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why are the arrows this way around in this galvanic cell?

why are the arrows this way around in this galvanic cell?



The image is a bit blurry but that's Fe II and Fe III. FeCl2 and FeCl3

In a reactivity series, iron(Fe) is closer to potassium, and lead is closer to gold.. So Iron has higher oxidation potential. Lead has higher reduction potential.

So i'd expect electrons to go from Iron to Lead, based on the reactivity series. (though the arrows showing direction of electrons, have electrons not going that way, but going from lead to iron.

Looking at reduction potentials.

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/electpot.html

Pb2+(aq) + 2e- -> Pb(s) -0.13

Fe2+(aq) + 2e- -> Fe(s) -0.41

Fe3+(aq) + 3e- -> Fe(s) -0.04

Fe3+(aq) + e- -> Fe2+(aq) 0.77

According to that, Fe3+ has the higher reduction potential at 0.77 and -0.04
And Fe2+ has the lowest reduction potential of all, so the highest oxidation potential.

So electrons could go from Pb2+ to Fe3+

Or, electrons could go from Fe2+ to Pb2+

So i'm a bit confused by the reduction potentials as to which way round the external circuit, the electrons would go. Whether from Pb to Fe, or Fe to Pb.

And if I go by the reactivity series, then the arrows seem like the wrong way round

Thanks
Original post by gazbo1
why are the arrows this way around in this galvanic cell?

The image is a bit blurry but that's Fe II and Fe III. FeCl2 and FeCl3
In a reactivity series, iron(Fe) is closer to potassium, and lead is closer to gold.. So Iron has higher oxidation potential. Lead has higher reduction potential.
So i'd expect electrons to go from Iron to Lead, based on the reactivity series. (though the arrows showing direction of electrons, have electrons not going that way, but going from lead to iron.
Looking at reduction potentials.
http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/electpot.html
Pb2+(aq) + 2e- -> Pb(s) -0.13
Fe2+(aq) + 2e- -> Fe(s) -0.41
Fe3+(aq) + 3e- -> Fe(s) -0.04
Fe3+(aq) + e- -> Fe2+(aq) 0.77
According to that, Fe3+ has the higher reduction potential at 0.77 and -0.04
And Fe2+ has the lowest reduction potential of all, so the highest oxidation potential.
So electrons could go from Pb2+ to Fe3+
Or, electrons could go from Fe2+ to Pb2+
So i'm a bit confused by the reduction potentials as to which way round the external circuit, the electrons would go. Whether from Pb to Fe, or Fe to Pb.
And if I go by the reactivity series, then the arrows seem like the wrong way round
Thanks

The two reduction half equations are:
Fe3+(aq) + e- <==> Fe2+(aq) = +0.77V
Pb2+(aq) + 2e- <==> Pb(s) = -0.13V

For a spontaneous cell reaction E(cell) = E(red) - E(ox) must be positive
For this E(ox) must be the Pb||Pb2+ half cell, i.e. this is the half-cell in which oxidation is taking place (loss of electrons)
So the electrons flow from the lead half cell to the iron half cell and the overall cell reaction is:
Pb(s) + Fe3+(aq) ==> Pb2+(aq) + Fe2+(aq)
Original post by gazbo1
why are the arrows this way around in this galvanic cell?

The image is a bit blurry but that's Fe II and Fe III. FeCl2 and FeCl3
In a reactivity series, iron(Fe) is closer to potassium, and lead is closer to gold.. So Iron has higher oxidation potential. Lead has higher reduction potential.
So i'd expect electrons to go from Iron to Lead, based on the reactivity series. (though the arrows showing direction of electrons, have electrons not going that way, but going from lead to iron.
Looking at reduction potentials.
http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/electpot.html
Pb2+(aq) + 2e- -> Pb(s) -0.13
Fe2+(aq) + 2e- -> Fe(s) -0.41
Fe3+(aq) + 3e- -> Fe(s) -0.04
Fe3+(aq) + e- -> Fe2+(aq) 0.77
According to that, Fe3+ has the higher reduction potential at 0.77 and -0.04
And Fe2+ has the lowest reduction potential of all, so the highest oxidation potential.
So electrons could go from Pb2+ to Fe3+
Or, electrons could go from Fe2+ to Pb2+
So i'm a bit confused by the reduction potentials as to which way round the external circuit, the electrons would go. Whether from Pb to Fe, or Fe to Pb.
And if I go by the reactivity series, then the arrows seem like the wrong way round
Thanks

The reactivity series fails in this instance - you are using a tool for comparing the tendency of metals to form ions to compare the tendency of one metal (Pb) to form (Pb^2+) ions and one ion (Fe^3+) to be converted to another ion (Fe^2+).

If you look at the cathode in the diagram, you can see that there is no solid iron - it’s a platinum rod immersed in a solution containing FeCl3 and FeCl2. This means you don’t need to consider the electrode potentials for the processes involving solid iron - the cathode reaction must therefore be Fe^3+ + e^- <=> Fe^2+.

Charco has already explained all you need to know up to A level/equivalent, but if you want to really understand, it’s probably worth looking at things from a thermodynamic standpoint (since electrode potentials work on a purely thermodynamic basis).

My horrible attempt to explain the thermodynamics nonsense

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