electrochemical cells Flashcards
what happens when a rod of a metal is dipped into a solution of its own ions
an equilibrium is set up between the solid metal and the aqueous metal ions
what is the half equation for zinc (s) to zinc (II)
Zn(s) –> Zn2+(aq) +2e-
what would the half equation for copper be (from copper (II) to copper (III)
Cu2+ ⇌ Cu3+(aq) +e-
what is the simplest salt bridge made of
filter paper soaked in saturated solution of KNO3 (potassium nitrate)
why are salt bridges important?
- complete circuit, but avoid further metal/ion potential as it does not perform electrochemistry
- allows ions to move and balance the charge
- do not react with electrodes
- helps to prevent the cell from taking its reaction to equilibriuk
what symbol is used to represent a salt bridge in standar notation
||
what type of species goes on the outside (furthest from the salt bridge) in standard cell notation
the most reduced species
what does | indicate
a phase change (eg from solid to aqueous)
how would an aluminium/copper cell be represented
Al(s)|Al3+(aq) || Cu2+(aq)|Cu(s)
what happens at the left hand electrode
- oxidation occurs
- half cell most negative Eo value
what hapenns at the right hand electrode
- reduction occurs
- half cell with the most positive Eo value
which side of the cell has the most negative Eo value?
what happens to the metal with the most negative Eo value
- left hand electrode
- oxidation
what conditions is needed in standard hydrogen electode
temp = 298K
pressure = 100kPa
[H+] = 1.00 moldm-3
what is the standard hydrogen electode used for
- Eo value for S.H.E is defined as 0
- other cells can be compared for there Eo value
why are other standard electrodes (other than the SHE) used occasionally
- cheaper/easier/quicker to use
- provide just as good reference
- platinum is expensive (as an electrode)
if the Eo value is more negative, what does that mean in terms of oxidising/reducing power
better reducing agent (easier to oxidise)
if Eo value is more positive, what does it mean in terms of oxidising/reducing power
better oxidising agent (easier to reduce)
what factors will change Eo values?
- conc of ions
- temp
what happens if you reduce the conc of the ions in the left hand half cell
- equilibrium moves to left to oppose change
- release more electrons
- Eo value of left is more negative (more reduced)
- EMF of cell increases
how do you calculate the EMF of a cell from Eo valeus
Eo cell = EoRHS (oxidised) - EoLHS (reduced)
when would you use a platinum electrode
when both oxidising and reducing forms of the metal are in aqueous solution
why is platinum chosen
inert so doesnt take part in elecrochemistry
good conductor to complete circuit
how would you predict if a reaction would occur in the electrochemical cells
- take the 2 half quations
- find the species that is being reduced (this is the right hand is being reduced)
- calculate its Eo vaue minus the Eo value of the species that is being oxidesed (the left hand cells)
- if the Eo>0, the reaction occurs (not feasible if the overall reaction is negative)
what was the first commercial cell made from (Daniell cell)
zinc/copper (II)
