ELECTROCHEMICAL CELLS Writing of equations representing oxidation and reduction half reactions and redox reactions Electrolytic and galvanic cells; Writing.

ELECTROCHEMICAL CELLS• Writing of equations representing oxidation and reduction half reactions and redox reactions

• Electrolytic and galvanic cells;

Writing Equations

• Relation of current and potential to rate and equilibrium;

• Understanding of the processes and redox reactions taking place in cells;• Standard electrode potentials;

Calculating Ecell

ElectrolysisRust

 Industrial applications

________

Metal Electrodes. Metal atoms can leave their

electrons (weakly held) and go into solution.

_M__

+ +++

+++

+

+

+++

++

+++

OH

H

OH

H

OH

H

M(s) M+(aq) + e-

e-

e-

e-

e-

e-

e-

e-

M+

M+

An electric potential develops between the solution(….) and electrode(……). Move the red charges to show the electric potential.

O-

H+

H+

M+

M+

-

+

________

Direct Contact.

_Zn__

+ +++

+++

+

+

++

+

++

+O

H

H

OH

H

OH

H

Zn(s) Zn2+(aq) + 2e-

e-

e-

e-

e-

e-

e-

e-

Cu2+

Cu2+

The copper ions are reduced to copper metal and the zinc metal is oxidised to zinc ions.

Cu2+

Cu2+

(SO42-)

Cu2+

OH H

OH

H

OH

H

Cu2+(aq) + 2e- Cu(s)

• The zinc rod would become layered with copper

• The blue solution would become colourless

Copper has a STRONGER attraction for electrons than Zinc.

State which ions would be OXIDISED AND WHICH WOULD BE REDUCED

Electrochemical Cell. Two metals can react indirectly.

_____

Zn_

_

Cu

V

-

KNO3(aq)

+

> e-

Zn Zn2+ + 2e-Cu2+ + 2e- Cu

CuSO4ZnSO4

Salt bridgeanode cathode

• Electrons flow through the external circuit from Zn to Cu.

• Ions flow through the salt bridge (barrier).

• Zinc metal is being .................

• Copper ions are being …………...

• Observations: The zinc electrode would be ………….- the copper electrode would ……………….

Decide which electrode would be negative and which positive and put the charges over the correct electrode.

Electrochemical Cell. Two metals can react indirectly.

_____

Zn_

_

Cu

V

_

KNO3(aq)

+

>

Zn Zn2+ + 2e-Cu2+ + 2e- Cu

Zn2++ 2e- --> Zn

Cu2+ + 2e- --> CuAttract elelctons strongly

Attract elelctons weakly

CuSO4ZnSO4

Salt bridge

anodecathode Zn + Cu2+ Zn2+ + Cuoxidation

reduction REDUCING AGENTOXIDISING AGENT

OXIDISING AGENT

REDUCING AGENT

Top right reacts with bottom left.

e-

Move the labels until they show the correct locations of oxidizing & reducing agents.

ELECTROCHEMICAL CELLS GENERAL A METAL IN CONTACT with a SOLUTION OF ITS IONS is

known as A HALF-CELL.

M M+ + e- N+(aq) + e N(s)

When TWO HALF – CELLS are COMBINED an ELECTROCHEMICAL CELL IS OBTAINED.

OXIDATION takes place at the POSITIVE/NEGATIVE (delete incorrect one) electrode.

PLACE THE TWO SYMBOLS AT THE APPROPRIATE PLACE ON THE

DIAGRAM

M+(aq)

M+(s)

N+(aq)

N+(s)

V

- e-

CELL DIAGRAMS The cell undergoing reduction (more +ve Eθ value) is assigned as the

R.H.S. electrode.

M1(s) M1n+(aq) M2

n+(aq) M2(s)

The solid line represents the phase boundary solid metal & the aqueous ions.

The double dashed line represents the salt bridge. The cell potential is calculated by:

Eθcell = Eθ

RHS EθLHS

(do not change the signs of the Eθ values )

Phase boundary

salt bridge Phase boundary

+

-

ELECTROCHEMICAL CELLS

Electrons flow from –ve to +ve in the external circuit.

The salt bridge allows the flow of ions and completes the circuit.

…….. ……….. // …………. …………. Zinc is ……………& copper is …………….

The Daniell cell :

Galvanic cell converts chemical energy to electrical energy

E0

0.34

-0.76

?

Reactions written as …………………..

Positive potentials ………….. electrons are good ………………...... AGENTS.

Negative potentials …………… electrons are good …………………… AGENTS.

ELECTROCHEMICAL SERIESElectrochemical half-cell potentials (Eθ) are listed from +ve to –ve values.

Zn2+ + 2e- Zn

Cu2+ + 2e- Cu

ElectricPotentials

(….)

Zn ………………. so is MORE ……………….

(….)

Reactions written as reductions.

Positive potentials accept electrons are good OXIDISING AGENTS.

Negative potentials donate electrons are good REDUCING AGENTS.

ELECTROCHEMICAL SERIESequilibrium E° (volts)

1.5

0.8

0.34

0

-0.13

-0.44

-0.76

-1.66

-2.37

-2.71

-2.87

-2.92

-3.03

Electrons flow in external circuit

Ele

ctro

ns

flow

in e

xter

nal

cir

cuit

.

Electrochemical half-cell potentials are listed from +ve to –ve Eθ values.

Oxidizing Agents

REDUCING AGENTS

Reactions take placeTop LEFT to bottom RIGHT

ELECTROCHEMICAL SERIES

Difference = 0.8 – (-2.71)

STANDARD HYDROGEN ELECTRODE

……….. electrode H2 gas pressure of ……….

(………. kPa) Solution of …..M H+ ions

(HCl) …………K Eθ = ……………v S.H.E. is ………. to set up. ……………….. standard

electrodes used.

DETERMINATION OF CELL POTENTIALS

Eθ values are determined using a ……………………………… electrode. The S.H.E is always the …………………. electrode. Standard Conditions : …………………………………………………. A secondary standard electrode the ………………. electrode can be used.

Eθcell= …………………………………………

Eθcell= ………………. – ……….

N+(aq)

N+(s)

Unknown electrodeStandard electrodeHigh resistance voltmeter

Salt bridge

If a cell potential of 1.45v is obtained

with metal N. Then the electrode

potential of N+ /N is .................................. .................................. .................................. ........................

..........

DETERMINATION OF CELL POTENTIALS

If the calomel electrode (+0.241 V ) is used as a standard with another electrode the cell potential is -1.901V.

By means of a calculation, identify the other metal.

M+(aq)

M+(s)

Unknown electrodeCalomel electrode

Salt bridge

High resistance voltmeter

SPONTANEOUS REACTIONS

N+(aq)

N+(s)

Positive electrode

M+(aq)

M+(s)

Negative electrode

Salt bridge

High resistance voltmeter E0 M - ve

E 0.15 VE -0.25 V

NON SPONTANEOUS REACTIONS

N+(aq)

N(s)

M+(aq)

M(s)

Salt bridge

High resistance voltmeter

N+(aq)

E 0.15 VE 0.25 V

NON SPONTANEOUS REACTIONS

N+(aq)

N(s)

M+(aq)

M(s)

Salt bridge

High resistance voltmeter

N+(aq)

WORKED EXAMPLES1. The redox couples Mn2+(aq) Mn(s) and U3+(aq) U(s) have

Eθ values of –1.18 and –1.79 respectively. Determine Eθcell

when they are combined and write an equation for the reaction that takes place.

2. The redox couples Co2+(aq) Co(s) and Ni2+(aq) Ni(s) have Eθ values of –0.28v and –0.23v respectively. Write an equation for the spontaneous reaction and use the Eθ

values to justify your answer.

EXAMINATION QUESTIONSUse the standard Eθ values for the half-equations shown below to answer the questions that follow.

MnO4-(aq)+8H+

(aq)+5e- Mn2+(aq)+ 4H2O(l) Eθ = +1.70v

Cl2(g) + 2e- 2Cl-(aq) Eθ = +1.36v

Tl3+(aq) + 2e- Tl+(aq) Eθ = +1.25v

AgCl(s) + e- Ag (s) + Cl-(aq) Eθ = + 0.22v Determine Eθ

cell for the reaction of MnO4-(aq) with Tl+(aq) in acidic

solution and write an equation for the reaction. Why is it not possible to use HCl acid in the reaction above. Use

the Eθ values to justify your answer. Construct an equation for the reaction of Cl2(g) with Ag(s) and

calculate the Eθ value for the reaction.

Electrolytic cell: Converts…

Electrolysis

Copper chloride

Electrolysis - Lead Bromide

PbBr2

Electrolysis - Lead Bromide

PbBr2

Purifying Copper

Identify the anode cathode etc

-+

Extraction of Aluminium

ELECTROCHEMICAL CELLS• Writing of equations representing oxidation and reduction half reactions and redox reactions

• Electrolytic and galvanic cells;

Writing Equations

• Relation of current and potential to rate and equilibrium;

• Understanding of the processes and redox reactions taking place in cells;• Standard electrode potentials;

Calculating Ecell

ElectrolysisRust

 Industrial applications