Electrochemical Cells (College Board AP Chemistry)

• 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 (eg CuSO₄), due to the following processes:

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

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

• Note that a chemical reaction is not taking place – there is simply a potential difference between the rod and the solution

Creating an EMF

• If two different electrodes are connected, the potential difference between the two electrodes will cause a current to flow between them
  • Thus an electromotive force (EMF) 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)

• The circuit must be completed by allowing ions to flow from one solution to the other
• This is achieved using a salt bridge
  • This is often a piece of filter paper saturated with a solution of an inert electrolyte such as KNO₃ (aq)
• The EMF 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

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