redox (2) Flashcards
wwhat oxidation number to uncombined elements have
0
what oxidation number is aluminium
+3
what oxidation number does hydrogen have
+1
-1 in metal hydrides eg NaH
what oxidation number does chlorine have
-1
in a compound with F or O it has positive value eg ClF3 it is +3
what oxidation number does oxygen have
-2
in peroxides it is -1 eg H2O2
+2 in OF2
what is an oxidising agent
accepts electrons from a species being oxidised therefore is reduced
what is a reducing agent
donates electrons to species being reduced, therefore is oxidised
rules for ionic half equations
1) balance all species
2) balance oxygen using H2O
3) balance hydrogen using H+
4) balance charges on species being oxidised using e-
what is a half cell/electrode
putting a metal in a solution of its own ions to set up an equillibrium
what is electrode potential
how readily electrons are lost and can stick onto ions in solution (how good of a reducing agent a metal is)
what is a salt bridge made up of usually
KNO3
why use a salt bridge in measuring electrode potential
maintains electrical neutrality within the cell by providing ions to either side depending on the charge built up in each half cell
solution they are soaked in should not react with either of the solutions in the half cells
why not use a wire instead of salt bridge
to avoid further metal/ion potentials in the circuit and also to provide/remove ions to balance charge on either side
describe a standard hydrogen electrode
-hydrogen gas at 100KPa
-hydrogen ions in solution at 1moldm-3
-platinum electrode for electron exchange with hydrogen ions
-298K temperature
what is the potential of a hydrogen electrode
0
which electrode is always the hydrogen electrode
left hand
what does the voltmeter measure in a half cell
the difference in electron build up on the surface of the metals involved in the 2 half cells
rules for drawing cell notation
-more positive cell on right except when using hydrogen
-double line = salt bridge
-single line = state change
-most oxidised species written closest to salt bridge
does an oxidising agent donate or remove electrons
removes electrons from other species to oxidise it
why is a reference electrode necessary (hydrogen electrode)
it provides a stable, known potential that allows changes in a cell to be attributed to the working electrode
how to calculate emf values
positive (runs right) - negative value (runs left)
emf always positive unless hydrogen electrode is in it as it is always on the left
how do you use emf to tell if reaction is feasible
if its positive the reaction is feasible
what is meant by the term oxidation number
the charge that atom would have if the compound was composed of ions
what is a disproportionation reaction
where an element in a single species is being simultaneously oxidised and reduced
what is E cell directly proportional to
total entropy change and to lnK
limitations of predictions made using standard electrode potentials
departure from standard conditions- temp or conc may change slightly from standard conditions
reaction may have slow rate due to high Ea so it is unfeasible
describe an acid fuel cell
hydrogen gas is fed into the anode, where it is split into protons and electrons. The protons pass through a semi-permeable membrane to the cathode, where they combine with oxygen and electrons to create water
describe alkaline fuel cell
oxygen enters the cell from the cathode side and hydrogen enters from the anode side. oxygen receives the electrons coming through the external circuit, forming hydroxide ions in the electrolyte
equations in acid fuel cell
H2 -> 2H+ + 2e-
1/2 O2 + 2H+ + 2e- -> H2O
equations in alkaline fuel cell
H2 + 2OH- -> 2H2O + 2e-
1/2 O2 + H2O + 4e- -> 4OH-
why are fuel cells good
only waste product is water
very efficient compared to combustion engines
lightweight
don’t require fossil fuels
why are fuel cells not good
not widely available
hydrogen often made from methane (non renewable)
explosive when stored badly
why can you not add starch indicator too early to a redox titration
it forms complexes with the iodine and prevents all of it from reacting
if E cell is positive then what is lnK and s total
positive
how to set up an electrochemical cell
1) obtain metals under investigation and clean with sandpaper to remove surface impurities
2) some metals have grease on surface from handling, wash with propanone and wear gloves
3) place each metal in solution of ions of the same metal
4) make salt bridge with filter paper soaked in KNO3
why use platinum metal in your half cell
inert but electrically conductive
why can’t hydrochloric acid be used in KMnO4 titration
MnO4- would oxidise Cl- to Cl2 so affect the volume of KMnO4 required for titration
why should dilute sulfuric acid be used and not conc acid in KMnO4 titration
conc acids are oxidising agents themselves so affect volume of KMnO4 required
equation for KMnO4 titration
MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O
colour change for KMnO4 titration
purple-> pale pink
equation for Fe2+ and MnO4- titration
5Fe2+ + MnO4- + 8H+ -> 5Fe3+ + Mn2+ + 4H2O
reacting ratio of Fe2+ to MnO4-
5:1
equation for C2O42- and MnO4- titration
5C2O42– + 2MnO4+ 16H+ -> 10CO2 + 2Mn2+ + 8H2O
reacting ratio of C2042- to MnO4-
2.5 : 1
