electrochemical cell, Lecture notes of Physical Chemistry

Download electrochemical cell and more Lecture notes Physical Chemistry in PDF only on Docsity!

Electrochemical cells

Topic 6C

Electrochemical cells

• Electrochemical cells consist of two electrodes in contact with an electrolyte
• Galvanic cell – electrochemical cell that produces electricity as a result of the spontaneous reaction
• Electrolytic cell – electrochemical cell in which a non-spontaneous reaction is driven by an external source
• Types of electrodes:

Redox reactions

• Consider a simple example of a strip of Zn metal placed in a solution of CuSO 4
• Electrons will spontaneously transfer from Zn to Cu 2+ to form Zn 2+ and Cu
• The shiny Zn surface turns dark brown as copper accumulates on the surface and the blue colour of the solution (due to the Cu 2+ ions) will fade as these are consumed

Redox reactions

• The electron transfer can be represented by two half reactions
• Oxidation half reaction:
• Reduction half reaction:

Electrochemical cells

• We can use an electrochemical cell to separate the two distinct half reactions
• This forces the electrons lost by the Zn(s) to flow through an external circuit before being acquired by the Cu 2+ ions
• Figure (a) – salt bridge provides an electrical connection through which the ions can migrate to balance the electrical charge
• Figure (b) – an ion permeable bridge allows ions to migrate between the two compartments

Reaction quotient

• We can express the composition of an electrode compartment in terms of the reaction quotient, Q , for the half reaction
• We can ignore the electrons in these equations when writing an expression for Q
• Example: The reduction of O 2 to H 2 O in acid solution: O 2 (g) + 4 H

(aq) + 4 e- → 2 H 2 O(l) jane.ferguson@smu.ca 8 Here we’re assuming the activity of water is 1, and the oxygen behaves as a perfect gas CHEM 2313

Electrochemical cells

• Galvanic cell
  • Cathode has higher potential than the anode
  • The species undergoing reduction withdraws electrons from the cathode, leaving a relative positive charge on it, which corresponds to the high potential
  • At the anode, oxidation results in the transfer of electrons to the electrode, giving it a relative negative change (low potential)
• Electrolytic cell
  • Requires an external source of electrical energy to force a chemical reaction
  • A voltage is applied between the two electrodes to make the redox reaction occur jane.ferguson@smu.ca 10 When a spontaneous reaction takes place in a galvanic cell, electrons are deposited on the anode and collected from the cathode, and there is a net flow of current which can be used to do work CHEM 2313

Varieties of electrochemical cells

• Electrolyte common to both electrodes
• Simplest type of electrochemical cell

Liquid junction potentials

• In a cell with two different electrolyte solutions in contact, there is an additional source of potential difference across the interface of the two electrolytes - Liquid junction potential, E lj
• All electrolyte concentration cells have a liquid junction, electrode concentration cells do not

Liquid junction potentials

• To minimize the contribution of the liquid junction to the potential difference, we can connect the electrolyte compartments using a salt bridge
• If the ions in the salt bridge have similar mobilities, then the liquid junction potentials at either end are largely independent of the concentrations of the two dilute solutions, and will nearly cancel jane.ferguson@smu.ca 17 The salt bridge is an inverted U- tube full of concentrated salt solution in a jelly CHEM 2313

Cell notation example

Cell notation example