Voltaic and electrochemical cells

This page covers one Form 5 content standard from Redox Equilibrium: voltaic and electrochemical cells. This is the mirror image of electrolysis. In electrolysis, electricity forces a redox reaction; in a voltaic cell, a spontaneous redox reaction produces electricity. Get the direction of electron flow and the terminal labels right and this becomes one of the most reliable topics in the chapter.

What a voltaic cell is

A voltaic cell (also called a chemical cell or galvanic cell) converts chemical energy into electrical energy using a spontaneous redox reaction. The simplest version is two different metals dipped as electrodes into an electrolyte and connected by a wire through a bulb or voltmeter. Because the two metals differ in reactivity, one is more willing to give up electrons than the other, and that difference drives a current.

A more advanced version, the Daniell cell, uses two half-cells, zinc in zinc(II) sulfate and copper in copper(II) sulfate, joined by a salt bridge that completes the circuit while keeping the solutions from mixing.

Terminals, electron flow and half-equations

The rules follow directly from the reactivity series:

• The more reactive metal loses electrons more readily, so it is oxidised and becomes the negative terminal (the anode of the cell).
• The less reactive metal is the positive terminal (the cathode), where reduction happens.
• Electrons flow through the external wire from the negative terminal (more reactive metal) to the positive terminal (less reactive metal). Conventional current flows the opposite way.

For a magnesium-copper cell in copper(II) sulfate, for example:

• Negative terminal (magnesium, oxidised): Mg → Mg²⁺ + 2e-
• Positive terminal (copper, reduction): Cu²⁺ + 2e- → Cu

Magnesium dissolves and the copper electrode gains a coating of copper, exactly as in a displacement reaction, because a voltaic cell is a displacement reaction with the electron transfer routed through a wire.

Cell voltage and the electrochemical series

The voltage (potential difference) of a simple cell depends on how far apart the two metals are in reactivity: the greater the gap, the higher the voltage. So a magnesium-copper cell gives a higher voltage than a zinc-copper cell, because magnesium and copper are further apart than zinc and copper. By measuring the voltage of many pairs, chemists arrange metals into the electrochemical series, essentially the reactivity series ordered by electrode potential. The metal that is always oxidised (the negative terminal) is the more reactive one, higher in the series.

Voltaic cell versus electrolytic cell

Keep the two kinds of cell clearly apart, because the exam often pairs them. A voltaic cell contains two different metals and needs no external power, the spontaneous reaction produces electricity, and its negative terminal is the site of oxidation. An electrolytic cell contains electrodes connected to an external power supply that forces a non-spontaneous reaction; its cathode (negative electrode) is the site of reduction. The energy change is opposite too: a voltaic cell turns chemical energy into electrical energy, while an electrolytic cell turns electrical energy into chemical energy. Being able to state these differences cleanly is a frequent short-answer question.

Worked example

Question. A simple cell is made from a magnesium strip and a copper strip in dilute sulfuric acid, connected through a voltmeter. State which strip is the negative terminal, give the direction of electron flow, write the half-equation at each electrode, and say which metal is oxidised.

Step 1, compare reactivity. Magnesium is more reactive than copper, so magnesium releases electrons more readily.

Step 2, assign terminals. The more reactive metal is the negative terminal, so magnesium is the negative terminal and copper is the positive terminal.

Step 3, direction of electron flow. Electrons flow through the external wire from the magnesium (negative) to the copper (positive).

Step 4, half-equations.

• Negative terminal (magnesium): Mg → Mg²⁺ + 2e- (loss of electrons = oxidation).
• Positive terminal (copper): here hydrogen ions from the acid are reduced, 2H⁺ + 2e- → H₂ (gas bubbles at the copper).

Step 5, which is oxidised. Magnesium loses electrons, so magnesium is oxidised (it is the reducing agent and the negative terminal).

Answer. Magnesium is the negative terminal; electrons flow from magnesium to copper through the wire; Mg → Mg²⁺ + 2e- at magnesium and 2H⁺ + 2e- → H₂ at copper; magnesium is oxidised.

Practice question

Two simple cells are set up in the same electrolyte: cell 1 uses zinc and copper; cell 2 uses magnesium and copper. Predict which cell produces the higher voltage and explain why. In each cell, which metal is the negative terminal?

Answer. Cell 2 (magnesium and copper) produces the higher voltage, because magnesium and copper are further apart in the reactivity series than zinc and copper, and a bigger reactivity difference gives a bigger voltage. In both cells the more reactive metal is the negative terminal, zinc in cell 1, magnesium in cell 2.

Exam tip

Fix one sentence in your memory: the more reactive metal is the negative terminal, is oxidised, and is where electrons flow FROM. Everything else follows. Do not confuse a voltaic cell with electrolysis: in a voltaic cell the negative terminal is where oxidation happens, which is the opposite of the negative electrode (cathode) in electrolysis where reduction happens, a classic SPM Chemistry trap. For voltage-comparison questions, always justify with the reactivity gap, not just “magnesium is reactive”. Draw the electron-flow arrow on any cell diagram; it earns marks and stops careless direction errors.

Where this fits in the chapter

The voltaic cell ties together displacement, the reactivity series and the idea of electron transfer at a distance. The final standard, the reactivity series and metal extraction, uses the same ordering of metals you build here.

• Up to the chapter hub: Redox Equilibrium.
• Sideways: the chapter’s worked examples and the key terms list for cell definitions.

Our online 1-to-1 SPM Chemistry teachers, teaching in English from RM50 per hour, focus on the one distinction students most often reverse, the negative terminal being the site of oxidation in a voltaic cell, so your cell answers stay consistent under exam pressure.