Metals in Contact With Aqueous Solutions
When a metal rod is placed in a beaker containing a solution of its own metal ions, two solutions can arise.
1. Metal atoms leave the rod and become metal ions in solution
M = M + e–
2. Metal ions leave the solution and become metal atoms on the surface of the rod.
M + e– = M
In the first case the electrons are released when the metal ions are formed stay on the rod, so the rod carries a negative charge. We say it has a negative potential. Zinc and Magnesium are examples of metals that behave in this way.
In the second instance, electrons are attracted out of the rod into the solution so the rod carries a positive charge. We say it has a positive potential. Copper and Silver are examples of metals that behave in this way.
The examples shown above each exhibits a half cell. This is because any two of them can be joined to form a whole cell.
This figure shows an attempt to link tow half cells to make a whole cell.
– The electrons repel each other from election rich zinc to electron deficient copper
– Ions (electrons) transfer negative charge through the electrolyte in the salt bridge without the two solutions mixing together.
– The salt bridge completes the circuit. This is called a Daniell Cell. The salt bridge is a tube containing (or a strip of filter paper soaked in) an aqueous solution of a good electrolyte example. Potassium Nitrate KNO3 or Potassium Chloride KCl. The salt bridge is usually made up with Potassium Nitrate solution because the salt does not react with other ions commonly used in electro chemical cells.
This enables charge to be transferred without the solution zinc sulphate and copper sulphate being mixed.
NB. The electrons pass through the wire while the ions move along the salt bridge.
Electrolytes are solutions, which decompose at the electrode when an electric current passes through. However, there is no decomposition in the salt bridge.
Functions Of The Salt Bridge
– One obvious function of the salt bridge is to complete the circuit without allowing the two solutions to mix.
– It has another important function. Without it the zinc half-cell would slowly become positively charged as electrons leave it, and the copper half cell would become negatively charged. With the salt bridge in place ions from the salt bridge are able to move in and out of the solutions to neutralize any build up of charge.
Non-Metals in Contact With Aqueous Solutions
It is convenient to make half cells of non-metals such as chlorine and hydrogen. Hydrogen half cell would consist of a layer of hydrogen atoms in contact with hydrogen ions in solution just as a copper electrode consists of copper atoms with in contact with copper ions in solutions.
A layer of immobile hydrogen atoms is created by solidifying it at negative 260 degrees celcius which will be in contact with hydrogen ions in solution.
Eg. HCL, H+, Cl –
Platinum is used to hold the layer of hydrogen atoms. Platinum absorbed molecules of gases onto its surface that is, it holds them in place when they come in contact with the metal.
In a commercial hydrogen electrode the platinum is not shinny but black. The surface of the electrode is porous and pitted. This creates a very large surface area for the hydrogen to be absorbed into.
The Standard Hydrogen Electrode (S.H.E)
The electromotive force (E.M.F) of the daniell cell is the maximum potential difference between the potential of the zinc half-cell and the potential of the copper half -cell. The question is how do we find the potential of each half-cell individually?
To do this we use the standard hydrogen electrode, which is given a potential of zero volts. When it is connected to another half cell the E.M.F between the S.H.E and the second half-cell is equal to the potential of the second half-cell.
The S.H.E is actually the standard hydrogen half-cell.
It consist of hydrogen gas bubbling over a platinum electrode immersed in a solution of hydrochloric acid supplying hydrogen ions.
The Reaction that takes place is 2 H+ + 2 e- = H2
The platinum electrode simply produces an inert metal connection between hydrogen gas and hydrogen ions in solution.
When the S.H.E is being used to establish the potential of a Redox system, standard conditions must apply. These are:
– The hydrogen must be at a pressure of 1ATM
– The condition of the hydrogen ions must be 1mol dm3
– The temperature must be 25 degrees Celsius (298k)
Under these conditions the potential is defined as exactly zero volts.