ELECTROCHEMICAL CELLS, TWO & THREE ELECTRODE CELL SYSTEMS, SOLVENTS AND SUPPORTING ELECTOLYTES

CHM 4102ELECTROCHEMISTRY

GROUP 1

NORSHAFIDAH BT ABU SHAFIAN151897GOH RUO ZHEN 152008SITI ZAHARAH BT SYED RAMLI152197 HEE WAI SUM 152584LU CHING CHING 153165LING KAI SING 153168LYE FUI FANG 153560ARINA BT IRMAN153487NUR SYAZLIANA BT MALIK 153367THEN PAY KEE 154380

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ELECTROCHEMICAL CELLS

INTRODUCTION

An electrochemical cell is a device in which electron transfer in a redox reaction are made to pass through an electric circuit.

Oxidation process – loss of electron, the substance oxidized is the reducing agent.

Reduction process – gain of electron, the substance reduced is the oxidizing agent.

Two types of cell : Galvanic cell / voltaic cell Electrolytic cell

TWO & THREE ELECTRODE CELL

SYSTEMS

GALVANIC CELL

•A galvanic cell is an electrochemical cell that produces electricity as a result of the spontaneous reaction.

•Also called as voltaic cell

Component of Galvanic cell

The 2 metals are connected by a wire

The 2 containers are connected by a salt bridge

A voltmeter is used to detect voltage generated

example:

i- Zn metal in an aqueous solution of Zn2+

ii- Cu metal in an aqueous solution of Cu2+

Galvanic cell

What happens at zinc electrode?

Zn is more electropositive than Cu Zn has a tendency to release electron

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

Zn dissolves Oxidation occurs at Zn electrode

Zn2+ ions enter ZnSO4 solution

Zn is the negative electrode (anode)

What happens at copper electrode?

Cu2+(aq) + 2e- Cu(s) The electron move from negatives to positive

terminal Cu2+ ions from the solution accept electrons and the

blue colour of copper(II) solution fades Cu is deposited Reduction occurs at the Cu electrode Cu is the positive electrode (cathode)

Cell Notation

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

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

Zn(s) + Cu2+(aq) Zn2+(aq) + Cu(s)

Also can be represented as:

Zn(s) Zn2+(aq) Cu2+(aq) Cu(s)

Electrolytic cell

An electrolytic cell is an electrochemical cell in which a non- spontaneous reaction occur.

It is made up of two electrodes immersed in an electrolyte

A direct current is passed through the electrolyte from an external source

Molten salt and aqueous solution are commonly used as electrolytes

Differences between Electrolytic and Galvanic cell

Characteristic

Electrolytic cell Galvanic cell

Energy change Electrical energy Chemical energy

Chemical energyElectrical energy

Electric current and reaction

Electric current results in a chemical reaction

Chemical reaction produces an electric current

Cathode :Anode:

Negative terminalPositive terminal

Positive terminalNegative terminal

Negative terminal Cation receives electrons from the cathode

Electrons are released at the negative terminal

Positive terminal Anions release electrons to the anode

Electrons are received by the positive terminal

THREE ELECTRODE SYSTEM

Include the working electrode, reference electrode, and the auxiliary electrode.

The three electrodes are connected to the power source, which is a specially designed circuit for precise control of the potential applied to the working electrode and often called a potentiostat or polarograph.

This electrode system is important in voltammetry. Voltammetry is an electrochemical technique in which

the current-potential behaviour at an electrode surface is measured.

Auxiliary Electrode

Counter or Auxiliary electrode : electrode in the cell that completes the current path.

All electrochemistry experiments (with non-zero current) must have a working – counter pair.

Auxiliary electrode makes sure that current does not pass through the reference cell. It makes sure the current is equal to that of the working electrode's current.

Reference electrode

Serve as experimental reference points.

Specifically they are a reference for the potential (sense) measurements.

Reference electrodes should hold a constant potential during testing.

Example: Saturated Calomel, Silver/Silver Chloride, Mercury/Mercury (mercurous)

Oxide, Mercury/Mercury Sulfate, Copper/Copper Sulfate, and more.

Working Electrode

Working electrode is the designation for the electrode being studied.

In corrosion experiments, this is likely the material that is corroding.

In physical echem experiments, this is most often an inert material— commonly gold, platinum or carbon—which will pass current to other species without being affected by that current.

SOLVENTS AND SUPPORTING ELECTOLYTES

ELECTROLYTE

Electrochemical reactions occur in a medium, a solvent containing a supporting electrolyte which is mobile and support current flow.

A medium containing mobile ions must exist between the electrodes in an electrochemical cell to allow for measurement of the electrode potential.

Electrolyte provides the pathway for ions to flow between and among electrodes in the cell to maintain charge balance.

Electrolytes

Liquid Electrolytes

- Include molten salts and appropriate solvents

Solid Electrolytes

- Solids and some of those are crystalline

solids

Liquid Electrolyte

sMolecular

Liquids

Aqueous (water)

Mixed aqueous (water and cosolvent)

Nonaqueous (organic or inorganic solvent)

Ionic Liquids

Molten salts and usually used at relatively high temperatures

Mixtures of organic halides with aluminium trichloride

Atomic Liquids

Super Atomic Electrolyte (SPE)

Metallic mercury

Blend of a solvating polymer and a salt or a nonaqueous electrolyte solution

Exhibit various liquid electrolytes properties

SOLVENT

Choice-solubility of the analyte , its redox activity, and by solvent properties(electrical conductivity, electrochemical activity, and chemical activity)

The solvent should not react with the analyte (or products) and should not undergo electrochemical reactions over a wide potential range.

PROPERTIES OF SOLVENTS

Physical Chemical

Boiling point

Melting point

Vapor pressure

Heat of vaporization

Relative permittivity

Acidity

Basicity

EFFECT OF SOLVENT PROPERTIES ON CHEMICAL REACTION

Solvents with WEAK ACIDITY Solvents with STRONG ACIDITY

• Solvation to small anions is difficult -Small anions are reactive • Proton donation from solvent is difficult -pH region is wide on the basic side -Strong bases are differentiated -Very weak acids can be titrated• Reduction of solvent is difficult -Potential region is wide on negative side -Strong reducing agent is stable in the solvent -Strong oxidizing agent is stable in the solvent -Substances difficult to reduce can be reduced

• Solvation to small anions is easy -Small anions are nonreactive • Proton donation from solvent is easy -pH region is narrow on the basic side -Strong bases are leveled -Very weak acids cannot be titrated• Reduction of solvent is easy -Potential region is narrow on negative side -Strong reducing agent is unstable in the solvent -Strong oxidizing agent is unstable in the solvent -Substances difficult to reduce cannot be reduced

Solvents with WEAK BASICITY Solvents with STRONG BASICITY

• Solvation to small cations is difficult -Small cations are reactive

• Proton acceptance by solvent is difficult -pH region is wide on the acidic side -Strong acids are differentiated -Very weak bases can be titrated

• Oxidation of solvent is difficult -Potential region is wide on positive side -Strong oxidizing agent is stable in the solvent -Substances difficult to oxidize can be oxidized

• Solvation to small cations is easy -Small cations are nonreactive

• Proton acceptance by solvent is easy -pH region is narrow on the acidic side -Strong acids are leveled -Very weak bases cannot be titrated

• Oxidation of solvent is easy -Potential region is narrow on positive side -Strong oxidizing agent is unstable in the solvent -Substances difficult to oxidize cannot be oxidized

Solid Electrolytes

1. A large number of the ions of one species should be mobile. This requires a large number of empty sites, either vacancies or accessible interstitial sites. Empty sites are needed for ions to move through the lattice.

2. The empty and occupied sites should have similar potential energies with a low activation energy barrier for jumping between neighboring sites. High activation energy decreases carrier mobility, very stable sites (deep

potential energy wells) lead to carrier localization.

3. The structure should have solid framework, preferable 3D, permeated by open channels. The migrating ion lattice should be “molten”, so that a solid framework of the

other ions is needed in order to prevent the entire material from melting.

4. The framework ions (usually anions) should be highly polarizable. Such ions can deform to stabilize transition state geometries of the migrating ion

through covalent interactions.

Liquid Electrolytes VS. Solid Electrolytes

Liquid electrolytes show generally better leveling capabilities for both temperature and concentration discontinuities and allow for small volume changes due to chemical or electrochemical reactions.

Liquid electrolytes maintain a permanent interfacial contact at the electrolyte or electrode interface and have generally higher conductivities.

Liquid electrolytes is capable to dissolve the reaction products; they may hence be used in electro synthesis as reaction media.

Liquid electrolytes are potential gassing and leakage problems in cells, and the higher effort in assembling cells.

Solid electrolytes often offer cationic or anionic transport in contrast to liquid electrolyte, where anions and cations are contributing to the conductivity. Avoids the need for a separator. However, their electronic conductivity may be detrimental in some applications

What to consider in choosing electrolytes?

Conductivity Mobility of active species Temperature Chemical thermal stability Electrochemical stability Solubility Viscosity

An electrolyte containing chemical species that are not electroactive (within the range of potentials used)

which has an ionic strength and conductivity much larger than those due to the electroactive species added to the electrolyte.

Inert electrolyte / inactive electrolyte

The typical concentration of the supporting electrolyte is 0.1 to 1.0 mol/kg

Supporting Electrolyte

Functions

Maintain constant ionic strength and constant pH

↑ conductivity of the solution

eliminate the contribution of the analyte to the migration current & ↓transport number

of electroactive species

↓ resistance

Functions

Change metal ions in the sample to the metal-ion complexes with different electrochemical

properties

Determine the useable potential range of

polarographic & voltammetric measurement.

Maintain constant of the activity coefficients and the

diffusion coefficients

Example of supporting electrolyte

Acids HCl, HNO3, H2SO4, H3PO4, Citric acid

Bases NaOH, KOH, TBAOH, NH4OH

Buffers Citrate, Tartate, Acetate, Phosphate, Borate

Non-aqueous Solvents

Alcohols, Acetonitrile, DMF, DMSO-containing dissolved salts for conductivity

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