5.114 Consider the following unbalanced oxidation-reduction reactions in aqueous solution:

Ag+(aq) + Li(s) --> Ag(s) + Li+(aq)

Fe(s) + Na+(aq) --> Fe2+(aq) + Na(s)

K(s) + H2O(l) --> KOH(aq) + H2(g)

(a) balance each of the reactions.

Ag+(aq) + Li(s) --> Ag(s) + Li+(aq)

this is balanced - the Ag and Li are both balanced, as are the charges (net charge of +1 on both sides.) what is oxidized and what is reduced here??

next:

Fe(s) + Na+(aq) --> Fe2+(aq) + Na(s)

we have to balance both atoms and charge!!!! there's a charge of +2 on the right,and only +1 on the left. This is because iron is being oxidized to iron (II) and sodium cation is being reduced. Write these out....

Fe(s) ---> Fe2+(aq) + 2e-

Na+(aq) + e- --> Na(s)

(be sure and review Ch 4, oxidation - reduction reactions, if you need to!) Now, to combine these reactions, we must make the electrons cancel - to do this we multiply the reduction of Na+ x 2 and add the reactions:

Fe(s) + 2Na+(aq) --> Fe2+(aq) + 2Na(s)

now charge and atoms are balanced.

K(s) + H2O(l) --> KOH(aq) + H2(g)

just balance it, baby.

2K(s) + 2H2O(l) --> 2KOH(aq) + H2(g)

(c) By using data in appendix C, calculate ΔHo for each of the reactions.

for

Ag+(aq) + Li(s) --> Ag(s) + Li+(aq)

for

Fe(s) + 2Na+(aq) --> Fe2+(aq) + 2Na(s)

and lastly

2K(s) + 2H2O(l) --> 2KOH(aq) + H2(g)

(c) Based on the values you obtain for ΔHo, which of these reactions would you expect to be thermodynamically favorable? (that is, which would you expect to be spontaneous?)

exothermic reactions are generally spontaneous and favorable, so the first and the third would be expected to be spontaneous.

(d) Use the activity series to predict which of these reactions should occur. Are these results in accord with your conclusion in part (c) of this problem?

remember the activity series on p. 141? Recall that any elemental metal will react with (be oxidized by) the ions of the species below is. So...

Li is at the very top of the series and should react with Ag+ - consistent with ΔHrxn= -384.4 kJ.

Ag+(aq) + Li(s) --> Ag(s) + Li+(aq)

Fe(s) + 2Na+(aq) --> Fe2+(aq) + 2Na(s)

Fe is below sodium - therefore, Na(s) will react with Fe2+ but Na+ will not react with Fe(s). Consistent with ΔHrxn = 392.3 kJ.

2K(s) + 2H2O(l) --> 2KOH(aq) + H2(g)

this is a little bit different - we don't interpret a reaction like this based on the activity series. You've seen this reaction in lab - was what you saw consistent with ΔHrxn= -393 kJ??

DAC 7/9/08