What is the effect on the emf of the cell shown in Figure 20.9, which has the overall reaction Zn(s) + 2H+ (aq) → Zn2+(aq) + H2(g), for each of the following changes?
(a) The pressure of the H2 gas is increased in the cathode half-cell.
(b) Zinc nitrate is added to the anode half-cell.
(c) Sodium hydroxide is added to the cathode half-cell, decreasing [H+].
(d) The surface area of the anode is doubled.