Electron transfer Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + ... - Spada UNS

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KESETIMBANGAN ELEKTROKIMIA (Electrochemistry Equilibrium)

Dr. Mohammad Masykuri, M.Si.

Chemistry Education Study Program Teacher Training and Education Faculty Sebelas Maret University (UNS)

Website: http://masykuri.staff.fkip.uns.ac.id, email: [email protected]

Kimia Fisika 2

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Teacher Training and Education Faculty

Sebelas Maret University M. Masykuri_Phisical Chemistry 2

Feb, 2020

Zn strip Zn strip

With time, Cu plates out onto Zn metal strip.

Electrons are transferred from Zn to Cu

²⁺

, but there is no useful electric current.

• Zn is oxidized and is the reducing agent Zn(s)  Zn 2+ (aq) + 2e-

• Cu 2+ is reduced and is the oxidizing agent Cu 2+ (aq) + 2e-  Cu(s)

Copper Plating

CuSO4

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9V

Copper wire

Steel washer Paper clip

Copper sulfate solution (CuSO )

Copper Plating

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Atom transfer

HCl (g) + H 2 O (l)  Cl - (aq) + H 3 O + (aq)

Electron transfer

Zn(s) + Cu 2+ (aq)  Zn 2+ (aq) + Cu(s)

Transfer Reaction

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Electron transfer

Zn(s) + Cu 2+ (aq)  Zn 2+ (aq) + Cu(s)

L oss of E lectrons = G ain of E lectrons = - 2 e-

+2 e-

Zn metal

Cu

²⁺

ions Zn metal

H

₂

O

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Chemical Change  Electric Current

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Rusting

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Reaction of Magnesium with Oxygen Mg (s) + ½ O 2 (g)  MgO (s)

Simple example of oxidation-reduction

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Schematic view of the

processes involved in

redox reactions.

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Tabel Elektronegativitas

Elektronegativitas atom mendasari adanya perbedaan muatan

listrik relatif antar atom, termasuk atom-atom logam sehingga

mendorong terjadinya reaksi REDOKS.

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Sel Electrochemistry

Apabila dua logam berbeda, Cu dan Ag masing-masing dicelupkan dalam larutan Cu

²⁺

dan Ag

⁺

, dan dihubungkan dengan jembatan garam KNO

₃

(aq): ternyata terjadi BEDA POTENSIAL 0,46 V

SEL ELEKTROKIMIA!

Aliran elektron

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Sel Elektrokimia

Gambaran secara atomik proses yang terjadi dalam sel galvanis:

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Sel Elektrokimia

Penulisan diagram sel elektrokimia:

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POTENSIOMETRI

mempelajari hubungan konsentrasi dengan pootensial.

• Elektrode pembanding (refference electrode)

• Elektroda indikator (indicator electrode)

• Alat pengukur potensial

Alat-alat yang diperlukan dalam metode potensiometri:

Potensiometri:

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Potensial Elektroda Standard (Eo)

Dalam teori listrik dan elektronik, voltase adalah parameter beda potensial yang diukur antara 2 point dalam suatu rangkaian.

Pengukuran absolut potensial (voltase) pada hanya 1 point tidak memiliki makna, karena itu harus digunakan suatu referen.

Dalam istilah listrik, referen dikenal sebagai “ground”.

Dalam elektrokimia, sebagai referen digunakan standard hydrogen electrode (SHE).

Suatu potensial elektroda standard, Eo, mengukur kecenderungan proses

reduksi yang terjadi pada suatu elektroda pada kondisi standard (konsentrasi

larutan~ 1.0 M atau, untuk gas, tekanan 1 bar {~1 atm}).

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Standard Hydrogen Electrode (SHE)

a

_H2

= 1.0 ~ P

_H2

= 1.0 bar ~ 1.0 atm a

_H3O+

= a

_H+

= 1.0 ~ [H+] = 1M

|| H

⁺ _(aq)

(1M) | H

_2(g)

(1 atm) | Pt H

⁺ _(aq)

+ 2e

^-

H

_2(g)

Eo = 0.00 V

Sering ditulis sebagai:

Eo

H+(aq)/H2(g)

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Mudah tereduksi (sifat oksidator

meningkat)

Mudah teroksidasi (sifat reduktor

meningkat)

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The potential of an electrochemical cell under standard conditions may be calculated by

Eo cell = Eo cathode – Eo anode

where the Eo’s are standard reduction potentials taken from a table.

The cathode is the electrode at which reduction occurs (electrons on left side of equation, oxidation state decreasing).

The anode is the electrode at which oxidation occurs (electrons on right side of equation, oxidation state increasing).

Also for a spontaneous reaction, Eocell > 0, as we will see shortly.

Potensial Elektroda Standard (Eo)

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 Is there a relationship between E cell and DG for a redox reaction?

 Relationship between E cell and DG:

 DG = -nFE cell

• F = Faraday constant = 96500 C/mol e

^-

’s, n = # e

^-

’s transferred redox rxn

• 1 J = CV

• DG < 0, E

_cell

> 0 = spontaneous

 Under standard state conditions:

 D G° = -nFE° cell

Equilibrium Constants from E cell

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Equilibrium Constants from E cell

 Relationship between E cell and D G:



DG = -nFE

_cell

• F = Faraday constant = 96500 C/mol e

^-

’s, n = # e

^-

’s transferred redox rxn

• 1 J = CV

• DG < 0, E

_cell

> 0 = spontaneous

 Under standard state conditions:



DG° = -nFE°

_cell

and



DG° = -RTlnK

so

 -nFE° = -RTlnK

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D H° D S°

Calorimetric Data

D G°

Electrochemical Composition Data

Data E° cell

Equilibrium constants

K

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Contoh-contoh

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Contoh-contoh

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Persamaan Nernst

Previously, we talked about standard

electrode potentials in which everything was in its standard state.

Very rarely are things in standard state!

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1) Draw the condensed cell diagram for the voltaic cell pictured at right.

2) Calculate the value of E

_cell

.

Soa l 1 Soa l 1

Aplikasi Persamaan Nernst

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Aplikasi Persamaan Nernst (cont.)

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Aplikasi Persamaan Nernst (cont.)

Soa l 2

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Aplikasi Persamaan Nernst (cont.)

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Menentukan K eq

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Sel Konsentrasi

Both half-cells are the same chemical system, just different concentrations.

The driving force (i.e. the EMF) is provided by the difference in

concentrations.

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Sel Konsentrasi

This concentration cell behaves as a pH meter! Other

concentration cells can be used to measure unknown

concentrations of other species (i.e. potentiometry).

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Penentuan K sp

From measured E

_cell

, determine K

_sp

.

Solution:

Set up Nernst equation with Ag

⁺

(xM) at anode, 0.1M at cathode.

Solve Nernst equation to get x.

x = [Ag

⁺

] = S, [I

^-

] = S

K

_sp

= S

²

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Elektroda Pembanding

Elektrode pembanding (refference electrode):

elektrode dengan harga potensial setengah sel yang

diketahui, konstan, dan sama sekali tidak peka terhadap komposisi larutan yang sedang diselidiki

Penghantar listrik

Tabung dalam mengandung

campuran Hg, Hg

₂

Cl

₂

dan KCl jenuh

KCl jenuh Lubang kecil

Penyekat

1. Elektroda Kalomel (Calomel Electrode)

Setengah sel elektrode kalomel dapat ditunjukan sebagai,

 Hg

₂

Cl

₂

(sat’d), KCI (x M)  Hg Reaksi:

Hg

₂

CI

_{2 (s)}

+ 2 e

^¯

 2 Hg

_(l)

+ 2 CI

Harga potensial SCE adalah 0,244 V

pada 25

^o

C

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Elektroda Pembanding

1. Elektroda Silver/Silver Chloride

Reaksinya:

Diagram selnya:

Jika menggunakan larutan jenuh KCl, elektroda Ag/AgCl

memiliki potensial = +0.197 V at 25 °C.

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Elektroda Indikator

Elektrode indikator (indicator electrode/working electrode):

Merupakan pasangan elektrode pembanding yaitu elektroda yang potensialnya bergantung pada konsentrasi zat yang

sedang diselidiki

Elektroda indikator dibagi menjadi dua kategori, yaitu : 1. Elektroda logam

2. Elektroda membran

Elektroda logam dapat dikelompokkan ke dalam:

a) elektroda jenis pertama (first kind),

b) elektroda jenis kedua (second kind),

c) elektroda jenis ketiga (third kind),

d) elektroda redoks

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