chm 112 summer 2007 m. prushan chapter 18 electrochemistry
TRANSCRIPT
CHM 112 Summer 2007 M. Prushan
Chapter 18Electrochemistry
CHM 112 Summer 2007 M. Prushan
Oxidation–Reduction
Redox 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 reaction
The 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 K
nF
Eocell = standard cell potential
R is the gas constant (8.3145 J mol–1 K–1)
T is the Kelvin temperature
n is the number of moles of electrons involved in the reaction
F 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 …