Electrode Potentials (Chapter 23.4, 23.5, 23.6) Flashcards
What is a voltaic cell?
A type of electrochemical cell that converts chemical energy into electrical energy
What is required for a voltaic cell?
Chemical reactions that transfer electrons from one species to another (redox reactions) - bc electrical energy results from the movement of electrons
What does a half-cell contain?
The chemical species present in a redox half-equation
How is a voltaic cell made?
By connecting together two different half-cells, allowing electrons to flow
What is necessary in a voltaic cell and why?
The chemicals in the two half-cells must be kept apart - if they were allowed to mix, electrons would flow in a uncontrolled way and heat energy, instead of electrical, would be released
What do metal/metal ion half-cells consist of?
A metal rod dipped into a solution of its aqueous metal ion
What is the place where the metal is in contact with its ions called?
The phase boundary
What happens at the phase boundary in a metal/metal ion half-cell?
An equilibrium will be set up in which the forwards reaction show reduction and the reverse reaction shows oxidation e.g. Zn2+ + 2e- <=> Zn
What happens in an isolated half-cell?
There is no net transfer of electrons either into or out of the metal
What does the direction of electron flow depend on when two half-cells are connected?
The relative tendency of each electrode to release electrons
Describe ion/ion half-cells
- They contain ions of the same element in different oxidation states e.g. a mixture of Fe2+ and Fe3+
- e.g. redox equilibrium: Fe3+ + e- <=> Fe2+
- There is no metal to transport electrons ∴ an inert metal electrode made out of platinum (Pt) is used
What happens in a cell with 2 metal/metal ion half-cells connected?
The more reactive metal releases electrons more readily and is oxidised
Describe the electrodes in an operatic voltaic cell
- The electrode with the more reaction metal loses electrons and is ∴ oxidised - this is the negative electrode
- The electrode with the less reactive metal gains electrons and is ∴ reduced - this is the positive electrode
Define the standard electrode potential
The e.m.f (electron motive force) of a half-cell connected to a standard hydrogen half-cell under standard conditions of 298K , solution concentrations of 1mol/dm3 and pressure of 100 KPa
What does the standard electrode potential measure?
The tendency of a species to be reduced and gain electrons
What is the standard half-cell?
A half-cell containing hydrogen gas and a solution of H+ ions with a platinum electrode to allow electrons into and out of the half-cell
What are the standard conditions for a half-cell?
- Concentration of solutions = 1M
- Temperature = 298K (25 degrees)
- Pressure = 100KPa (1 atm)
What is the electrode potential of a standard hydrogen half-cell?
0V
What does the sign of an electrode potential show?
The sign of the half-cell connected to the standard hydrogen half-cell and the relative tendency to gain electrons compared with the hydrogen half-cell
Describe how to measure a standard electrode potential?
1) the half-cell electrode is connected to a standard hydrogen electrode by a wire, allowing a controlled flow of electrons
2) the two solutions are connected with a salt bridge which allows ions to flow and typically contains a concentrated solution of an electrolyte that does not react with either solution e.g. a strip of filter paper soaked in aqueous potassium nitrate (KNO3)
What do you need to label on a voltaic cell diagram?
1) solutions and concentrations
2) electrodes (the metals and charges)
3) voltmeter
4) salt bridge
The more negative the electrode potential value…
1) the greater the tendency to lose electrons and undergo oxidation
2) the less the tendency to gain electrons and undergo reduction
The more positive the electrode potential value…
1) the greater the tendency to gain electrons and undergo reduction
2) the less the tendency to lose electrons and undergo oxidation
What elements tend to have negative electrode potential values?
Metals
What elements tend to have positive electrode potential values?
Non-metals
What can you conclude in general about electrode potentials?
1) the more negative the electrode potential value the greater the reactivity of the metal in losing electrons
2) the more positive the electrode potential value, the greater the reactivity of a non-metal in gaining electrons
What is the e.m.f. of a cell called?
The cell potential, Ecell
What can the standard Ecell be recorded from the voltmeter?
Using two standard half cells, connected by a salt bridge with the electrodes connected to a voltmeter and wire
How do you work out the standard Ecell?
electrode potential of positive electrode - electrode potential of negative electrode
How can you use electrode potentials to predict redox equations?
- An oxidising agent takes electrons way from the species being oxidised ∴ oxidising agents are reduced and are on the left of the equation
- A reducing agent adds electrons to the species being reduced ∴ reducing agents are oxidised and on the right the equation
Where is the strongest oxidising agent in a table of equations?
At the bottom on the left
Where is the strongest reducing agent in a table of equations?
At the top on the right
How can you predict the feasibility of redox reactions from electrode potential values?
- A reaction should take place between an oxidising agent and a reducing agent, provided that the redox system of the oxidising agent has a more positive electrode potential than the redox system of the reducing agent
- e.g. an oxidising agent will not react if there are no redox systems with a less positive electrode value
Describe the direction of reaction of redox systems
- The redox systems with the more positive electrode potential value will react from left to right
- The redox system with the more negative electrode potential value will react from right to left
What are the two limitations of predictions using electrode potential values?
1) reaction rate
2) standard conditions
3) concentration
Why is reaction rate a limitation when making predictions using electrode potential values?
- Even though electrode potentials may indicate thermodynamic feasibility of a reaction, they give no indication of the rate of reaction
- ∴ a reaction deemed feasible may not actually be so, if the reaction has a very large activation energy and ∴ a very slow reaction rate (like with deltaG)
Why are standard conditions a limitation when making predictions using electrode potential values?
- Actual conditions may be different from standard conditions used to record standard electrode potential values - this will affect the value of the electrode potential
- Standard electrode potentials apply to aqueous equilibria but many reactions that take place are not aqueous
Why is concentration a limitation when making predictions using electrode potential values?
- Electrode potentials are measured using concentrations of 1M but many reactions take place using concentration or dilute solutions - if the concentration of the solution is not 1M, then the value of the electrode potential will be different from the standard electrode potential
- e.g. in a Zn half cell, if [Zn2+] is greater than 1M, the equilibrium will shift to the right, removing electrons from the system and making the electrode potential less negative
- if [Zn2+] is less than 1M, the equilibrium will shift tot eh left, increasing the number of electrons in the system and making the electrode potential more negative
- Any change to the electrode potential will affect the value of the overall cell potential
What is the key-requirement for modern day cells and batteries?
For the two electrodes to have different electrode potentials
What are the 3 types of cells?
1) primary
2) secondary
3) fuel
Describe primary cells
- Non-rechargeable and designed to be used once only
- Electrical energy is produced by oxidation and reduction at the electrodes, but the reactions cannot be reversed
- Eventually the chemicals will be used up, voltage will gall, the battery will go flat and the cell is discarded or recycled
- Typically alkaline, based on Zn/MnO2 and a KOH alkaline electrolyte
- Uses: low-current, long-storage devices e.g. wall clocks and smoke detectors
Why are secondary cells rechargeable?
Bc the cell reaction producing the electrical energy can be revered during recharging - the chemicals in the cell are ∴ regenerated and the cell can be used again
What are three examples of secondary cells?
1) lead-acid batteries used in car batteries
2) nickel-cadmium cells and nickel-metal hydride cells - cylindrical batteries used in radios and torches
3) lithium-ion and lithium-ion polymer cells used in modern appliances e.g. laptops and cameras
Describe lithium ion and lithium-ion polymer cells
- Negative electrode: Li => Li + e-
- Positive electrode: Li+ + CoO2 + e- => LiCoO2
- Limitations of lithium ion cells: can become unstable at high temperatures and have ignited laptops and phones ∴ care must be taken in their recycling as lithium is a very reaction metal
- Risks of lithium based cells: toxicity and fire
- Benefits of lithium based cells: flexible so can come in various shapes and sizes
Describe fuel cells
- They use the energy from the reaction of a fuel and oxygen to create a voltage
- The fuel and oxygen flow into the fuel cell, the products flow out and the electrolyte remains in the cell
- They can operate continuously, provided that the fuel and oxygen are supplied into the cell
- They do not have to be recharged
Why are hydrogen/hydrogen-rich (e.g. methanol) fuel cells very common?
They produce no CO2 during combustion, only water (using an acid or alkali electrolyte)
What are the equations for an alkali hydrogen fuel cell?
- Oxidation: H2 + 2OH- => 2H2O + 2e-
- Reduction: 1/2O2 + H2O + 2e- => 2OH-
- Overall: H2 + 1/2O2 => H2O
What are the equations for an acid hydrogen fuel cell?
- Oxidation: H2 => 2H+ + 2e-
- Reduction: 1/2O2 + 2H+ + 2e- => H2O
- Overall: H2 + 1/2O2 => H2O
What is the overall equation and Ecell for an acid and alkali hydrogen fuel cell?
Overall equation: H2 + 1/2O2 => H2O
Ecell = 1.23V
What direction do electrons flow in?
Negative electrode to positive electrode
Describe gas/ion half-cells
- Electrode: Pt
- Gas at 1atm
- e.g. Cl2 + 2e- => 2Cl-
What does the salt bridge do?
It allows ions to flow
Why is a platinum electrode used?
It is unreactive but conducts electricity so that electrons can be transferred
When is a reaction feasible?
When the Ecell is positive
Why are weak acids not used as a source of H+ ions for a hydrogen half-cell?
Because the [H+] is unpredictable and keeps changing
What are 2 advantages and two disadvantages of using hydrogen fuel cells?
- Advantages: 1) they operate continuously
2) they release no greenhouse or toxic gases e.g. CO2 - Disadvantages: 1) hydrogen gas is very flammable and ∴ dangerous to store
2) the fuel cell reaction has a slow reaction rate
