> i have understood that oxidation occurs at anode, which is fitting.
> but Zn is used as a cathode also. Why?

Remember the bottom line definition of an anode. An anode is the electrode
toward which anions in the cell migrate. If zinc and copper electrodes are
place in a sulfuric acid electrolyte and the the electrodes are connected
externally with a conductor, there will be a flow of (conventional) current
from the copper to the zinc through the external conductor. Sulfate anions
will migrate toward the zinc where the zinc will be oxidized and solubilized
by the anions when zinc sulfate is formed.

On the other hand, if external current is forced to flow from the zinc to
the copper, then the zinc becomes positive and will attract cations, thereby
turning the zinc electrode into a cathode. This process tries to turn the
cell into a secondary cell. A Daniell cell demonstrates this principle.
Because of side reactions, a Daniell cell probably did not perform well as a
secondary cell.

When physicists started using low pressure gas and vacuum tubes, the terms
anode and cathode were appropriated from electrochemistry. The tube would be
considered a cell. The anode was the electrode into which external
(conventional) current flowed while the cathode was the electrode from which
external current flowed.

I hope I got my signs straight.

Bill

> i have understood that oxidation occurs at anode, which is fitting.
> but Zn is used as a cathode also. why?

***{One example would be for the purpose of hydrogen loading. When an
HOH gains an electron at a metal cathode, the process involves the
separation of HOH into H+ and OH-, with the OH- being repelled and the
H+ being attracted. If the H+ travels into the crystal lattice before
acquiring an electron, it can wedge there, because an H+ is much smaller
than a neutral hydrogen atom and can penetrate deeply into most crystal
lattices before meeting up with an electron. Thus by using a metal as a
cathode (i.e., as the negative terminal), hydrogen atoms can be loaded
into the crystal lattice of the metal. Sometimes such loading can
involve very large quantities of hydrogen being confined in surprisingly
small amounts of space. I have read, for example, about palladium being
loaded up to a 1:1 ratio of H atoms to Pd atoms. Supposedly, if you
apply a match to such a piece of Pd, it will burn very intensely,
deloading as it burns. It is, of course, only the  hydrogen gas that
burns, not the palladium.

To see how much hydrogen gas can be stored in this way, let’s run some
numbers. A cube of Pd 10 cm on a side would weigh 12,023 gm, and a mole
would mass out at 106.42 gm. Thus such a cube would contain 12023/106.42
= 112.98 moles. Each mole, by Avogadro’s law, would contain 6.022×10^23
atoms of Pd, so the cube of Pd would contain (112.98)(6.022×10^23) =
6.803×10^25 atoms of Pd. And, at 1:1 loading, it would also contain
6.803×10^25 atoms of H.

How much gas is that at STP? Well, that is 3.402×10^25 molecules of H2,
or 56.49 moles. At STP, each mole of a gas occupies 22.4 liters, so the
total volume of the gas will be about (22.4)(56.49) = 1265 cubic liters.
That’s 1.265 m^3. Heat released upon combustion would be 286 kJ/mol, or
about 16.2 MJ per L of the Pd in which it was stored. By way of
contrast, gasoline releases 34.8 MJ/L when burned. Thus a liter of Pd
stores about half as much energy as a liter of gasoline.

In theory, therefore, hydrogen loading could enable Pd to be used to
store hydrogen for fuel purposes. In practice, however, the economics do
not work. Pd is currently trading on commodity exchanges for about $350
per Troy ounce, or $11.25/gm. That means the palladium fuel tank needed
to hold 1265 liters of hydrogen will cost about (12023)(11.25) =
$135,000, and that’s only the energy equivalent of about half a gallon
of gasoline. To be equivalent to a 20 gallon tank, 40 times as much Pd
would be needed, costing $5.4 million!

Of course, I didn’t check those calculations (boring), so they could be
wrong. Maybe it’s a great idea after all!

–Mitchell Jones}***

*****************************************************************
If I seem to be ignoring you, consider the possibility
that you are in my killfile. –MJ

<divij_urdb…@yahoo.co.in> wrote:
> i have understood that oxidation occurs at anode, which is fitting.
> but Zn is used as a cathode also. why?

That would be an unusual application. I can picture magnesium being used to
protect galvanized pipe. In that case, Mg would be the anode and FORCE the
zinc to be a cathode, thereby protecting the zinc against corrosion. That
process is called cathodic protection.

Bill
— Fermez le Bush–about two years to go.

"Fred Kasner" <fkas…@sbcglobal.net> wrote:
> The process is independent of what is occuring:
> discharging of charging. Reduction at the cathode; oxidation at the
> anode. Electronics conventions have to parallel this.

Electronic conventions do indeed do that. But because electronic vacuum
tubes have built in rectification, the more positive electrode is almost
always the anode if something electronic is happening. It is possible to
make the anode negative with respect to the cathode, but there will be no
electronic current flow under those circumstances.

Bill
— Fermez le Bush–about two years to go.

> i have a doubt:
> Zn is a metal, in dry cell, its used as a cathode, rightly so. in
> leclanche cell, its used as an anode. it gains e-. can anyone please
> explain this.

In electricity, the terms are for the directions of classic current
(+).  So think in terms of holes.

- Hide quoted text — Show quoted text -

hanson wrote:
> "Divij Rao" <divij_urdb…@yahoo.co.in> wrote in message
> news:1174576674.574526.236900@o5g2000hsb.googlegroups.com…
>> i have a doubt:
>> Zn is a metal, in dry cell, its used as a cathode, rightly so. in
>> leclanche cell, its used as an anode. it gains e-. can anyone please
>> explain this. any views and comments are welcome.
>> Divij

> [hanson]
> There have always been arguments over the cathode/anode thing.
> — When, as you point out in the battery, during its use/discharge
> cycle Zn is turned into Zn2+ & the metal is used up, it is an ANODE.
> But during the charge cycle in a battery, the negative pole attract
> and discharges the Zn2+ into Zn metal the old anode becomes
> the CATHODE….. ahahahaha…. Bottom line (generally):
> at the Cathode ([-] pole, e- flow out) you reduce ions to metal;
> at the Anode ([+] pole e- collect) you oxidize metal to ions.
> … and now continue to argue with ifs and buts, that it is relative,
> depending on the potential (voltage) & polarity where either side
> can be either the Anode or Cathode like in electrolytic refining of
> metals, or electronics in Diodes and more complex gismos…
> ahahaha… ahahahnson

If you use the definitions that are prevalent in electrochemistry the
cathode is the electrode at which reduction occurs (associate the "c"
for cathode with the "c" in the word reduction.) The anode is the
electrode at which oxidation occurs (loss of electrons from the metal
that is the anode.) Generally speaking the anode is a fairly chemically
active metal. The process is independent of what is occuring:
discharging of charging. Reduction at the cathode; oxidation at the
anode. Electronics conventions have to parallel this.
FK

news:1174576674.574526.236900@o5g2000hsb.googlegroups.com…
>i have a doubt:
> Zn is a metal, in dry cell, its used as a cathode, rightly so. in
> leclanche cell, its used as an anode. it gains e-. can anyone please
> explain this. any views and comments are welcome.
> Divij

[hanson]
There have always been arguments over the cathode/anode thing.
— When, as you point out in the battery, during its use/discharge
cycle Zn is turned into Zn2+ & the metal is used up, it is an ANODE.
But during the charge cycle in a battery, the negative pole attract
and discharges the Zn2+ into Zn metal the old anode becomes
the CATHODE….. ahahahaha…. Bottom line (generally):
at the Cathode ([-] pole, e- flow out) you reduce ions to metal;
at the Anode ([+] pole e- collect) you oxidize metal to ions.
… and now continue to argue with ifs and buts, that it is relative,
depending on the potential (voltage) & polarity where either side
can be either the Anode or Cathode like in electrolytic refining of
metals, or electronics in Diodes and more complex gismos…
ahahaha… ahahahnson

> Zn is a metal, in dry cell, its used as a cathode, rightly so. in
> leclanche cell, its used as an anode. it gains e-. can anyone please
> explain this.

Oxidation occurs at the anode.  Zinc gets oxidized in the Leclanche
cell, and it loses electrons to form Zn++.

As it should be.

<divij_urdb…@yahoo.co.in> wrote:
> i have a doubt:
> Zn is a metal, in dry cell, its used as a cathode, rightly so. in
> leclanche cell, its used as an anode. it gains e-. can anyone please
> explain this.

> any views and comments are welcome.

> regards,
> Divij

A dry cell IS a Leclanche cell. As pointed out by someone else, the anode is
where oxidation takes place. In a primary battery, such as a dry cell, it is
the negative terminal. In a secondary cell, the same electrode can be either
an anode or a cathode depending upon whether the cell is getting charged or
discharged.

I believe that the confusion arises from electronics. Various electron tubes
have cathodes that emit electrons and anodes, also called plates, that
collect them. In vacuum tubes anodes put electrons into the external circuit
just as the zinc of a Leclanche cell does.

The nomenclatures in electrochemistry and electronics appear to be
consistent. In old days, there were few electronic devices that had
electrodes that could act as anodes or cathodes. There was no way that cold
anodes emitted electrons in a way that was not detrimental. Secondary
emission of electrons from screen grids of tetrodes is one exception. These
days, there are solid state devices, triacs for example, that do allow for
electrons to flow either way in the external circuitry. That net effect is
that the terms anode and cathode are finding less use in electronics.

Bill
— Fermez le Bush–about two years to go.