Electrode potentials Flashcards
How do batteries convert chemicals energy into electrical energy?
- Using redox reactions
- The oxidation reaction and reduction reaction but occur in isolation in half cells
- The two half cells are joined by an external circuit (form electron donor to electron acceptor) to make a complete electrochemical cells and this achieved by a salt bridge
What does a half cell contain?
The chemical species present in a redox half equation
What would happen if the chemicals in the two half cells were not kept apart?
Electrons would flow in an uncontrolled way and what energy would be related rather than electrical energy
What is the simplest half cell (metal/metal ion)?
- A metal rod dipped into solution of its aqueous metal ion, represented by Zn2+(aq)I Zn (s)
- At the phase boundary where the metal is in contact with its ions, an equilibrium is set up
Which way is the forward reaction?
reduction
What is an ion/ion half cell?
-Ions of same element in different oxidation state, with inert metal electrode made of platinum
What happens in an isolated half cell? What does the direction of electron flow depends on?
- No transfer of electrons either into or out of the metal
- When two half cells are connected, the direction of electron flow depends upon the relative tendency of each electrode to release electrons (to be reduced), or how far to the right the position of equilibrium of each half-cell lies, this is a relative scale, with each half-cell having a standard electrode potential value
What happens in a cell with two metal/metal ion half-cells connected?
- The more reactive metal releases electrons more readily and is oxidised
1. The electrode with more reactive metal loses electrons and is oxidised and this is the negative electrode
2. The electrode with the less reactive metal gains electrons and is reduced and this is the positive electrode
What sit ehe starry electrode potential?
The electromotive force of a half cell compared with a standard hydrogen half cell, measured at 298L with solution concentrations of 1moldm^3 and a gas pressure of 100kPa
What happens if the standard electrode potential is positive?
The half cell has a greater tendency to gain electrons than the hydrogen cell and the metal becomes the positive electrode
What happens if the standard electrode potential is negative?
The half cell has a lower tendency to gain electrons than the hydrogen cell and the meta becomes the negative electrode
How do you measure standard electrode potential? What is a salt bridge?
- Half cell connected to s standard hydrogen electrode ((solution of H+(aq) ions and H2(g) and an inert platinum used to allow electrons into and out of half cell)
1. The two electrodes are connected by a wire to allow a controlled flow of electrons
2. The two solutions are connected with a salt bridge which allows ions to flow. The salt bridge typically contains a contracted solutions on an electrolyte that does not react with either oesltuion, e/g/ strip of filter paper soaked in aqueous potassium nitrate
What do you need to remember in diagrams?
- Label standard conditions
2. Salt bridges
How can you calculate standard cell potential directly from standard electrode potentials?
E cell = E(positive electrode) - E (negative electrode)
-Standard symbols on E’s
What can standard electrode potentials be used for?
- Predicting the feasibility of a chemical reaction
- A reaction is energetically feasible is the dleatEstandard is positive (more positive than the redox system of reducing agent)
What are oxidising agents?
- They are reduced
- On the left
- Removes electrons from species being oxidised
What are reducing agents?
- They are oxidised
- On the right
- Adds electrons to species being reduced
How do you know feasibility from a pari of redox systems?
- The redox system with the more positive (less negative) Estandard value will react left to right and gain reactions (reduced)
- The redox system with the less positive (more negative) Standard value will react right to left and lose electrons
What are the limitation of predictions using Estandard values?
- Electrode potentials show thermodynamic feasibility and they give no indication of the rate of the reaction (kinetics)
- Activation energy - Electrode potentials are measured under standard conditions, the actual conditions a reaction takes place under may differ form this
- In reality, many reaction required more concentrated or dilute solutions than 1moldm^-3 and this will change the cell potential
- May not happen in aqueous
What happens for changes in concentration for the redox equilibrium and standard cell electrode potential for zinc?
Zn2+ (aq) + 2e- (eq arrow) Zn(s)
- If the conc of Zn2+ (aq) is greater than 1moldm^-3 the equilibrium will shift to the right, removing electrons form the system, and making the electrode poetical less negative
- In concentrations of Zn2+ (aq) less than 1moldm^-3 the equilibrium will shift to the left increasing electrons in the system and making electrode potential more negative
- Any change to the electrode potential villa affect the value of the overall cell potential
What do secondary cell do?
-They store electrical energy as chemical energy
-The cell reaction can be reversed during recharging
Examples: lead-acid batteries, nickel cadmium, lithium ion
Describe lead-acid battery
- When a current flows the products formed at the electrodes are insoluble solids that stay where they are instead of dissolving the electrolyte
- This means that the electrode processes involved can be reversed as the cell is recharged
- When recharging the anode is connected to the native terminal of the charging source
- On recharging the PbSO4 on the anode gains electrons and on the cathode loses electrons
What are the equations in a lead-acid battery?
Anode: Pb(s) + SO42- (aq) (eq arrow) PbSO4 (s) + 2e-
Cathode: PbO2 (s) + 4H+ + SO42- (aq) + 2e- (eq arrow) PbSO4 (s) + 2H2O (l)
Overall: Pb(s) + PbO2(s) + 2H2SO4 (aq) (eq arrow) 2PBSO4 (s) + 2H2O (l)
To the right is discharging and left is charging in overall
Describe lithium ion battery
- Many mobile phones and laptop computers use rechargeable lithium batteries
- ADV: low density, very light cells (lightest metal)
- DisADV: flammable, reacts very readily with oxygen water
- Solution: electrodes with lithium ions inserted into a crystal lattice of another material
What are the equation in a lithium ion battery?
Anode: 2MNO2 (s) + 2Li+ (s) + 2e- -> Mn2O3 (s) + Li2O (s)
Overall: 2Li (s) + 2MnO2 (s) _> Mn2O3 (s) + Li2O (s)
What are primary cells?
- Non rechargeable and decided to be sued once only
- Electrical energy produced by oxidation and reduction at the electrodes
- Eventually chemicals used up and voltage will fall and batteries will go flat and the cell will be discarded or recycled
1. Most use in low-current long-storage devices such as wall clocks or smoke detectors
What are most modern primary cells?
-Alkaline and based on zinc and manganese dioxide, and a potassium hydroxide alkaline electrolyte
What are the equations in primary cells?
ZnO2 (s) + H2O (l) + 2e- (eq arrow) Zn(s) +2OH-(aq) Ecell=1.2 8V
2MnO2 (s) + H2O (l) + 2e- (eq arrow) Mn2O3 (s) +2OH-(aq)
Oxidation: Zn (s) +2OH- (aq) -> ZnO (s) + H2O (l) +2e-
Reduction: 2MnO2 (s) + H2O (l) +2e- -> Mn2O3 (s) +2OH- (aq)
Cell reaction: Zn(s) + 2MNO2 (s) -> ZnO (s) + MnO3 (s)
E cell: 1.43V
What are fuel cells?
-Generate electricity form an electrochemical reaction in which oxygen fuels (e.g. hydrogen) combine to form water
and creates a voltage
1. The fuel and oxygen flow into the fuel cell and the products flow out and the electrolyte remains in the electrolyte remains in the cell
2. Fuel cells can operate continuously provided that the fuel and oxygen are supped into the cells
3. Fuel cells do not have to be recharged as gases are supplied continuously
What fuels are used in fuel cells?
- Many different fuels can be used, but hydrogen is most common
- Hydrogen fuel cells produce no carbon dioxide during combustion, with water being the only combustion product
- Fuel cells using many other hydrogen-rich fuels, such as methanol are also being developed
Describe hydrogen fuel cells
- A hydrogen cell can have either an alkali or acid electrolyte
- The cell voltages are both 1.23V despite the alkali and acid cells having different redox systems and half equations
What are the equations in a hydrogen fuel cell with acidic electrolyte?
Anode: 2H2(g) -> 4H+ (aq) + 4e-
Cathode: O2 (g) + 4H+ (aq) +4e- -> 2H2O (l)
Overall: 2H2(g) + O2 (g) -> 2H2O (l)
What are the equations in a hydrogen fuel cell with alkaline electrolyte?
Anode: 2H2(g) + 4OH-(aq) -> 4H2O (l) 4e-
Cathode: O2 (g) + 2H2O (l) +4e- -> 2OH- (aq)
Overall: 2H2(g) + O2 (g) -> 2H2O (l)
