electrochem group 1 new (1)
TRANSCRIPT
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
WE are
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
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+
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.
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