Electrochemical Cells (College Board AP® Chemistry): Study Guide
Electrochemical Cells
Redox reactions involve simultaneous oxidation and reduction as electrons flow from the reducing agent to the oxidising agent
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
Oxidation takes place at the anode and reduction takes place at the cathode regardless of the type of cell
But depending on the type of cell, the polarity changes
In voltaic cells, the anode is negative
In electrolytic cells, the anode is positive
Comparing Voltaic & Electrolytic Cells Summary Table
| Negative | Positive |
---|---|---|
Voltaic cell | anode | cathode |
oxidation | reduction | |
Electrolytic cell | cathode | anode |
reduction | oxidation |
Examiner Tips and Tricks
Students often confuse the redox processes that take place in voltaic cells and electrolytic cells
An easy way to remember is the phrase RED CAT and AN OX
REDuction takes place at the CAThode!
OXidation takes place at the ANode
Galvanic (Voltaic) & Electrolytic Cells
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
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 (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
Worked Example
Writing a cell diagram
If you connect an aluminium electrode to a zinc electrode, the voltmeter reads +0.94V and the aluminium is the negative.
Write the conventional cell diagram of the reaction.
Answer:
Al (s) ∣ Al3+ (aq) ∥ Zn2+ (aq) ∣ Zn (s) E cell = +0.94 V
It is also acceptable to include phase boundaries on the outside of cells as well:
∣ Al (s) ∣ Al3+ (aq) ∥ Zn2+ (aq) ∣ Zn (s) ∣ E cell = +0.94 V
Electrolytic cells
Electrolytic cells can be constructed using a beaker or crucible as the cell depending on whether the ionic compound is in solution or molten
Electrolysis of a molten ionic compound

Positive ions move to the negative electrode and negative ions move to the positive electrode
In electrolytic cells, the substance that the current passes through and splits up is called the electrolyte
The electrolyte contains positive and negative ions
What happens to these ions during electrolysis?
Negative ions move to the anode and lose electrons - this is oxidation
Positive ions move to the cathode and gain electrons - this is reduction
Electrically neutral atoms or molecules are released
Electrolysis of molten lead bromide
The reactions which take place at the electrodes can be shown by half equations
When the positive lead ions move to the cathode, they gain electrons in a reduction reaction:
Pb2+(aq) + 2e- ⇌ Pb (s)
Similarly, when the negative bromide ions move to the anode they lose electrons in an oxidation reaction:
2Br- (l) - 2e- ⇌ Br2 (l)
Sometimes oxidation reactions are written with '+2e-' on the right of the arrow instead of '-2e' on the left
In this case, the alternative half equation is:
2Br- (l) ⇌ Br2 (l) + 2e-
Since metals are always cations and non-metal are always anions, it is easy to predict the products of electrolysis of molten salts:
Metals will always be formed at the cathode and non-metals at the anode
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