electrochemical & electrolytic cells Flashcards
dichromate reduction half equation
Cr2O7(2-) + 14H(+) +6e –> 2Cr(3+) +7H2O
permanganate reduction half equation
MnO4(-) + 8H(+) + 5e –> Mn(2+) + 4H2O
direct redox reaction
reactants mixed together in a single beaker: both oxidation and reduction occur in the same vessel (usually results in release of thermal energy)
indirect redox reaction
two reactants in different containers and ox and red occur simultaneously
explain that there is no spontaneous reaction between a reductant and an oxidant
(reductant) is a weaker reductant than the conjugate reductant of (oxidant) and hence no spontaneous reaction will be observed
(oxidant) is a weaker oxidant than the conjugate oxidant of (reductant) and hence no spontaneous reaction will be observed
oxidation
loss of e-
always at anode
reductant undergoes oxidation
increase in oxidation number
reduction
gain of e-
always at cathode
oxidant undergoes reduction
decrease in oxidation number
rules for determining oxidation number
- atom in elemental form =0
- simple ion = charge of ion
- Hydrogen is +1 unless in metal hydrides (eg NaH or CaH2) and it is -1
- oxygen is usually -2 except when in peroxide compounds (eg. H2O2, baO2) = -1 and when bound to flourine = +2
- neutral compound must add to 0
- polyatomic ion must equal charge of that ion
- in covalent compounds (without oxygen/hydrogen)more electronegative element has neg ox number. this is equal to charge of that ion
galvanic cell
electrochemical cell in which chemical energy from spontaneous redox reactions is converted into electrical energy
anode
negatively charged electrode in a galvanic cell where oxidation occurs
cathode
positively charged electrode in a galvanic cell where reduction occurs
salt bridge
connection which allows the flow of ions between two half cells to maintain electrical neutrality of the cell
standard half cell
cell consisting of both members of the conjugate redox pair and an electrode at standard conditions
electrode
electrical conductor that is either unreactive or involved in a reaction
standard electrode potential
potential of the half reaction (reduction), given in volts, relative to the standard hydrogen electrode
structural ft of galvanic cells
two half cells (allows chem energy -> electrical energy)
salt bridge prevents polarisation and completes circuit
each half cell contains conjugate redox pairs
electrons flow through external circuit
electric potential calculation
E overall = E(oxidant) - E(reductant)
E overall = E(cathode) - E(anode)
if sodium chloride forms in a reaction it is a…
a precipitate
explain why iron is protected from rusting when blocks of Mg or Zn are attached to it
Because Zn(s) and Mg(s) will preferentially oxidise when exposed to oxygen because they are stronger reductants than Fe(s), then electrons transfer from the Mg and Zn and protect the Fe from degradation. (anodic protection)
fuel cell
a special type of galvanic cell where there is a continuous supply of reactants allowing for the continuous conversion of chemical energy to electrical energy
general features of a fuel cell
O2 inlet at cathode (+) fuel inlet at anode (-) porous electrodes for large SA:V and faster rate of reaction net reaction is a combustion reaction electrolyte can be alkaline or acidic
adv of fuel cells
high energy conversion efficiency low chemical pollution (H2 based fuels particularly) fuel flexibility quiet continuous supply of electricity
disadvantage of fuel cells
Distribution storage and transport of hydrogen can be difficult (highly flammable and needs high pressure)
Manufacturing process and materials can be expensive
need reliable and continual supply of fuel
primary cells
non-rechargeable electrochemical cells in which a chemical reaction generates electrical energy
