Chapter 20A: Electrochemical Cells Flashcards
Define electrochemistry.
It is the study of the inter-conversion of chemical and electrical energy, which occurs in cells in which redox reactions produce or utilise electrical energy.
Define electrochemical cell (also known as galvanic/voltaic cell).
Cells in which the redox reaction is spontaneous and the energy released is converted to electrical energy.
What is the electron flow for electrochemical cells?
Electrons are produced at the negative anode, where oxidation takes place, and flows to the positive cathode, where reduction takes place.
What is the function of the salt bridge? (2)
1) Maintain electrical neutrality in the two half cells: It contains ions that are inert to the redox reaction, where its anions react with the excess cations and its cations react with the excess anions. Without a salt bridge, it would give rise to a charge imbalance in the cell and current will cease.
2) complete the circuit (allowing electrons to flow both ways)
Does the voltage of electrochemical cells last infinitely?
As the overall reaction proceeds, the system will approach equilibrium and the voltage would eventually decrease to zero.
What is electrode potential?
When one end of a metal is connected to a terminal of a voltmeter and the other end of the voltmeter is connected to a reference electrode (standard hydrogen electrode), the value on the voltmeter is the electrode potential.
How can electrode potential determine reactivity?
The more reactive the metal, the more the position of the equilibrium will lie to the left (more ions formed) and the greater amount of accumulated negative charge.
What does a standard hydrogen electrode comprise of? (3)
Also standard conditions for measuring electrode potential
1) a platinum wire and a piece of platinum foil covered with finely divided platinum (platinised platinum) = an inert surface for the redox reaction to occur
2) a solution of 1 moldm-3 H+ (aq) ions where the platinum electrode is immersed in
3) hydrogen gas at 298K and 1 bar.
Define standard electrode potential (Eº).
The standard electrode potential of a half-cell is the value of the standard electric motive force of the relevant half cell when all components are in their standard states, measured relative to the standard hydrogen electrode.
(EMF: energy to push 1 charge through an electric circuit, which is converted to electrical energy)
What are the 3 types of standard electrode potential measurement?
1) Eº of a metal in contact with its ions in an aqueous solution
2) Eº of a non-metal in contact with its ions in aqueous solution
3) Eº involving ions of the same element in different oxidation states
How do we use Eº values to compare oxidising/reducing strengths of species? (2)
Oxidised species + ne- ≈ reduced species
1) the more positive the Eº value, the more favourable the forward reduction half reaction, which means that the oxidised species has a greater ease of reduction. Thus, the oxidised species is a stronger oxidising agent.
2) the more negative the Eº value, the more favourable the backward oxidation half-reaction, which means the reduced species has a greater ease of oxidisation. Thus, the reduced species is a stronger reducing agent.
Define standard cell potential (Eºcell).
It is the potential difference between the two half-cells measured at standard conditions.
How can we predict the type of reaction taking place in each half-cell? (4)
1) write down the half-reaction equations that represent the equilibrium established in each half cell.
2) compare their Eº values and assign [R] to the more positive Eº value.
3) assign [O] to the less positive Eº value and reverse the ionic equation.
4) add the two half equations to give the overall cell reaction.
How can we predict the feasibility of a redox reaction? (3)
1) select ALL appropriate reduction half-reactions where each species appear and write them down.
2) if the two species being mixed are on the same side, redox reaction cannot occur. If the two species are on opposite sides of the half-equations, the one of the LEFT will be REDUCED and the one on the RIGHT will OXIDISED. A redox reaction is then possible.
3) calculate the Eºcell. If it is positive, the reaction is energetically feasible and spontaneous under standard conditions and vice versa. If it is zero, the reaction is at equilibrium and there is no net reaction in either direction.
How is Gibbs free energy (∆G) related to Eºcell?
Electrochemical cells generate electricity by making use of a spontaneous redox reaction that takes place within the cell. The change in ∆G is then converted into electrical energy.
A spontaneous redox reaction (∆G < 0) would mean that Ecell > 0.
∆Gº = –nFEºcell (F=96500 C mol-1)
