CHM 112 Summer 2007 M. Prushan

Chapter 18Electrochemistry

CHM 112 Summer 2007 M. Prushan

Oxidation–ReductionRedox involves the transfer of electrons

CHM 112 Summer 2007 M. Prushan

Half-Reactions

In any oxidation–reduction reaction, there are two half-reactions

Oxidation takes place when a species loses electrons to another species

Cu(s) Cu2+(aq) + e–

EOS

Reduction takes place when a species gains electrons from another species

Zn2+(aq) + e– Zn(s)

CHM 112 Summer 2007 M. Prushan

Voltaic CellsA half-cell consists of an electrode immersed in a solution of ions

CHM 112 Summer 2007 M. Prushan

An Illustration …

CHM 112 Summer 2007 M. Prushan

The solutions in the two half-cells are joined by a salt bridge

Voltaic Cells

CHM 112 Summer 2007 M. Prushan

Metal wires connect the electrodes to the terminals of an electric meter called a voltmeter

Voltaic CellsThe salt bridge keeps the two half-cells in contact with one another so that there can be a flow of electrons

CHM 112 Summer 2007 M. Prushan

Important Electrochemical TermsAn electrochemical cell is a device that combines two half-cells with the appropriate connections between electrodes and solutions

A voltaic cell is an electrochemical cell in which electric current is generated from a spontaneous redox reactionThe anode is the electrode at which oxidation occurs and the cathode is the electrode at which reduction occurs

The cell potential (Ecell) is the potential difference that propels electrons from the anode to the cathode

CHM 112 Summer 2007 M. Prushan

Cell Diagrams – Conventions

Place the anode on the left side of the diagram

Place the cathode on the right side of the diagram

Use a single vertical line ( | ) to represent the boundary between different phases, such as between an electrode and a solution

Use a double vertical line ( || ) to represent a salt bridge or porous barrier separating two half-cells

CHM 112 Summer 2007 M. Prushan

An Example Cell Diagram

CHM 112 Summer 2007 M. Prushan

Criteria for SpontaneousChange

If Ecell is positive, the reaction in the forward direction (from left to right) is spontaneous

If Ecell is negative, the reaction is nonspontaneous

If Ecell = 0, the system is at equilibrium

When a cell reaction is reversed, Ecell and G change signs

CHM 112 Summer 2007 M. Prushan

Go = –RTlnKeq = –n × F × Eocell

oeqcell lnRTE KnF

Eocell = standard cell potential

R is the gas constant (8.3145 J mol–1 K–1)T is the Kelvin temperaturen is the number of moles of electrons involved in the reactionF is the faraday constant

Equilibrium Constantsfor Redox Reactions

CHM 112 Summer 2007 M. Prushan

General Values and Meanings

Reaction behavior can be predicted using the following information …