6N.3 7th edition

[NatBrown1I]

How do you know which half reactions to use for this question?

[Iona Pay]

You can tell what half reactions to use by looking at the cell diagram - anode (oxidation) on the left side || cathode (reduction) on the right side. For example, part A:

Pt(s)|H2(g)|HCl(aq)||HCL(aq)|H2(g)|Pt.

From looking at this cell diagram, we can see that both sides of the cell diagram involve H2 and H+ (HCl is a strong acid and disassociates entirely, leaving us with H+ and Cl- ions in solution). Thus:

Oxidation (left): H2 ---> 2H+ + 2e-
Reduction (right): 2H+ + 2e- ---> H2