a2 redox and sep

8/12/2019 A2 Redox and SEP

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(b) Electrode processes

Candidates should be able to:

show awareness that electrode processes represent oxidations and reductions;

Electrode processes involve oxidations and reductions e.g. during electrolysis, oxidation takes place

at the anode and reduction takes place at the cathode.

Many industrial processes involve electrolysis e.g. reactive metals such as sodium and aluminium are

extracted electrolytically, the metal ions gaining electrons at the cathodes of the electrolytic cells.

(d), (e), (f) Recall, use and describe redox systems

Candidates should be able to:

(c)recall and use the redox systems specified below, including the appropriatecolour change and ion/electron half-equations.

Cu!

(aq) " Cu(s); #n!

(aq) " #n(s); $!

(aq) " $(g) %t; &e'!

(aq), &e!

(aq) "

%t; n*+

(aq), n!

(aq) " %t; (g) " (aq) ( Cl , r , );

(d)use redox systems in addition to those in (d), for which all rele0ant informationis supplied;

(f) describe simple electrochemical cells in0ol0ing;

(i) metal/metal ion electrodes, and

(ii) electrodes based on different oxidation states of the same element.

Consider the changes that occur when small pieces of inc metal are dropped into a!ueous

copper("") sulfate. "f the mixture is stirred, gradually the blue colour of the solution begins to fade

and a reddish deposit is seen on the pieces of inc and at the bottom of the beaker.

# redox reaction has taken place.

$n(s) $n%&

(a!) & %e

Cu%&

(a!) & %e Cu(s)

'verall reaction $n(s) & Cu%&

(a!) $n%&

(a!) & Cu(s)

he a!ueous inc ion is colourless and the blue colour of the a!ueous copper("") ion disappears as

the ions are reduced to the metal. "f the two halves of the reaction are kept separate, chemical energy

is changed into electrical energy producing a voltage.


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%

flow of electrons high resistance voltmeter

salt bridge

copper foilinc foil

a!ueous a!ueousinc sulfatecopper("") sulfate

he salt bridge is needed to allow ions to flow from one solution to the other while the two solutions

themselves are kept separate.

Electrons are released at the inc electrode and will flow through the external wire to the copper

electrode where copper ions accept electrons.

he emf of this cell is of the order of . volts.

*y convention the cell is represented by a cell diagram but note that a cell diagram is not the same

asa diagram of a cell which is the diagram above.

his cell diagram representing the above system issubstance being

substance being reduced

oxidised

$n(s) $n%&

(a!) Cu% &

(a!) Cu(s)

+ve electrode &ve electrode

his cell diagram represents

$n(s) & Cu%&

(a!) Cu(s) & $n%&

(a!)


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5

Some Standard Electrode Potentials

6eaction 1 7 8

9i&(a!) & e 9i(s) .5

1

&

(a!) & e

1(s) %./%Ca

%&(a!) & %e Ca(s) %.:;

n%&

(a!) &.?

Cu%&

(a!) & e Cu&(a!) &.;

Cu%&(a!) & %e+ Cu(s) &.5

Cu&(a!) & e Cu(s) &.?%

"%(s) & %e %" (a!) &.?5

=e&

(a!) & e =e%&

(a!) &.::

2g%%&

(a!) & %e %2g(l) &.0

#g&(a!) & e #g(s) &.0

2g%&

(a!) & %e 2g(l) &.0?

%2g%&

(a!) & %e 2g%%&

(a!) &./


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?

'5(a!)

#g


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;

!ther examples of redox cell chemistry

(i)

=e

3t %/0 1

=e>'5(a!)

Mn'5 (a!). mol dm,

. mol dm

2&(a!) . mol dm

Mn%&(a!) . mol dm

=rom the table above

1 78=e

&(a!) & e =e

%&(a!) & .::

Mn'5 (a!) & 02&(a!) & ?e Mn

%&(a!) & 52%'(l) & .?

Mn'5A

has the more positive1 value so this reaction proceeds in the forward direction andacidified potassium manganate(8"") oxidises iron("").

6earranging

?=e%&(a!) ?=e&(a!) & ?e 1 @ .:: 8(note change in sign)

Mn'5 (a!) & 02&(a!) & ?e Mn

%&(a!) & 52%'(l) 1 @ & .?8

'verall Mn'5 (a!) & 02&(a!) & ?=e

%&(a!) Mn

%&(a!) & 52%'(l) & ? =e

&(a!)

1 @ & .:5 8

he =e electrode is negatively charged and the 3t electrode is positively charged meaning thatelectrons would flow from the iron to the platinum if the cell were short circuited.


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:

(ii) >tate what the spontaneous reaction would be in the following set up.

3t

3t

=e%&

7=e&

(a!) each 1*r(a!. mol dm

+

*r%(l) . mol

=rom the table of standard redox potentials

%=e&

(a!) & %e %=e%&

(a!) & .:: 8

*r%(l) & %e %*r+(a!) & .: 8

%=e%&

(a!) %=e&

(a!) & %e 1 @ .:: 8 (note change in sign)

*r%(l) & %e %*r (a!) 1 @ &.: 8

he spontaneous reaction is

*r%(l) & %=e

%&(a!)

%*r (a!)&%=e

,&(a!)

1 @ &.8

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