A2-Level Electrochemistry

  • Metals react by losing electrons and forming positive ions.

  • The reactivity of a metal refers to how easily it can lose electrons (and form positive ions).

  • When a reactive metal is placed in water, metal atoms lose electrons and form positive ions. The lost electrons have nowhere to go, so they build up on the surface of the metal. Eventually, an equilibrium is established between the ions in solution and the metal itself.

  • A build-up of charge on a surface creates an electrical potential. The size of the electrical potential is determined by the equilibrium between the ions in solution and the metal solid – this is called an electrode potential.

  • Highly reactive metals will lose electrons easily and there will be a greater build-up of electrons on the surface of the metal.

  • The more electrons on the surface of the metal, the more negative the electrode potential.

  • Zinc is more reactive than iron, so it would have a greater buildup of electrons on its surface – giving it a more negative potential.

  • The solution, metal strip (electrode) and container are called a half-cell.





Electrochemistry (The Basics)


Electrochemistry is the combination of chemical reactions and the study of electricity.


Once the basic principles are understood, electrochemistry becomes relatively straightforward. These notes may seem long, but it is vital that the basics are fully understood before attempting exam questions and revising.


The basics


When metals react, they become positive ions by losing electrons. Different metals lose electrons more easily than others, making them more reactive. A good way to think about the reactivity of a metal is as a way of saying how easily a metal forms a positive ion. A highly reactive metal (potassium, for example) easily forms positive ions, whereas an unreactive metal (gold, for example) does not easily form positive ions. Therefore, how many positive ions of a metal are formed in a reaction, and how many electrons are released, depend on the reactivity of the metal.



When placed in water, a metal will react to form positive ions and release electrons. The electrons will have nowhere to go, so they will build-up on the surface of the metal. Any build-up of charge on a surface can be described as an ‘electrical potential’. The piece of metal now has an electrical potential.





This potential will not change, as eventually, an equilibrium will form between the ions released in solution and the metal. The potential difference between the metal strip and solution creates an electrode potential and is different for all metals.




All metals react in the following way (although not all metals form a 2+ charge).


M  → M     + 2e


However, once there is a high enough concentration of positive ions in the solution, the metal ions can pick up two electrons from the surface of the metal and reform solid metal atoms. This is the reverse reaction from above.


M    + 2e  → M



If you place a piece of metal in a solution that contains ions of that metal, then the position of equilibrium will change and the electrode potential (potential difference between the metal strip and solution) will be different. This is known as a half-cell.


The absolute key thing to understand here is that this position of equilibrium can be changed!


Take iron and zinc. Zinc is more reactive than iron, so if both are placed in separate solutions containing their own ions, more zinc will form Zn  ions than iron will form Fe  ions.




If more Zn  ions are formed, more electrons are released onto the surface of the metal. More electrons on the surface of the metal means the metal has a more negative potential.

There would be fewer electrons on the iron metal strip, therefore the iron would have a potential that is not as negative as the zinc.


Electrochemistry is all about measuring how the potential of one half-cell compares to the potential of another half-cell. When two half-cells are connected together, an electrochemical cell is formed.