1 (805)

Prévia do material em texto

196 6 CHEMICAL EQUILIBRIUM
will decrease and so Ecell will increase . �is is con�rmed by the Nernst
equation. Treating the hydrogen as a perfect gas so that aH2 = pH2/p−○ ,
the Nernst equation for this cell is
Ecell = E−○cell −
RT
2F
ln( pR
pL
)
If pL is increased then Ecell will increase.
(c) �e reduction half-reactions for the cell are
R: MnO2(s) + 4H+(aq) + 2e− →Mn2+(aq) + 2H2O(l)
L: [Fe(CN)6]3−(aq) + e− → [Fe(CN)6]4−(aq)
�e overall cell reaction, a�er multiplying the le�-hand reduction half-
reaction by 2 so that both half-reactions have ν = 2, is
MnO2(s) + 4H+(aq) + 2[Fe(CN)6]4−(aq)
→Mn2+(aq) + 2H2O(l) + 2[Fe(CN)6]3−(aq)
If the pH in the right-hand compartment is decreased, corresponding
to an increase in the H+ concentration, then there will be an increased
tendency for the cell reaction as written tomove in the forward direction.
�erefore ∆rG for the cell will decrease and so Ecell will increase . �is
is con�rmed by the Nernst equation. Noting that pure solids and liquids
have aJ = 1, the Nernst equation for this cell is
Ecell = E−○cell −
RT
2F
ln
⎛
⎝
aMn2+a
2
[Fe(CN)6]3−
a4H+a
2
[Fe(CN)6]4−
⎞
⎠
If the pH is decreased then aH+ will increase and therefore Ecell will in-
crease.
(d) �e reduction half-reactions for the cell are
R: Br2(l) + 2e− → 2Br−
L: Cl2(g) + 2e− → 2Cl−
�e overall cell reaction is
Br2(l) + 2Cl−(aq)→ 2Br−(aq) +Cl2(g)
If the concentration of HCl in the le�-hand compartment is increased,
then there will be an increased tendency for the overall cell reaction as
written to shi� in the forward direction. �erefore ∆rG for the cell will
decrease and so Ecell will increase .�is is con�rmed by the Nernst equa-
tion. Noting that pure liquids have aJ = 1 the Nernst equation for this cell
is
Ecell = E−○cell −
RT
2F
ln(
a2Br−aCl2
a2Cl−
)
If HCl is added then aCl− will increase so Ecell will increase.
(e) TO DO/CHECK