Analysing voltaic Cell

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Malaysia SPM Form 4 Chemistry, Chapter 6: Electrochemistry.

[edit] Cells and Batteries

A device which converts chemical energy into electrical energy is called a cell or battery. Battery is a collection of cells.
A cell consists of a pair of dissimilar metals in an electrolyte.
Figure above shows an example of a simple voltaic cell consist of a magnesium electrode and a copper electrode immerse in magnesium sulphate solution.
When chemical reaction happens, the more reactive metal, magnesium, dissolves in the magnesium sulphate solution and become magnesium ions, thereby producing electrons, as shown in the half equation below:

Mg ---> Mg²⁺ + 2e

As electrons are produced, the magnesium acts as the negative electrode.
These electrons then travel to the copper electrode.
The hydrogen ions around the copper electrode receive the electrons and are discharged to produce bubbles of hydrogen gas:

2H⁺ + 2e ---> H₂

As electrons are taken in, the copper is the positive electrode.
This production and movement of electrons is electricity, so electrical energy has been generated and the galvanometer is deflected. *Overall, the chemical reaction can be represented by the ionic equation:

Mg + 2H⁺ ---> Mg²⁺ + H₂

In voltaic cell, the negative electrode is the anode whereas the positive electrode is the cathode, which is the opposite of the electrolytic cell.

The voltage of the cell depends on the difference in the reactivities of the two metals.
The greater the difference in reactivity, the bigger the voltage produced.
Magnesium and copper are far apart in the reactivity series, so the voltage is high.
However, as the magnesium dissolves rapidly in the magnesium sulphate solution, the reading of the galvanometer decreeases.
Also the copper electrode gets covered with bubbles of hydrogen gas, which prevent the electricity from flowing. This is called polarisation.
If you try the experiment with other less reactive metals such as zinc or iron, instead of magnesium, the reading of the galvanometer will be lower, but the cell will last longer.

The more reactive metal acts as negative electrode while the less reactive metal acts as positive electrode.
The negative electrode is anode while the positive electrode is cathode.

During the process, the more reactive metal (negative electrode) being ionized and form ion.

X ---> Xⁿ⁺ + ne

At the same time, negative ions are discharged at positive electrode. ( The ions are from the solution )
If there is more than one negative ion present, the ion situated at the lower position in the electrochemistry series will be chosen to be discharged.

Yⁿ⁺ + ne ---> Y

One of the first practical batteries is called the 'Daniel cell'.
It uses a half-cell of copper dipped in copper(II) sulphate, and in electrical contact with a 2nd half-cell of zinc dipped in zinc sulphate solution.

Anode

The zinc is the more reactive, and is the negative electrode, releasing electrons because on it zinc atoms lose electrons to form zinc ions,

Zn(s) ---> Zn²⁺(aq) + 2e-

Cathode

The less reactive metal copper, is the positive electrode, and gains electrons from the negative electrode through the external wire connection and here .. the copper(II) ions are reduced to copper atoms,

Cu²⁺(aq) + 2e- ---> Cu(s)

Zn(s) + CuSO₄(aq) ---> ZnSO₄(aq) + Cu(s)

or ionically:

Zn(s) + Cu²⁺(aq) ---> Zn²⁺(aq) + Cu(s)

A Daniel Cell can be substitute by a design as showing in diagram 1. The chemical reaction remains the same.
Using salt bridge can reduce the internal resistance of the cell.
The function of the salt bridge is to allow ions follow from one solution to another to complete the circuit.