Voltaic cells
- A voltaic cell generates a potential difference known as an electromotive force or EMF
- The EMF is also called the cell potential and is given the symbol E
- The absolute value of a cell potential cannot be determined, only the difference between one cell and another
- This is analogous to arm-wrestling: you cannot determine the strength of an arm-wrestler unless you compare them to the other competitors
- Voltaic (or Galvanic) cells generate electricity from spontaneous redox reactions, e.g.
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
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–
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- 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)
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- 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