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WJEC Chemistry 3.2 Redox Reactions
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Alicia Hennah
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Cards (10)
Potassium Dichromate (VI)
and
Potassium Manganate (VII)
are
oxidising
agents.
Thiosulfate
S2O3^2-
is a
reducing agent
and donates
electrons
to become oxidised.
Ion-electron half equations:
Acidified
manganate (VII)
ions-
MnO4
– (aq) + 8H+ (aq) + 5e– ⇌ Mn2+(aq) + 4H2O(l)
Colour change: purple to colourless
Ion-electron half equations
:
Acidified
dichromate
ions-
Cr2O7 2–
(aq) + 14H+ (aq) + 6e– ⇌ 2Cr3+(aq) + 7H2O(l)
Colour change
: orange to green
Ion-electron
half equations:
Iodine
to
Iodide
ions-
2(aq) + 2e– ⇌ 2I– (aq)
Colour change
: brown to colourless
Ion-electron
half equations:
Thiosulfate
ions-
2S2O3 2–(aq) ⇌ S4O6 2–(aq) + 2e–
Redox titration-Aqueous thiosulfate ions and aqueous
iodine
:
Thiosulfate solution placed in the burette.
Aqueous iodine pipetted into the conical flask.
The S2O3 2– solution is run into the flask until the colour fades to a pale-yellow.
Starch solution is added as an indicator, turning the mixture dark blue. The end point is when the blue colour is decolourised.
2(aq) + 2S2O3 2–(aq) → S4O6 2–(aq) + 2I– (aq)
Redox titration
-Aqueous
iodide
ions and aqueous
copper(II)
ions:
The blue solution loses its colour.
A white precipitate of
CuI
forms along with a brown solution of
I2
.
The I2 can then be titrated with
S2O3
2–.
2Cu2+
(aq) +
4I–
(aq) →
2CuI(s)
+ I2(aq)
Redox titration
-Aqueous dichromate ions and iron(II) ions:
Aqueous
Cr2O7
2– is placed in the burette and added to a conical flask where it oxidises aqueous
Fe2+
.
An indicator is required.
The colour change is orange to yellow.
Redox titration
-Aqueous
manganate(VII)
ions and
iron(II)
ions:
Potassium manganate(VII)
solution must be standardised by titration with a reducing agent such as
iron(II) ammonium sulfate
.
The aqueous
MnO4
– is placed in the burette.
Fe2+
solution pipetted into the conical flask with an excess of aqueous sulfuric acid.
Keep adding Fe2+ until the solution turns a permanent pink colour.