3/14 Redox Flashcards
Oxidation
loss of e-
reduction
gain of e-
O.N rules
1) uncombined elements have an O.N of 0
2) overall OS of compounds is 0
3) elemental ions ahve an O.S equal to charge
4) overall OS of moelcular ion equal to charge
5) group 1: always +1
6) group 2: always +2
7) aluminium: always +3
8) hydrogen: always +1 (except in metal hydrides, eg NaH where it is -1)
9) oxygen: always -2 (except in peroxides, eg H202 where it is -1)
10) group 7: always -1 (except when combined with a more electronegative element)
disproportionation
element simultaneously oxidised and reduced
oxidising agent
what has been reduced, electron acceptor
reducing agent
what has been oxidised, electron donor
redox in acidic conditions
any oxygen lost is H20 in products, add H+ ions to balance the H in H20
half cell
1) made up of a solid metal electrode and 1.0 mol dm-3 solution of X ions
2) an eq is set up between X(s) and X ions (aq)
3) eq can shift: forward reaction is reduction, reverse reaction is oxidation
combining half cells
1) any two half cells can be combined to create a potential difference (voltage)
2) put most negative E0 value at top
3) apply anticlockwise rule
4) high resistance voltmeter
5) salt bridge (AgNO3) completes circuit
LOAN
LHS
oxidation
anode
negative
RRCP
RHS
reduction
cathode
positive
convention
oxidised state on left and reduced state on right
double line to represent salt bridge
single line to represent different phase
half cells of ions in solution
Fe3+(aq) + e- –>
standard hydrogen electrode
1) the redox power of half cells is relative
2) redox potential of all half cells measured against the SHE
3) platinum:unreactive and porous: large SA fro e- transfer
4) SHE connected to another half cell and potential difference measured
5) 2H+(aq) + 2e- –> H2(g) 0.00v
6) 298K, 1.0mol dm-3 H+ ions eg HCL, H2 gas pumped in at 100kPa
down the electrochemical series
1) increase in oxidising power
2) eq lies to right
3) ions readily gain electrons
4) easily reduced
5) last one is strongest O.A Ag+ (aq)
up the electrochemcial series
1) increase in reducing power
2) eq lies to left
3) atoms readily lose e-
4) easily oxidised
5) first one is strongest R.A Li (s)
calculating emf
most positive - most negative
oxidised cell value - reduced cell value
emf always postivie
greatest EMF value
largest difference in E0 values
smallest EMF value
smallest difference in E0 values
Li(s)
stronger reducing agent
most easily oxidised
most negative E0 value
Ag+ (aq)
strongest oxidising agent
moest easily reduced
most postive E0 value
Predicting feasibility of reactions
predictin if reaction will occur (spontaneous)
only happens if reactants are top right and bottom left
limitations to predicting feasibility using E0 values
all E0 values are based on standard conditions: if any of these change then the Ecell values will also change
eg Cu will react with conc h2so4 (10moldm-3) but not with 1 moldm-3
kinetics (rate) of reaction may be so slow that no real reaction is seen due to high AE
effect of changing concentration
1) chateliers principle: when conc or temperature changed
2) increase conc of reactant and decrease conc of product: Ecell increases
3) decrease conc of reactant and increase conc of product: Ecell decreases
4) no E0 value because deviated from standard conditions