6.2 Chemical Cell
A chemical cell is an electrochemical device designed to convert chemical energy directly into electrical energy. It typically consists of two different metals immersed in an electrolyte and linked through an external circuit.
Learning Objectives
- Define a simple chemical cell and identify its essential components.
- Determine the positive terminal and negative terminal of a simple chemical cell using the electrochemical series.
- Explain the chemical reactions, electron flow, and energy conversion occurring in a simple chemical cell.
- Describe how alternative sources such as fruits, plant parts, and seawater can be used to generate electrical energy.
Simple Chemical Cell Mechanism and the Electrochemical Series
A magnesium-copper cell converts chemical energy into electrical energy as electrons flow from the more electropositive metal to the less electropositive metal.
Short Explanation
What is a simple chemical cell?
A chemical cell is a device that converts chemical energy into electrical energy. It requires two different metals immersed in an electrolyte.
The metal positioned higher in the electrochemical series is more electropositive, readily donates electrons, and functions as the negative terminal. The metal positioned lower serves as the positive terminal.
Electron flow. Electrons flow through an external wire from the negative terminal to the positive terminal, creating an electric current. Meanwhile, cations in the electrolyte that are less electropositive receive these electrons at the positive electrode surface.
Identical metals produce zero voltage. If identical metals are used, no potential difference is generated, resulting in a voltmeter reading of 0 V.
Structure of a Simple Chemical Cell
A simple chemical cell consists of two different metals, an electrolyte, and an external circuit.
A simple chemical cell consists of:
Two different metals
These act as electrodes. For example, magnesium and copper.
An electrolyte
An ionic solution such as copper(II) sulphate solution, CuSO₄, or sodium chloride solution, NaCl.
An external circuit
Connecting wires and a voltmeter or switch complete the circuit.
The Electrochemical Series and Determining Terminals
The more electropositive metal is the negative terminal; the less electropositive metal is the positive terminal.
The electrochemical series arranges ions in order of increasing electropositivity from bottom to top:
| Position | Ion | Electropositivity |
|---|---|---|
| Top | K⁺ | Most electropositive |
| Na⁺ | ||
| Ca²⁺ | ||
| Mg²⁺ | ||
| Al³⁺ | ||
| Zn²⁺ | ||
| Fe²⁺ | ||
| Sn²⁺ | ||
| Pb²⁺ | ||
| H⁺ | ||
| Cu²⁺ | ||
| Bottom | Ag⁺ | Least electropositive |
Negative terminal: The more electropositive metal (higher in the series) has a greater tendency to donate electrons.
Positive terminal: The less electropositive metal (lower in the series).
Reactions in a Magnesium-Copper Chemical Cell
Magnesium donates electrons at the negative terminal; copper(II) ions receive electrons at the positive terminal.
At the negative terminal (Magnesium, Mg)
Magnesium is more electropositive than copper. Magnesium donates electrons, forms magnesium ions (Mg²⁺), and dissolves into the electrolyte.
Electron flow
Released electrons flow through the external circuit from the negative terminal (magnesium) to the positive terminal (copper). This electron flow generates electrical energy.
At the positive terminal (Copper, Cu)
Electrons arriving at the copper strip are accepted by copper(II) ions (Cu²⁺) from the electrolyte because Cu²⁺ is less electropositive than H⁺ ions. Solid copper is formed and deposited onto the copper strip.
Energy conversion
Chemical energy → Electrical energy.
Activity 6.2: Building a Simple Chemical Cell
Different metals produce a voltage reading; identical metals produce 0 V.
Materials & Apparatus: Sandpaper, two magnesium ribbons, two copper strips, 1.0 mol dm⁻³ sodium chloride solution (NaCl), measuring cylinder, beaker, connecting wires with crocodile clips, switch, voltmeter.
Procedure
- Clean the metal strips with sandpaper.
- Measure and pour 150 cm³ of 1.0 mol dm⁻³ NaCl solution into a beaker.
- Immerse a magnesium ribbon and a copper strip into the solution.
- Connect them to a voltmeter using connecting wires.
- Close the switch and record the voltage reading.
- Repeat by using pairs of the same metals: magnesium–magnesium and copper–copper.
Observations / Concept: A cell with different metals (e.g., magnesium–copper) produces a voltage reading because of the difference in electropositivity. Pairs of identical metals (magnesium–magnesium or copper–copper) produce a reading of 0 V because there is no difference in electropositivity.
Activity 6.3: Generating Electrical Energy from Alternative Sources
Fruits, plant parts, and seawater can act as electrolytes in simple chemical cells.
Simple chemical cells can be built using fruits, other plant parts, or seawater as the electrolyte. Chemical energy from these natural sources is converted into electrical energy.
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the energy conversion that occurs in a simple chemical cell?
Between zinc and copper, which metal acts as the negative terminal in a simple chemical cell, and why?
Why does a cell constructed using two copper strips immersed in sodium chloride solution give a voltmeter reading of 0 V?
Common Mistakes
- Confusing the terminals of a chemical cell with an electrolytic cell: in a simple chemical cell, the more electropositive metal is the negative terminal, whereas in electrolysis, the anode connected to the battery's positive terminal is positive.
- Assuming electrons flow from the positive terminal to the negative terminal; electrons always flow through the external circuit from the negative terminal to the positive terminal.
- Believing any two pieces of metal will generate a voltage; if both pieces of metal are identical, no voltage is produced (0 V).
Concept Misunderstandings
An external electrical power source like a dry cell or battery is required to operate a simple chemical cell.
A chemical cell generates its own electricity through spontaneous chemical reactions between the two different metals and the electrolyte; it does not require a power source.
The metal lower in the electrochemical series donates electrons.
The metal higher in the electrochemical series is more electropositive and therefore donates electrons more readily.
Summary
- A simple chemical cell consists of two different metals immersed in an electrolyte.
- It converts chemical energy to electrical energy.
- The more electropositive metal serves as the negative terminal (donates electrons).
- The less electropositive metal serves as the positive terminal.
- Electrons flow through the external circuit from the negative terminal to the positive terminal.
- Pairs of identical metals produce 0 V.
- Electrolytes can be sourced from conventional salt solutions or natural materials such as fruits, plant parts, and seawater.
Short Activity
A. Objective Quiz
1 In a simple chemical cell using magnesium and copper electrodes immersed in copper(II) sulphate solution, which statement correctly explains the role of magnesium?
2 What reading will be observed on the voltmeter if two identical copper strips are immersed in 1.0 mol dm⁻³ sodium chloride solution?
B. Fill in the Blanks
3 In a simple chemical cell, the energy conversion that takes place is from chemical energy to energy.
4 In the external circuit of a chemical cell, electrons flow from the negative terminal to the terminal.
C. Drag and Drop: Match the Component or Observation to Its Function or Description
Drag each component or observation to its correct function or description. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Component or Observation to Its Function or Description
Write the correct component or observation next to each function or description.
| Function / Description | Component / Observation |
|---|---|
| 1. The more electropositive metal that donates electrons and dissolves as ions. | |
| 2. The less electropositive metal towards which electrons flow in the external circuit. | |
| 3. Results in a voltmeter reading of zero because there is no difference in electropositivity. | |
| 4. Alternative natural source that can act as an electrolyte to generate electricity. |
Answer choices: Negative terminal • Positive terminal • Pair of identical metals • Seawater or fruit juice
