Electrochemical Cells (College Board AP® Chemistry): Study Guide

Electrochemical cells

• Redox reactions involve simultaneous oxidation and reduction

  • Electrons flow from the species that is oxidized to the species that is reduced

• Which way electrons flow depends on the reactivity of the species involved

• Redox chemistry has very important applications in electrochemical cells, which come in two types:

  • Voltaic or galvanic cells

  • Electrolytic cells

Reduction and oxidation in cells

• Regardless of the type of cell:

  • Oxidation takes place at the anode

  • Reduction takes place at the cathode

• But depending on the type of cell, the polarity changes 

  • In voltaic cells, the anode is negative

  • In electrolytic cells, the anode is positive 

• Identify the anode and cathode by where oxidation and reduction happen, not by the sign of the electrode

Comparing galvanic and electrolytic cells

Galvanic (voltaic) & electrolytic cells

Voltaic cells

• A galvanic (voltaic) cell generates a potential difference called the cell potential, given the symbol E

• The potential of a single electrode cannot be measured on its own

  • A voltmeter measures the difference between two electrodes, which is the cell potential

• Galvanic (voltaic) cells generate electricity from a thermodynamically favored redox reaction, for example:

Zn (s) + CuSO₄ (aq) → Cu (s) + ZnSO₄ (aq)

• Instead of electrons being transferred directly from the zinc to the copper ions, a cell is built which separates the two redox processes

• Each part of the cell is called a half-cell

• If a rod of metal is dipped into a solution of its own ions, an equilibrium is set up

  • For example

Zn (s)  ⇌  Zn²⁺ (aq) + 2e^– 

Zinc metal in a solution of zinc sulfate

When a metal is dipped into a solution containing its ions, an equilibrium is established between the metal and its ions

• This is a half-cell and the strip of metal is an electrode

• The position of the equilibrium determines the potential difference between the metal strip and the solution of metal

• The Zn atoms on the rod can deposit two electrons on the rod and move into solution as Zn²⁺ ions:

                   Zn (s) ⇌ Zn²⁺ (aq) + 2e^– 

• This process would result in an accumulation of negative charge on the zinc rod

• Alternatively, the Zn²⁺ ions in solution could accept two electrons from the rod and move onto the rod to become Zn atoms:

                  Zn²⁺ (aq) + 2e^– ⇌ Zn (s)

• This process would result in an accumulation of positive charge on the zinc rod

• In both cases, a potential difference is set up between the rod and the solution

  • This is known as an electrode potential

• A similar electrode potential is set up if a copper rod is immersed in a solution containing copper ions (e.g. CuSO₄), due to the following processes:

Cu²⁺ (aq) + 2e^– ⇌ Cu (s)  – reduction (rod becomes positive)

Cu (s) ⇌ Cu²⁺ (aq) + 2e^– – oxidation (rod becomes negative)

• No net reaction takes place in a single half-cell

  • The equilibrium sets up a potential difference between the rod and the solution

Creating a cell potential

• If two different electrodes are connected, the potential difference between the two electrodes will cause a current to flow between them

  • Thus a cell potential is established and the system can generate electrical energy

• A typical electrochemical cell can be made by combining a zinc electrode in a solution of zinc sulfate with a copper electrode in a solution of copper sulfate

Electrochemical cell 

The zinc-copper voltaic cell (also known as the Daniell Cell)

• A salt bridge containing a strong electrolyte such as KNO₃ (aq) joins the two half-cells

• Its ions migrate to keep each half-cell electrically neutral

  • Cations (K⁺) move toward the cathode half-cell

  • Anions (NO₃⁻) move toward the anode half-cell

• Electrons never travel through the salt bridge

  • They travel only through the external wire, from the anode to the cathode

• The cell potential can be measured using a voltmeter

  • Voltmeters have a high resistance so that they do not divert much current from the main circuit

• The two half cells are said to be in series as the same current is flowing through both cells

• The combination of two electrodes in this way is known as a voltaic cell and can be used to generate electricity

• As the zinc-copper cell runs:

  • The zinc anode loses mass because Zn atoms become Zn²⁺ ions in the zinc sulfate solution

  • The copper cathode gains mass because Cu²⁺ ions from the copper(II) sulfate solution become Cu atoms

  • Electrons have almost no mass, so they do not carry mass from one electrode to the other

Conventional representation of cells

• Chemists use a type of shorthand convention to represent electrochemical cells

• In this convention:

  • A solid vertical (or slanted) line shows a phase boundary, which is an interface between a solid and a solution

  • A double vertical line (sometimes shown as dashed vertical lines) represents a salt bridge

    • A salt bridge has mobile ions that complete the circuit

    • Potassium chloride and potassium nitrate are commonly used to make the salt bridge as chlorides and nitrates are usually soluble

    • This should ensure that no precipitates form which can affect the equilibrium position of the half-cells

• The substance with the highest oxidation state in each half-cell is drawn next to the salt bridge

• The cell potential difference is shown with the polarity of the right-hand electrode

• The cell convention for the zinc and copper cell would be

Zn (s) ∣ Zn²⁺ (aq) ∥ Cu²⁺ (aq) ∣ Cu (s)   E _cell = +1.10 V

• This tells us the copper half-cell is more positive than the zinc half-cell so that electrons would flow from the zinc to the copper half-cell

• The same cell can be written as:

Cu (s) ∣ Cu²⁺ (aq) ∥ Zn²⁺ (aq) ∣ Zn (s)   E _cell = -1.10 V

• The polarity of the right-hand half-cell is negative, so we can still tell that electrons flow from the zinc to the copper half-cell