Electrochemical cells and the Daniell cell
A galvanic cell turns the chemical energy of a spontaneous redox reaction into electricity. In the Daniell cell, zinc is oxidised, copper ions are reduced, and E°cell = 1.10 V.
Two kinds of electrochemical cells
Electrochemistry studies how chemical energy and electrical energy change into each other. A galvanic (voltaic) cell uses a spontaneous redox reaction to produce electricity, as in a torch cell. An electrolytic cell uses electricity from outside to drive a non-spontaneous reaction, as in electroplating. In every cell, oxidation happens at the anode and reduction at the cathode.
How the Daniell cell works
- 1A zinc rod dips in ZnSO₄ solution and a copper rod dips in CuSO₄ solution. Each rod with its solution is a half-cell, also called a redox couple.
- 2Anode (negative), oxidation: Zn(s) → Zn²⁺(aq) + 2e⁻
- 3Electrons flow through the outer wire from zinc to copper. Conventional current flows the other way, from copper to zinc.
- 4Cathode (positive), reduction: Cu²⁺(aq) + 2e⁻ → Cu(s)
- 5The salt bridge, a U-tube of KCl or NH₄NO₃ set in agar-agar jelly, lets ions move between the solutions. It completes the inner circuit and keeps each solution electrically neutral.
- 6★Overall: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). With 1 M solutions the cell gives 1.10 V.
★ key step
How to draw the Daniell cell
- Draw two beakers side by side. Left: a zinc rod in ZnSO₄(aq). Right: a copper rod in CuSO₄(aq).
- Join the two solutions with an inverted U-tube labelled salt bridge (KCl in agar-agar).
- Connect the two rods with a wire through a voltmeter.
- Label zinc anode (−) and copper cathode (+).
- Draw an arrow along the wire for electron flow from Zn to Cu, and an opposite arrow for current.
- Write each half-reaction under its beaker.
Writing a cell in short
Write the anode on the left and the cathode on the right. A single vertical line | marks a boundary between two phases, such as a metal and its solution. A double vertical line || marks the salt bridge. So the Daniell cell is Zn(s) | Zn²⁺(aq, 1 M) || Cu²⁺(aq, 1 M) | Cu(s). The cell potential is the potential difference between the two electrodes. When no current is drawn, it is called the EMF (electromotive force): E_cell = E_right − E_left, using reduction potentials.
The Daniell cell against an outside voltage
| Opposing voltage applied from outside | What happens |
|---|---|
| Less than 1.1 V | Electrons flow from Zn to Cu. Zinc dissolves and copper deposits: the cell works as a galvanic cell. |
| Exactly 1.1 V | No current flows, and the reaction stops. |
| More than 1.1 V | Electrons flow from Cu to Zn. Zinc is deposited and copper dissolves: the cell now works as an electrolytic cell. |
Galvanic cell vs electrolytic cell
In both, oxidation is at the anode and reduction at the cathode: An Ox, Red Cat.
The cell in a TV remote or a wall clock is a galvanic cell. A school-fair lemon battery, with a zinc nail and a copper coin pushed into a lemon, works on the same idea as the Daniell cell.
1 marks: “Write the cell notation for the Daniell cell.” Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s)
2 marks: “Give two functions of the salt bridge in a galvanic cell.” (i) It completes the inner circuit by letting ions move between the two solutions. (ii) It keeps both solutions electrically neutral, so the cell keeps working.
3 marks: “What happens in a Daniell cell when an outside opposing voltage is (i) less than, (ii) equal to and (iii) greater than 1.1 V?” (i) Galvanic: zinc dissolves, copper deposits, electrons flow from Zn to Cu. (ii) No current flows. (iii) Electrolytic: zinc deposits, copper dissolves, electrons flow from Cu to Zn.
An Ox, Red Cat: Anode oxidation, Reduction cathode. In a galvanic cell the anode sits on the Left and is Negative.