11. Electrode Potentials and Electrochemical Cells
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What is a redox reaction?
In a reaction if an element is oxidised then another must be reduced
What happens when a piece of metal is dipped into a solution of its metal ions?
An equilibrium is set up and there is a tendency • for the metal to form positive ions and go into solution • as well as for the metal ions in solution to gain electrons and form metal
What is a half cell?
When a piece of metal is dipped into a solution of its metal ions and an equilibrium is set up
What two things can happen when a piece of metal is dipped into a solution of its metal ions and an equilibrium is set?
• tendency for the metal to form positive ions and go into solution • tendency for the metal ions in solution to gain electrons and form metal
What is an electrode also known as?
A half cell
For a zinc rod immersed in Zn²⁺ ion solution, what could happen?
• each Zn on rod could deposit two electrons on the rod and move into solution as Zn²⁺ ions (-ve charge on rod) • Zn²⁺ ions in solution could accept 2 electrons and move onto rod to become Zn atoms (+ve charge on rod)
What happens when a zinc rod is immersed in Zn²⁺ ion solution and electrons are transferred?
A potential difference is set up between the rod and the solution
What is an electrode potential?
When a metal is dipped in a solution of its metal ions and a potential difference is set up between the rod and the solution
When will the potential difference of a half cell be greater?
When there is a greater tendency to give up electrons (forming ions)
What does an electrode potential indicate?
How readily the metal gives up electrons
Does a chemical reaction take place in a lone half cell?
No - there is simply a potential difference between the rod and solution
What does the potential difference in a half cell depend on?
• nature of the ions in solution • concentration of ions in solution • type of electrode used • temperature
What is required in a half cell where there is no solid metal involved in the half equation?
A metal electrode
Why is a metal electrode required in a half cell where there is no solid metal involved?
To allow the flow of electrons
When a half cell involves no solid metal, what metal is usually used as the electrode?
Platinum (Pt)
Why is platinum used for metal electrodes?
It is unreactive/inert
What are some types of electrodes?
• gas electrodes * redox electrodes
What is a gas electrode?
A gas and a solution of its ions
What is a redox electrode?
Two different ions of the same element present in solution
Can the potential of a half cell be measured directly?
No
What letter represents the potential of a cell?
E
How is the potential of a half cell measured?
Connected to another half-cell of known potential, and p.d. between the two half cells is measured
What is an electrochemical cell?
When two half cells are combined
What is formed when two half cells are combined?
An electrochemical cell