SCH 4U - Galvanic Cells

Galvanic Cells

The energy released in a spontaneous redox reaction can be used to perform electrical work.  This occurs in a galvanic cell (aka a voltaic cell) which is a device in which electron transfer is forced to take place through an external pathway, rather than directly between reactants.

Every galvanic cell consists of two beakers, filled with electrolyte solutions.  As well as, two electrodes (which are immersed in the solutions), an external circuit (wires) and usually a voltmeter (although a load, like a light bulb, could be placed here to harness the electrical energy produced by the cell).  Cells can be set up with many combinations of electrodes and electrolytes - this is just one possibility.          *****Let me take you through the cell*****  Starting on the left,  in the anode compartment, the solid zinc electrode undergoes oxidation to place zinc ions in solution (therby increasing the [zinc ion] in the electrolyte solution and decreasing the mass of the Zn electrode).  Two electrons are also produced in this process, which travel, via the external circuit, to the cathode compartment, where they are used in the reduction of the copper(II) ions in the electrolyte solution.  Solid copper is produced and it plates out onto the electrode (thereby decreasing the [copper(II) ion] in the electrolyte solution and increasing the mass of the Cu electrode).  The salt bridge closes the circuit and acts to  maintain electrical neutrality.  Notice that the oxidation reaction in the anode compartment produces positive zinc ions - so, negative nitrate ions flow from the salt bridge to balance out the charge.  Also, notice that the reduction reaction in the anode compartment uses up positive copper(II) ions - so, positive sodium ions flow from the salt bridge to balance out the charge.

    

The two solid metals (Zn & Cu) are called electrodes.  The electrode at which oxidation occurs is called the anode (Zn).  The electrode at which reduction occurs is called the cathode (Cu).

A voltaic cell consists of two half-reactions (reduction and oxidation).  Zn is oxidized at the anode to produce electrons, which flow through the external circuit to the cathode, where Cu²⁺ is reduced.  Oxidation of Zn introduced extra Zn²⁺ ions into the anode compartment – unless this positive charge is neutralized, no further oxidation can take place (also reduction of Cu²⁺ leaves excess negative charge in the cathode compartment).

Electrical neutrality is maintained by migration of ions through a salt bridge (a glass U-tube filled with cotton batting soaked electrolyte solution - usually sodium nitrate).  The Na⁺ ions migrate to the cathode and the NO₃^- ions migrate to the anode. The salt bridge also acts to close the circuit.

The resulting electrochemical cell can produce a voltage of 1.10 V

For the cell in which the reaction Zn°(s)   +   Cu²⁺(aq)   →   Zn²⁺(aq)   +   Cu°(s) takes place, it can be written in a short form, following the format below.  The │represents a phase boundary like that between the electrode and the electrolyte.  The ║represents a physical boundary like the salt bridge.

  anode(-) │electrolyte ║ electrolyte │cathode(+)

Zn(s)│Zn²⁺(aq)║ Cu²⁺(aq)│Cu(s)

TryIt! 

For the Cu(s)│Cu²⁺(aq)║ Ag⁺(aq)│Ag(s) cell:

1. Draw the cell, including beakers, specific electrodes, specific electrolytes, salt bridge , wires & voltmeter.
2. Label the anode & cathode.
3. Place the half cell reactions under the appropriate half cell.
4. Show the direction of electron flow.
5. Show the direction of ion flow.
6. Write out the net cell reaction.
   

Check out this video for the answer.

 

 

Cell Potential

So, why do electrons flow spontaneously through the external circuit?

Like all spontaneous processes, the answer involves energy.  It is energetically favourable for electrons to flow from the anode to the cathode of a voltaic cell, thus reducing their electrical potential energy.  This is similar to the way in which it is energetically favourable for a boulder to roll down a hill to reduce its gravitational potential energy.

In each half cell, the electrodes have different potential energies.  In the above cell, the E_P of the electrons is higher in the Zn electrode than in the Cu electrode.  If the electrodes are connected, the electrons lower their E_P by flowing from the Zn to the Cu electrode. 

The potential difference or voltage is measured in volts (V).  The potential difference between the electrodes of the cell is called the cell potential (E_cell or E°) or cell voltage.  The Zn/Cu cell is 1.10 V at standard conditions (1.0 M solns and 1 atm gases, usually 25°C).  So, 1.10 V is the standard cell potential (E°_cell).

Standard Electrode Potentials

E°_cell can be thought of as the sum of two half cell potentials (E°_ox and E°_red), the standard oxidation potential and the standard reduction potential:

E°_cell = E°_ox + E°_red

We can’t directly measure half cell potentials, so the standard reduction potential of the

2H⁺(aq)  +  2e^-   →   H₂(g)

half-cell has been assigned a value of 0 V and all other half cell potentials are measured relative to this.  This standard hydrogen electrode consists of platinum wire to serve as an inert surface for the cathode reaction.  The electrode is encased in a glass tube so H₂(g) can bubble over the platinum.

 

 

Still have questions?  Check out this video.

 Homework: #4-9 (found here)