WORLD OF MOLECULES

Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework**

Consortium leader

PÁZMÁNY PÉTER CATHOLIC UNIVERSITY

Consortium members

SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund ***

**Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben

***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg.

PÁZMÁNY PÉTER CATHOLIC UNIVERSITY SEMMELWEIS

UNIVERSITY

WORLD OF MOLECULES

ELECTROCHEMISTRY

(Molekulák világa)

(Elektrokémia)

KRISTÓF IVÁN

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1. Laws of thermodynamics 2. Chemical thermodynamics

3. Extensive and intesive quantities 4. Heat

5. Entropy 6. Enthalpy

7. Gibbs free energy 8. Equilibrium

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World of Molecules: Electrochemistry

Previously – Thermodynamics

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World of Molecules: Electrochemistry

the quotient of two extensives gives an intensive quantity

connecting two separate thermodynamic systems with different measures will result in the two

systems changing towards a common equilibrium during this

the extensives are added the intensives equlibrate

Previously - Intensive and extensive quantities

) (intensive )

(extensive volume

) (extensive mass

e.g. = density

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World of Molecules: Electrochemistry

speed distribution of of 1 million gas

molecules

at -100, 20 and 600 degrees °C increasing entropy

Previously - Maxwell-Boltzmann distribution of molecule speeds

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World of Molecules: Electrochemistry

Gibbs free energy of formation (Δg): the non-

mechanical work associated with the formulation of a compound from its elements.

• the chemical potential can be described:

• the Gibbs free energy of a reaction (similarly to heat of reaction)

Previously - Gibbs free energy

reactants , products

, ∑

∑ ^{⋅ − ⋅}

= Δ

j

j j

m i

i i

m n g n

g G

0

ln

i i

RT c

μ = μ +

0

ln

i i

RT x

μ = μ +

0

ln

i i

RT p

μ = μ +

1. Electrolytes

2. Electrochemistry 3. Concentration cells 4. Galvanic cell

5. Electromotive force

6. Standard electrode potentials 7. Redox reactions

8. Electrolysis

Table of Contents

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World of Molecules: Electrochemistry

World of Molecules: Electrochemistry

• deals with the changes and transformation of the electric and chemical energy into each other

• usually occurring at a solid-liquid interface

• charge carriers: electron: in solids (e.g. metals), or ion: in liquids, molten salts

• at the interface of phases there is a change of the type of charge carriers

• spontaneous: galvanic cells

• forced: electrolysis

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Electrochemistry

World of Molecules: Electrochemistry

solutions where there are ions that can move around freely and carry charge

in water: solvated ions, usually solution of acids, bases or salts

solvation occurs because water molecules are dipoles and orient themselves around charged substances

molten salts (have free charge carriers as well) their descriptor is electrical conductivity

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Electrolytes

−

+

+

⎯→

⎯

)

(s

Na

Cl

NaCl

World of Molecules: Electrochemistry

if a metal electrode is in an electrolyte containing its ions a redox reaction occurs due to the

difference in oxidation states of the same material

• reduced form → oxidized form + z*e ^- the simplest electrochemical system is a

concentration cell, where two electrodes are in contact with their respective solutions

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Metal electrode in an electrolyte

− + + ⋅

→ Cu e

Cu _{( s ) (} ² _{aq )} 2

World of Molecules: Electrochemistry

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Concentration cell

Cu electrode | 0,05M CuSO

|| 0,2M CuSO

| Cu electrode

) ( 2

) ( 2

) ( )

( _s | 0 . 05 M Cu _aq || 0 . 2 M Cu _aq | Cu _s

Cu ^{+ +}

electrode the

of n dissolutio anodic

2e M)

05 . 0 ( :

oxidation Cu

→ Cu

+

copper metal

of deposition cathodic

2e M)

2 . 0 ( :

reduction Cu

+

→ Cu

World of Molecules: Electrochemistry

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Galvanic cell

http://en.wikipedia.org/wiki/File:Galvanic_cell_with_no_cation_flow.png

) ( 2

) ( 2

) ( )

( | 1 M n || 1 M |

n _s Z _aq Cu _aq Cu _s

Z ^{+ +}

ANODE

2e :

oxidation Zn

→ Zn

+

CATHODE

2e :

reduction Cu

+

→ Cu

World of Molecules: Electrochemistry

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Hydrogen fuel cell

World of Molecules: Electrochemistry

Electrode potentials

• if the electron is not removed from the electrode

surface it would result in a local electron excess on the anode and Zn ²⁺ excess in the solution

• this results in an electrochemical double layer which generates a potential difference

• on the cathode the lack of electrons and the excess of sulphate ions (SO ₄ ^2- ) creates

a double layer

• these two connected will generate the electromotive force, described by the Nernst equation

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Electromotive force

ε

+

ε

0

0

ln 0,059

log

i

i

RT c zF

z c

ε ε ε ε

= +

= +

World of Molecules: Electrochemistry

1. Pt electrode 2. H ₂ gas

3. acid, where [H ⁺ ]=1 mol/l 4. hydroseal

5. place for counter electrode

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Standard Hydrogen electrode (SHE) - reference

0

0

0,059 log H ε

ε

=

⎡ ⎤

= ⋅ ⎣ ⎦

−

+ + ⋅

→ H e

H _{2 ( g )} 2 _{( aq )} 2

World of Molecules: Electrochemistry

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Standard electrode potentials

World of Molecules: Electrochemistry

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Standard electrode potentials

World of Molecules: Electrochemistry

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Standard electrode potentials

World of Molecules: Electrochemistry

The electromotive force is calculated from the anodic and cathodic standard electrode

potentials taking into consideration the concentrations of the species

e.g. for the Galvanic cell

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Electromotive force

0

0

ln 0,059

log

i

i

RT c zF

z c ε ε

ε ε

= +

= +

+

E= ε

− ε

= 0,36 V − − ( 0,76 ) 1,12 V = V

− + −

= ε ₁ ε ₂

E

World of Molecules: Electrochemistry

redox potential

• similar to the standard electrode potential but both species are in ionic form

• the Nernst equation for redox reactions

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Redox reactions

0

0, 0,

0, 0,

0

, és ln

ln ln

ln ln

ox red i

ox ox red red

ox red ox ox

red red

G z F RT c

RT c RT c z F

c c

RT RT

z F zF c zF c

μ μ ε μ μ

μ μ ε

μ μ

ε ε

Δ = − = ⋅ ⋅ = +

+ − − = ⋅ ⋅

= − + = +

⋅

0

0,059

log

red

c

z c

ε ε = +

World of Molecules: Electrochemistry

list of redox potentials e.g.

• Fe ²⁺ /Fe ³⁺ : +0,76V

• Sn ²⁺ /Sn ⁴⁺ : +0,15V

The higher the redox potential of a system

the more oxidizing the process it comprises

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Redox reactions

spontaneous

+ +

+

+ + ² ↔ ² + ⁴

3 2

2 Fe Sn Fe Sn

World of Molecules: Electrochemistry

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Exotic redox reactions – Belousov-Zhabotinskii oscillating reaction

http://en.wikipedia.org/wiki/File:Bzr_fotos.jpg

World of Molecules: Electrochemistry

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Pourbaix diagram (E – pH)

ε

compared to the standard hydrogen electrode,

at room temperature,

assuming 1 M solution

World of Molecules: Electrochemistry

• with the utilization of external force (electric current) the non-spontaneous reaction occurs in the system

• usually performed in an electrolytic cell

• aim: to separate elements

• e.g. molten salt electrolysis of NaCl → Na, Cl ₂ or water electrolysis to obtain H ₂ , O ₂

• Faraday’s law of electrolysis:

• we have to apply higher potential difference than the standard electrode potential of the system for the

electrolysis to occur (overpotential)

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Electrolysis

n I t

z F

= ⋅

⋅ ^{m = I} _F ^{⋅ t ⋅ M} _z

World of Molecules: Electrochemistry

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Electrolytic cell

ANODE

4e 4

2 :

oxidation H

O

→ O

+ H

+

CATHODE

2e 2

:

reduction H

+

→ H

World of Molecules: Electrochemistry

the current density can be expressed using the cathodic and anodic current densities:

substituting the j ₀ exchange current density

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Electrolyzing current - Butler – Volmer – Erdey-Grúz equation

T E n red

s ox T E

n ox

s

red k e n c k e

c n

j ^R

F R 0

F

0 F

F ^α − ^{− β}

=

⎟⎟ ⎠

⎜⎜ ⎞

⎝

⎛ −

= j ₀ e ^{R n F T ( E − E 0 )} e ^{− R n F T ( E − E 0 )}

j ^{α β}

0

if

₀

0 = j = j E − E =

j _{a k}

World of Molecules: Electrochemistry

It describes how the

electrical current on an electrode depends on the electrode potential, considering that both a cathodic and an anodic reaction occur on the same electrode

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Electrolyzing current - Butler – Volmer – Erdey-Grúz equation