redox 3 Flashcards
what are redox titrations used for
in redox
systems, the titration method can be adopted
to determine the strength of a reductant/
oxidant using a redox sensitive indicator.
explin redox titrtion using mno4 -
In one situation, the reagent itself is
intensely coloured, e.g., permanganate
ion, MnO4
–
. Here MnO4
–
acts as the self
indicator. The visible end point in this case
is achieved after the last of the reductant
(Fe2+ or C2O4
2–) is oxidised and the first
lasting tinge of pink colour appears at
MnO4
–
concentration as low as 10–6 mol dm–3
(10–6 mol L–1). This ensures a minimal
‘overshoot’ in colour beyond the equivalence
point, the point where the reductant and
the oxidant are equal in terms of their
mole stoichiometry.
what do we do if there is no self indicating reactant
If there is no dramatic auto-colour change
(as with MnO4
–
titration), there are indicators
which are oxidised immediately after the
last bit of the reactant is consumed,
producing a dramatic colour change. The
best example is afforded by Cr2O7
2–, which
is not a self-indicator, but oxidises the
indicator substance diphenylamine just
after the equivalence point to produce an
intense blue colour, thus signalling the
end point.
how too indicate end point with substance oxidisng iodine
This method relies on the facts that
iodine itself gives an intense blue colour with
starch and has a very specific reaction with
thiosulphate ions (S2O3
2–), which too is a redox
reaction:
I2(aq) + 2 S2O3
2–(aq)→2I–
(aq) + S4O6
2–(aq) (7.60)
I2, though insoluble in water, remains in
solution containing KI as KI3.
On addition of starch after the liberation
of iodine from the reaction of Cu2+ ions on
iodide ions, an intense blue colour appears.
This colour disappears as soon as the iodine
is consumed by the thiosulphate ions. Thus,
the end-point can easily be tracked and the
rest is the stoichiometric calculation only.
what is redox couple
A redox couple is
defined as having together the oxidised and
reduced forms of a substance taking part in
an oxidation or reduction half reaction.
how are redox couple represented
This is represented by separating the
oxidised form from the reduced form by
a vertical line or a slash representing an
interface (e.g. solid/solution). For example
in this experiment the two redox couples are represented as Zn2+/Zn and Cu2+/Cu.
In both cases, oxidised form is put before
the reduced form.
whatdoes salt bridge
We connect solutions
in two beakers by a salt bridge (a U-tube
containing a solution of potassium chloride
or ammonium nitrate usually solidified by
boiling with agar agar and later cooling to a
jelly like substance). This provides an electric
contact between the two solutions without
allowing them to mix with each other
It allows the completion of inner circuit the allowing the flow of ions without mixing of 2 solutions. Helps maintain electrical equality
what happens in daniell cll
Electrons
produced at the anode due to oxidation(LOAN)
of Zn travel through the external circuit
to the cathode where these reduce the
copper ions. The circuit is completed
inside the cell by the migration of ions
through the salt bridge. It may be noted
that the direction of current is opposite to
the direction of electron flow.
L=loss
o=oxidation
a=anode
n -ve charge
red= reduction, cat=cathode=cu
mechanism of daniel cell
- The transfer of electrons now does not
take place directly from Zn to Cu2+ but
through the metallic wire connecting the
two rods as is apparent from the arrow
which indicates the flow of current. - The electricity from solution in one beaker
to solution in the other beaker flows by
the migration of ions through the salt
bridge. We know that the flow of current
is possible only if there is a potential
difference between the copper and zinc
rods known as electrodes here.
what is electrode potential and standard electrode potential
The
electrode potential value for each electrode
process is a measure of the relative tendency
of the active species in the process to remain
in the oxidised/reduced form. A negative E
means that the redox couple is a stronger reducing agent than the H+/H2 couple. A
positive E means that the redox couple is a
weaker reducing agent than the H+/H2 couple.
what is electrochemical cell
electrochemical cell is a device by which the chemical energy of a spontaneous chemical racn is converted to electrical energy
what is an electrode
when a metal rod/strip is dipped in a solution of its own ions is called electrode
what is standard electrode potential
If the concentration of each species taking
part in the electrode reaction is unity (if any
gas appears in the electrode reaction, it is
confined to 1 atmospheric pressure) and
further the reaction is carried out at 298K,
then the potential of each electrode is said
to be the Standard Electrode Potential.
applications of standard electrode potentila
- predict feasibility of a cell reaction
- predict if hydrogen can be liberated
- compare reactivity of metal
what is electromostive force
the difference between the electrode potential of 2 half cells when no current is drawn.
E°cell= E°cathode-E°anode
cathode- metal whose potential is less -ve
anode- metal ehose potential is more -ve
