Electro chemistry

Prepared by

Odyssa Natividad RM. Molo

Electrochemistry

• Branch of chemistry that deals with electricity and its relation to chemical reaction

Electrochemical processes

- Redox rxns in which the energy released by a spontaneous rxn is converted to electricity or in which electricity is used to drive a nonspontaneous chemical rxn

Redox Reaction

• Oxidation-reduction reaction

• Chemical reactions in which the oxidation state of one or more substances changes

Oxidation state/number

- a (+) or (-) whole # assigned to an element in a molecule or ion on the basis of a set of formal rules; to some degree it reflects the (+) or (-) character of that atom

A Spontaneous Redox Rxn

Redox reaction

• Oxidation – Involves loss of electron

• Reduction – Involves gain of electrons

• Oxidizing agent/oxidant – Substance responsible for another substance to be

oxidized; one that undergoes reduction

• Reducing agent/reductant – Gives up electrons/undergoes oxidation; caused

another substance to be reduced

How to determine if redox or not?

• Keep tract of the oxidation number of all the elements involved in the reaction.

– Activity: Rules on assigning oxidation #s (p44 LM)

– Ex:

• Zn(s) + 2 H+(aq) Zn2+(aq) + H2(g)

• In any redox rxn, both oxidation & reduction must occur. If one substance is oxidized, another must be reduced.

Balancing Redox Reaction

• The law of conservation of mass must be observed.

• + the gains & losses of electrons must be balanced( Electrons are neither created nor destroyed in any chemical rxn)

• Two methods:

– Oxidation number method (LM p 44)

– Ion-electron method (LM p48)

ELECTROCHEMICAL CELL

Electrochemical cell

• Apparatus which converts chemical energy from spontaneous reaction to produce electricity

• Also called voltaic cell or galvanic cell

• Transfer of electron takes place through an external pathway rather than directly between reactants

• Based on the principle formulated by Alessandro Volta (Italian physicist) & Luigi Galvani (Italian physiologist)

Voltaic/galvanic cell diagram

Voltaic/galvanic cell diagram

Components of Electrochemical Cell • Electrodes

– simply where redox occurs (anode: oxidation; cathode: reduction) – Types: inert, membrane & metallic

• Charge carriers – Metal wire (usually wire with alligator clips)

• Where electrons pass through as current that be measured by a voltmeter

– Salt bridge • Maintains the electrical neutrality of the cell by transferring its cations

or anions • Usually made up of U-tube with an electrolyte soln that will not react

with other specie in the cell • Electrolyte is often incorporated into a paste or gel so that the

electrolyte solution does not pour out when U-tube is inverted. As oxidation & reduction proceed at the electrodes, ions from the salt bridge migrate to neutralize charge in the cell compartments. Whatever means is used to allow ions to migrate between half-cells, anions always migrate toward the anode and cations toward the cathode.

What happens in the electrodes?

What happens in the electrodes?

Types of Electrode • Inert electrode

– Usually made of C(graphite) or Pt (inert & nonreactive to components that undergo redox)

– Usually used when gases or liquids are formed as by-products of redox since the charge carrier cannot be connected to these

• Membrane electrode – Specialized; also called ion-selective electrode

– Detects electron transfer from or to a specific species

• Metallic electrode – Composed of metal strip & its metallic ion soln

– Not only act as sites of rxn but also participate in redox rxn

– Also called active electrodes

Cell emf

• Q: Why does electrons flow spontaneously in the way that they do?

• A: There is a “driving force” that pushes the electrons through an external circuit in a voltaic cell

• Comparison:

– Electron Flow to Flow of Water in Waterfall

Electron Flow to Flow of Water in Waterfall

• Water flows over the waterfalls because of its PE is lower at the bottom of the falls that at the top. Likewise, if there is an electrical connection between the anode & cathode of a voltaic cell, electrons flow from anode to cathode to lower their PE

Electromotive force, emf

• Electromotive = “causing electron motion”

• Potential difference between two electrodes of a voltaic cell which provides the driving force that pushes electron through the external circuit

Cell emf/cell potential

• Standard electrochemical cell potential; aka “cell voltage”

• Measured in volts

• E°cell = E°red(cathode) - E°red(anode)

• Standard condition: (with note sign)

– 1M conc for R & P in soln & 1 atm pressure for those that are gases at 25°C

Cell emf cont…

• Factors that cell emf depends on: – Specific rxns that occur at the cathode & anode

– Conc of reactants & products

– Temp (assumed to 25°C unless noted)

• Standard Reduction Potential (SRP), E°red – Standard electrode potential for reduction half rxn

• Standard Hydrogen Electrode, SHE – Reference half-rxn

– 2 H+(aq, 1M) + 2e- H2(g, 1atm) E°red = 0V

SHE

SRP

Points to remember on SRP

• SRPs are intensive properties or has intrinsic properties. Thus changing the stoichiometric coefficient in a half-rxn does not affect the value of SRP

– Ex: Zn2+(aq) + 2e- Zn(s) E°red = -0.76

2 Zn2+(aq) + 4e- 2 Zn(s) E°red = -0.76V

• The more positive the value of E°red, the greater the tendency for the reactant to be reduced, undergo reduction, & oxidize another specie

Practice Exercise

• Exercise 18.3 page 479 (refer to SRP page 478)

– Compute corresponding E°cell

– Indicate which rxn occurs in the anode & cathode

– Which electrode is consumed?

– Which is the OA? RA?

– Is the rxn spontaneous or not?

– Write the cell diagram

Spontaneity of Redox Rxn

• Determining std change in Gibb’s free energy of the rxn (G) makes it possible to determine the spontaneity of a rxn using this eqn:

G = -nFEcell where n = # of e- transferred during rxn

F = Faraday’s constant, 96500 C/mol

• G is (-) spontaneous; G is (+) nonspontaneous

Spontaneity cont…

• Voltaic cells use redox rxns that proceed spontaneously. Any rxn that can occur in a voltaic cell to produce a (+) emf must be spontaneous.

• General statement: – “A (+) value of E° indicates a spontaneous process

& a (-) value indicates a nonspontaneous one”

– E°= emf under std condition

– E= emf under nonstandard condition

Cell Diagram

• Conventional notation that shows the components of an electrochemical cell

• Conventions: (Std rule: ABC) – Anode = left side

– Cathode = right side

– Boundary • between diff phase (ex electrode & soln) = single vertical line (│)

• between half-cell compartment (salt bridge) = double vertical line (││)

• between diff species within the same soln = comma (,)

– Species in aqueous soln are placed on either side of the double vertical line

Practice Exercise

• Example 18.1 page 482

• Exercise 18.4 page 483

END OF PART 1