → Hg2+ + H2(g) are 
     anode: Hg(l) = Hg2+ + 2e-     -0.85
     cathode: 2H+ + 2e- = H2(g)    0.00
     E°cell = 0.00V - (-0.85V) = 0.85V
Therefore, the reaction will be spontaneous.