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
Which elements in the electrochemical series are the best reducing agents?
Those with very negative potentials
In terms of electrochemical cells, when do electrons flow?
When two different half reactions are connected as part of a complete circuit
In an electrochemical cell, which direction do electrons flow?
From the more reactive metal to the less reactive metal
In terms of electrons, what does it mean if a metal is more reactive?
It is more likely to give up electrons (oxidation)
In terms of electrons, what does it mean if a metal is less reactive?
It is more likely to gain electrons (reduction)
Which is the reducing half cell?
The one with the most negative value
Which is the oxidising half cell?
The one with the least negative value
What happens to electrons at the anode?
The metal loses electrons - so is oxidised to metal ions
What happens to electrons at the cathode?
The metal ions gain electrons - so is reduced to metal atoms
Is the anode oxidised or reduced?
Oxidised
Why is the anode negative?
The metal loses electrons - oxidation, so is oxidised to metal ions
Is the cathode oxidised or reduced?
Reduced
Why is the cathode positive?
As electrons are used up - metal ions gain electrons and so it is reduced to metal atoms
What must be noted about the charges of the electrodes in the electrochemical cell?
The positive and negative electrodes have opposite charges to those during electrolysis
What equation is used to find the voltage of a cell?
E cell = E₊ - E₋ = E right - E left
When calculating the voltage of a cell, what is subtracted from what?
The oxidation value (most negative value) is taken away from the reduction value
When will a reaction not take place in terms of electrochemical cells?
If a stated reaction has a negative electrode potential value
What happens if a stated reaction has a negative electrode potential value?
The reaction will not take place
What does || represent in the convention for writing cells?
A salt bridge
What represents a salt bridge in the convention for writing cells?
||
In the convention for writing cells, which electrode is always on the right hand side?
The reducing electrode (more negative)
In the convention for writing cells, which electrode is always on the left hand side?
The oxidising electrode (more positive)
In the convention for writing cells, which side does the reducing electrode go on?
Right
In the convention for writing cells, which side does the oxidising electrode go on?
Left
When will the oxidising electrode not be written on the left hand side?
When measuring potentials against primary or secondary standards, when the standard electrode is always the left hand electrode
In the convention for writing cells, which species is put closest to the salt bridge in the middle?
The most oxidised species
Do state symbols need to be included when writing out cells?
Yes
In the convention for writing cells, what happens if there are two species in the same state?
They are separated using a coma
In the convention for writing cells, what represents the boundary between two phases?
|
In the convention for writing cells, where is the metal electrode written?
On the outside
In the convention for writing cells, what happens if there is no solid metal in the half equation?
Platinum is used and included in cell notation
What do batteries contain?
Two separate half cells
What does the salt bridge in a battery do?
Allows ions to flow through without allowing complete mixing of the solutions
In a battery, what are the electrodes connected to?
The terminals of the battery
What is a non-rechargeable cell?
When the reactions taking place in the half cells are irreversible and has to be disposed of after their single use
What happens to the emf in non-rechargeable cells?
Emf drops over time as chemicals are used up
What happens, in a non-rechargeable cell, when one or more of the chemicals have been completely used up?
The cell is flat and emf is 0V
What are non-rechargeable cells useful for?
Gadgets which do not use a lot of power e.g. TV remotes and smoke alarms
Example of a non-rechargeable cell?
Zinc/carbon cells
What are the two half equations in a zinc/carbon cell?
- Zn(NH₃)₂²⁺ + 2e⁻ ⇋ Zn + 2NH₃
* 2MnO₂ + 2H⁺ +2e⁻ ⇋ Mn₂O₃ + H₂O
What is equation for the overall reaction in a zinc/carbon cell?
(Zn(NH₃)₂²⁺ + 2e⁻ ⇋ Zn + 2NH₃ and
2MnO₂ + 2H⁺ +2e⁻ ⇋ Mn₂O₃ + H₂O)
2MnO₂ + 2H⁺ + Zn + 2NH₃ → Mn₂O₃ + H₂O + Zn(NH₃)₂²⁺
When is a battery rechargeable?
When the reactions taking place in the half cells are reversible
Example of a rechargeable cell?
Lithium cells
What are rechargeable cells useful for?
Gadgets which use power frequently e.g. mobile phones and cars
How is a rechargeable battery recharged?
- another power supply with a larger emf is connected
- electrons are forced around the circuit in the opposite direction
- reversing the spontaneous chemical reaction regenerating the chemicals
Examples of the electrodes and electrolyte in a lithium cell?
- positive lithium cobalt oxide electrode
- negative graphite electrode
- electrolyte - lithium salt in organic solvent
What are the two half equations for a lithium cobalt oxide/graphite electrode lithium cell?
- Li⁺ + CoO₂ + e⁻ ⇋ LiCoO₂
* Li⁺ + e⁻ ⇋ Li
In a lithium cell, what is the overall reaction during discharge?
(Li⁺ + CoO₂ + e⁻ ⇋ LiCoO₂ and
Li⁺ + e⁻ ⇋ Li)
CoO₂ + Li → LiCoO₂
In a lithium cell, what is the overall reaction during re-charge?
(Li⁺ + CoO₂ + e⁻ ⇋ LiCoO₂ and
Li⁺ + e⁻ ⇋ Li)
LiCoO₂ → CoO₂ + Li
When discharging lithium cells, which direction do electrons flow?
Negative to positive electrode
When re-charging lithium cells, which direction do electrons flow?
Positive to negative electrode (forced through the external circuit)
When re-charging lithium cells, what happens to lithium ions?
They flow through the electrolyte towards the positive electrode to maintain the balance of charge
What type of battery is lead-acid?
Rechargeable
What type of battery is nickel-cadmium?
Rechargeable
What is a fuel cell?
A cell in which a chemical reaction between a fuel and oxygen is used to create a voltage
What are the energy transfers in a fuel cell?
Chemical to electrical energy
Why do fuel cells not run out or need charging?
They have a continuous supply of chemicals to the cell
What does a fuel cell consist of?
Two half cells connected by a semi-permeable membrane
What is the most common fuel cell?
The hydrogen fuel cell
What does the semi-permeable membrane in a hydrogen fuel cell allow for?
OH⁻ ions and water to pass through it but not the oxygen or hydrogen gas
What electrodes are used in hydrogen fuel cells?
Platinum
What do the platinum electrodes act as in hydrogen fuel cells?
A catalyst
Why do the platinum electrodes act as a catalyst in hydrogen fuel cells?
They provide a surface for the reaction
What the current available in the hydrogen fuel cell depend on?
The rate, hence the advantage of a catalyst
As well as a catalyst, what will affect the current generated from a hydrogen fuel cell?
Temperature and pressure
Does the hydrogen-oxygen fuel cell run in alkaline or acidic conditions?
Can run in either
For a hydrogen fuel cell run in alkaline conditions, what is the half equation for the negative electrode (anode)?
H₂ + 2OH⁻ → 2H₂O + 2e⁻
In electrochemical cells, is the anode positive or negative?
Negative
In electrochemical cells, is the cathode positive or negative?
Positive
For a hydrogen fuel cell run in alkaline conditions, what is the half equation for the positive electrode (cathode)?
O₂ + 2H₂O + 4e⁻ → 4OH⁻
For a hydrogen fuel cell run in alkaline conditions, what is the overall equation?
2H₂ + 4OH⁻ + O₂ + 2H₂O → 4H₂O + 4OH⁻
For a hydrogen fuel cell run in acidic conditions, what is the half equation for the negative electrode (anode)?
H₂ → 2H⁺ + 2e⁻
For a hydrogen fuel cell run in acidic conditions, what is the half equation for the positive electrode (cathode)?
O₂ + 4H⁺ + 4e⁻ → 2H₂O
For a hydrogen fuel cell run in acidic conditions, what is the overall equation?
2H₂ + 4H⁺ + O₂ + 2H₂O → 2H₂O + 4H⁺
For fuel cells working in alkaline conditions, what is used as the electrolyte?
A base e.g. KOH
For fuel cells working in acidic conditions, what is used as the electrolyte?
An acid e.g. H₃PO₄
Advantages of cells?
Portable source of electrical energy
Advantages of non-rechargeable cells?
Cheap
Advantages of rechargeable cells?
- less waste
- cheaper in the long run
- lower environmental impact
Advantages of fuel cells?
- water is the only waste product
- don’t need recharging
- very efficient
Disadvantages of cells?
Waste issues
Disadvantages of non-rechargeable cells?
Waste issues
Disadvantages of rechargeable cells?
Some waste issues (at end of life)
Disadvantages of hydrogen fuel cells?
- need constant supply of fuels
- hydrogen is flammable and explosive
- hydrogen is usually made using fossil fuels
- high cost of fossil fuels
