Electrochemistry
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what happens at the cathode
cathode is negative electrode so attracts cations they gain electrons (usually metal ions)
what happens at the anode
the anode is + so attracts -ve ions which donate e-
why does a voltage form
one side of the half cell has more of a tendancy to oxidise (release electrons) than the other So electrons will build up on this side creating a potential difference and one more -ve half cell than the other
why use a high resistance voltmeter
so electrons can’t flow between half cells
which side of half cell undergoes reduction
the one with the most +ve electrode
what is the SHE
electrode which is given the value of 0V by convention so it can be used to compare electrodes
why is Pt used in the SHE
it is a catalyst and can absorb H2 gas
why are standard conditions needed for SHE
otherwise the position of equilibrium will shift and the value will no longer be 0
what is the standard electrode potential
the potential difference when an electrode is connected to the SHE
which direction do the most +ve half equations go in
forwards (reduction)
E cell equation
Ecell= Erhs- Elhs
what will a spontaneous change have
a positive E cell value
where are the most powerful reducing agents found in the electrochemical series
on the right of the most negative end
Use data to explain why Fluorine reacts with water
Eo F2/F- > Eo O2/H2O
what is EMF a measure of
the position of eqm./ how likely it is for a reaction to occur
why does emf drop if current is allowed to flow
reactants get used up
effect of increasing temperature on emf
decrease (as most Ecells are exothermic)
what are the electrodes made from in a lithium ion cell
graphite and LiCoO2
reduction half eqn in lithium ion battery
Li+ + CoO2 + e- ⇌ LiCoO2
oxidation eqn in lithium ion battery
Li+ + e- ⇌ Li (goes backwards)
reaction at the anode of H2 alkaline fuel cell
2H2(g) + 4OH-(aq) → 4H2O(l) + 4e- (oxidation)
reaction at the cathode of the H2 alkaline fuel cell
O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
advantage of fuel cell
maintains same voltage less pollution greater efficiency
limitations of H2 fuel cells
hard to store expensive to make and dispose of