How can a chemical reaction produce electricity?
Electrochemical cells: how two different metals in an electrolyte produce a voltage, the role of the ion bridge, and using the electrochemical series to predict the direction of electron flow.
An SQA National 5 Chemistry answer on electrochemical cells, covering how two different metals in an electrolyte produce a voltage, the role of the ion bridge, the direction of electron flow, and using the electrochemical series to predict and explain the cell.
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What this key area is asking
The SQA wants you to describe how an electrochemical cell turns a chemical reaction into electricity, explain the role of the ion bridge, state the direction of electron flow, and use the electrochemical series to predict and explain the cell. It builds directly on oxidation and reduction from the metals key area.
How a cell produces electricity
Electron flow and the electrochemical series
So in a zinc-copper cell, zinc is higher than copper, so the zinc is oxidised and electrons flow from zinc to copper:
The role of the ion bridge
The size of the voltage
The further apart the two metals are in the electrochemical series, the larger the voltage the cell produces. A magnesium-copper cell gives a bigger voltage than a zinc-copper cell, because magnesium and copper are further apart in the series than zinc and copper.
Worked example: predicting a cell
Examples in context
Electrochemical cells are the chemistry of batteries, from the cell in a torch to the lithium cells in phones and electric cars, all of which turn a redox reaction into a useful current. A simple version is the "lemon battery", where two different metals pushed into a lemon (the electrolyte) produce enough voltage to light an LED. The same idea, run in reverse with an external supply, is electrolysis, which is used to extract very reactive metals, the subject of the next key area.
Try this
Q1. State what flows through the wires and what flows through the ion bridge. [2 marks]
- Cue. Electrons flow through the wires; ions flow through the ion bridge.
Q2. In a zinc-copper cell, which metal is oxidised? [1 mark]
- Cue. Zinc, because it is higher in the electrochemical series.
Q3. Explain why a magnesium-copper cell gives a larger voltage than a zinc-copper cell. [2 marks]
- Cue. Magnesium and copper are further apart in the electrochemical series than zinc and copper, so the voltage is larger.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 2019 style3 marksA cell is made by connecting a piece of zinc and a piece of copper, each in a solution of its own ions, joined by an ion bridge. Using the electrochemical series, state the direction of electron flow through the wires and explain why electrons flow that way.Show worked answer →
Markers reward the correct direction and the reasoning from the electrochemical series.
Electrons flow through the wires from the zinc to the copper.
Zinc is higher in the electrochemical series than copper, so zinc loses electrons more readily. The zinc atoms are oxidised, releasing electrons, which travel through the connecting wires to the copper. There the electrons are gained by copper ions, which are reduced to copper metal. Electrons always flow from the metal higher in the electrochemical series to the one lower down.
SQA N5 2021 style3 marksState the purpose of the ion bridge in an electrochemical cell, name the type of reaction taking place overall, and state what is used to measure the voltage produced.Show worked answer →
A 3 mark answer needs the role of the ion bridge, the reaction type, and the measuring device.
The ion bridge completes the circuit by allowing ions to move between the two solutions, which keeps the charges balanced so the cell can keep working.
The overall reaction is a redox reaction: oxidation happens at one metal and reduction at the other, with electrons transferred between them.
The voltage produced is measured with a voltmeter connected across the cell.
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Sources & how we know this
- SQA National 5 Chemistry Course Specification — SQA (2019)