standard electrode potentials

describe an electrochemical cell:

• 2 half cells - electrodes submerged in solns of own ions

• half cells connected by high resistance voltmeter salt bridge

what is the purpose of the high resistance voltmeter?

• measures p.d. between electrodes - this is the EMF/E_cell

• typically e^- would flow from more reactive to less reactive metal but high resistance inhibits e^- flow to maintain conc of ions in soln

what is the salt bridge?

• filter paper soaked in saturated soluble salt soln e.g. potassium nitrate

• OR a tube containing uncreative ions in an agar gel

what is the purpose of a salt bridge?

• helps to maintain electrical neutrality w/in internal circuit

• allows movement of ions to complete the circuit and so prevent charge buildup ∴ prevents cell from taking its reaction to equilibrium

• connects 2 ½ cells/electrodes but keeps solns separate

how does a salt bridge work?

to maintain electrical neutrality:

• -vely charged ion moves to +ve ½ cell via salt bridge

• +vely charged ion move to -ve ½ cell via salt bridge 

what happens when a piece of metal is submerged into a soln of its own ions?

• an equilibrium is set up

• there is a tendency for the metal to form +ve ions and go into soln

what happens if the PoE lies more to the left?

PoE lies more to the left:

• metal acquires -ve charge due to e^- buildup on metal

• so electrode/half-cell has -ve potential

what happens when PoE lies more to the right?

PoE lies more to the right: 

• metal acquires +ve charge as e^- have been used up to form metal from metal ions

• so electrode/half cell has +ve potential 

how does PoE change w/ reactivity?

• more reactive metals - tend to form Mⁿ⁺ ions ∴ -ve charge builds up on the metal

• unreactive metals - less likely to form an ion ∴ +ve charge builds up on the metal

describe what would happen in a cell that has reached equilibrium:

using cell seen in diagram as an example:

• e^- flow externally in the circuit from the -ve ½ cell to the +ve ½ cell

• Zn atoms in -ve ½ cell lose e^- so are oxidised

• e^- flow from Zn metal to Cu metal 

• Cu²⁺ ions in soln gain e^- to form Cu

• Cu forms on the Cu electrode - mass of Cu electrode increases

• some Zn metal forms Zn²⁺ ions - mass of Zn electrode decreases

• ∴ e^- can flow

which side is the +ve electrode on (and so which side is the -ve electrode on?

• +ve - RHS: more +ve E^θ

• (-ve - LHS: more -ve E^θ)

at the +ve electrode, which reaction occurs? (oxidation/reduction)

• reduction

• so oxidation occurs at the -ve electrode