3.2 redox reactions Flashcards
what is the oxidation number of V in HVO4?
H = +1
O4 = -2 (-8)
V = +7
what is the oxidation number of V in VO6 2-?
O6 = -2 (-12)
V = +10
if oxidation is loss of e-, what side is the e- in a half equation?
RHS
if reduction is gain of e-, what side is the e- in a half equation?
LHS
what can the gain and loss of electrons be shown by?
half equations
what are redox reactions?
reactions that involve the transfer of electrons
(both oxidation and reduction occurs)
what is the half equation for the oxidation of lithium to lithium ions?
Li —> Li+ + e-
write the half equation from VO3 - —> V 2+
oxidation numbers : +5 -2(-6) —> +2
+5 to +2 = reduction
so e- LHS
VO3 - + 3e- —> V 2+
3e- + VO3 - —> V 2+ + 3H2O
6H + + 3e- + VO3 - —> V 2+ + 3H2O
write the half eqution from WO4 2- —> W 3+
oxidation numbers : +6 -2(-8) —> +3
WO4 2- + 3e- —> W 3+
3e- + WO4 2- —> W 3+ + 4H2O
8H+ + 3e- + WO4 2- —> W 3+ + 4H2O
how can overall redox equations be made?
by combining the half equations
- make electron number the same
- cancel out electrons
- equal half equations to each other
e.g
Fe 2+ —> Fe 3+ + e- [x5]
MnO4 - + 8H + + 5e- —> Mn 2+ + 4H2O
= 5Fe 2+ —> 5 Fe 3+ + 5e-
MnO4 - + 8H+ + 5Fe 2+ —> Mn 2+ + 4H2O + 5Fe 3+
what is an oxidising agent?
- a species which can accept electrons from another species
- it itself is reduced
what is a reducing agent?
- a species which can donate electrons to another species
- is oxidised
what are the three specific half equations you need to know?
- manganate reaction
MnO4 - (aq) + 8H+ (aq) + 5e- —> Mn 2+ (aq) + 4H2O (l)
purple —> pink - dichromate reaction
6e- + Cr2O7 2- (aq) + 14H + (aq) —> 2Cr 3+ (aq) + 7H2O (l)
orange —> green - thiosulfate
2S2O3 2- (aq) —> S4O6 2- (aq) + 2e-
colourless —> colourless (no colour change)
what is the manganate half equation you need to know?
what is the colour change?
MnO4 - (aq) + 8H + (aq) + 5e- —> Mn 2+ (aq) + 4H2O (l)
purple —> pink
what is the dichromate half equation you need to know?
what is the colour change?
6e- + Cr2O7 2- (aq) + 14H + (aq) —> 2Cr 3+ (aq) + 7H2O (l)
orange —> green
what is the thiosulfate half equation you need to know?
what is the colour change?
2S2O3 2- (aq) —> S4O6 2- (aq) + 2e-
colourless —> colourless
what are disproportionation reactions?
redox reactions where the same element/species is both oxidised and reduced
e.g 2H2O2 —> 2H2O + O2
-1 -2 0
an impure sample of iron of mass 2.55g was dissolved in dilute sulfuric acid and the solution made up to 250cm^3. the solution contained iron (II) ions together with the impurities. portions of 25cm^3 of this solution were titrated with potassium manganate (viii) solution of concentration 0.02 mol dm^-3. the average titre was 28.50cm^3. calculate the % purity of the sample of iron
MnO4 - + 8H + + 5e- —> Mn 2+ + 4H2O
Fe 2+ —> Fe 3+ + e- [x5]
MnO4 - + 8H + + 5Fe 2+ —> Mn 2+ + 4H2O + 5Fe 3+
27.20/1000
0.00205
n = 5.576x10^-5 —(x5)—> 2.788x10^-4
5/1000 dm^3
conc = 0.05576
what is the difference between a redox titration and an acid-base titration?
- redox titrations follow the same procedure
- the only difference being the type of reaction that takes place. as one of the reactants in a redox titration invariably changes colour, then there is USUALLY no need for an indicator
titre results are said to be concordant if they are within ____ of each other?
0.20 cm^3
(should obtain 3 concordant results)
apart from the 3 main half equations you need to know, what are 2 other common ones?
Fe 2+ —> Fe 3+ + e-
pale green —> pale yellow
I2 + 2e- —> 2I-
brown —> colourless
(OR blue-black to colourless if starch indicator added)
what is a half equation?
- an equation that tells you what is happening at one of the electrodes during an electrochemical reaction
what is a redox titration?
- a titration of a reducing agent by an oxidising agent (or vice versa)
what is the ionic equation for the reaction between acidified Fe2+ and potassium manganate (VII)?
5Fe2+ + MnO4 - + 8H+ —> 5Fe3+ + Mn2+ + 4H2O
what is potassium manganate (VII)?
a strong oxidising agent
explain what happens during the redox reaction between Cu2+ and I- ions
- in this reaction, I- isnt a strong enough reducing agent to completely reduce the Cu2+ ions, so they are only reduced to Cu+ ions
give the chemical equation for the reaction between Cu2+ ions and I- ions
2Cu2+ + 4I- —> I2 + 2CuI
what happens when iodine is titrated with thiosulfate ions?
- iodine is reduced by the thiosulfate reducing agent
- the thiosulfate ions are oxidised by the iodine oxidising agent
what is the chemical equation for the redox reaction that takes place between iodine and thiosulfate ions?
I2 + 2e- —> 2I-
2S2O3 2- —> S4O6 2- + 2e-
I2 + 2S2O3 2- —> 2I- + S4O6 2-
