1.9 electrode potentials and cells Flashcards
(29 cards)
required practical
what is an electrochemical cell made of?
2 different metals dipped in salt solutions of their own ions and connected by a wire (external circuit)
salt bridge
2 reactions- redox process
whats a salt bridge?
eg strip of filter paper soaked in KNO3(aq).
allows ions to flow between half cells and balance out charges- completes the circuit
what is the cell potential/ cell EMF
voltage between the 2 half-cells (electrons flow from most reactive metal to least.
what is electrode potential?
how easily a metal is oxidised
the direction each reaction goes in depends on how easily a metal is oxidised
what does a negative cell potential mean?
a metal thats easily oxidised
what does a positive cell potential mean?
a metal thats harder to oxidise
how do you write the conventional representation of cells (cell notation)
R/O//O/R
- more negative electrode potential on the left
- phase boundary if diff states, comma if same state
- // is the salt bridge
- pt electrode goes on the end if r and o are both aq
how do you write half equations for electrodes?
look at conventional representation and write equation from that.
reduction happens at positive electrode and oxidation happens at negative electrode
how do you calculate cell potential?
Ecell= E right- E left
should always be positive value as more negative value is being taken away from more positive
what factors affect the electrode potential?
reversible so equilibrium position is affected by temp, pressure, conc
changing eq changes cell pot
why do you use standard conditions when measuring electrode potential?
so you get the same value for electrode potentials and you can compare values for diff cells
what is the standard hydrogen electrode?
way to measure electrode potential
hydrogen gas is bubbled through solution of aq H+ ions. Pt electrode used as a platform for redox reactions
Must use standard conditions
what is the standard electrode potential of a half-cell?
voltage measured under standard conditions when half cell is connected to standard hydrogen electrode
how do you measure standard electrode potential?
SHE is always on left despite other being more + or more -
SHE has an electrode pot of 0.00V so the voltage reading for the whole cells will be the same as the E right hand side
what are standard conditions?
solutions of ions must have a conc of 1.00mol dm-3
298K/ 25c
100kPa
what is the electrochemical series?
- list of electrode potentials for diff electrochemical half cells in numerical order
- the half equations are written as reduction
when 2 half-equations are put together in an electrochemical cell, which direction do they go in?
the one with the more negative electrode pot goes in the oxidation direction (backwards) and the one with the more positive electrode pot goes in the reduction direction (forwards)
how can you use the electrochemical series to calculate standard cell potential when 2 half-cells are connected?
E cell= E reduced- E oxidised
(reduced has more + electrode pot, oxidised has more - electrode pot)
how can you use electrode potentials to predict which direction a redox reaction will go in?
1) find the 2 half equations and write them both as reduction
2) use electrochemical series to see which has more - electrode pot- write this as oxidation
3) combine the 2 equations
4) will give + overall E value. rection wont happen other way round
how to work out if a redox reaction is feasible or not?
work out the cell emf
if its +, it is feasible
if its -, it is not feasible
what is a non-rechargeable cell?
cell that uses irreversible reactions eg dry cell alkaline battery
you can make reactions in non-rechargeable cells run backwards in the right conditions, so why not?
it could leak/ explode because a zinc electrode forms the casing of the battery and this becomes thinner as zinc is oxidised
what are rechargeable cells?
use reversible reactions
eg nickel-cadmium and lead-acid
