11. Electrode Potentials and Electrochemical Cells Flashcards
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
Are the two half reactions that make up a cell in equilibria?
Yes
What is the position of equilibria affected by?
- temperature
- concentration
- pressure
When can cell potentials only be compared?
Under standard conditions
Why must cell potentials be compared under standard conditions?
Different conditions could change the position of equilibrium
What are the standard conditions that cell potentials are measured under?
- 298K
- 1 moldm⁻³ solution of ions
- 100 kPa
How is the full potential difference (emf) of cells measured?
Under zero-current conditions
How are zero-current conditions achieved in order to measure the emf of a cell?
By using a high resistance voltmeter
What does S.H.E. stand for?
Standard hydrogen electrode
What had to happen before the potential of any half-cells could be measured?
A potential had to be assigned to one particular half cell (so the potential of all other electrodes could be measured against it)
What is the potential of the SHE?
0V by definition
What is the SHE known as?
The primary standard
Why is the SHE known as the primary standard?
It is the potential to which all others are compared
What does the standard hydrogen electrode consist of?
- a platinum electrode with hydrogen gas of standard pressure (100 kPa) bubbling over its surface
- a solution containing H⁺ ions of standard concentration (1 moldm⁻³)
What acids could be used in the standard hydrogen electrode?
- 1 moldm⁻³ HCl/HNO₃
* 0.5 moldm⁻³ H₂SO₄
When a half cell is connected to the SHE, what happens?
The p.d. between the two electrodes causes a current to flow between them
When finding the potential of a half cell under test, which side is the standard electrode?
The left hand electrode
What can the SHE/primary standard also be known as?
The standard electrode
How is the circuit between the SHE and a half cell completed?
By using a salt bridge to allow ions to flow from one solution to another
What is a salt bridge usually made from?
A piece of filter paper saturated with a solution of an inert electrode, e.g. KNO₃ (aq)
Why can charge be transferred through the salt bridge?
The ions in it move
What does the transfer of charge through the salt bridge counter balance?
The electron flow
What must be true of the salt used in a salt bridge?
Must not react with either of the solutions in the half cells
What is the symbol for the standard electrode potential?
E^θ
What is the standard electrode potential?
The potential difference between the electrode and the hydrogen electrode under standard conditions
Why are electrode potentials sometimes known as reduction potentials?
The equilibria are written with the electrons on the left
What is the electrochemical series?
When the electrode potentials are recorded in order of increasing/decreasing potential
What is it called when the reduction potentials are recorded in order of increasing/decreasing potential?
The electrochemical series
What do very positive potentials mean?
These elements are good at attracting electrons (by taking them from something else which is oxidised)
Which elements in the electrochemical series are the best oxidising agents?
Those with very positive potentials
What do very negative potentials mean?
These elements are good at giving away electrons (by giving them to something else which is reduced)
What happens to reducing power as the value of potential decreases?
It increases
What happens to reducing power as the value of potential increases?
It decreases