When you compare cells from a table, find the more reactive metal in each pair, then compare the voltages under the same conditions. Finish with one sentence on what the table cannot decide.
This lesson is part of SPM Science electrochemistry. It uses the conduction idea from explaining current flow in an electrolyte.
How does a simple cell produce a voltage?
A simple cell has two different metals dipped in an electrolyte and joined by a wire. The more reactive metal loses electrons more readily, so it becomes the negative terminal.
Electrons flow through the wire from that metal to the less reactive one. The bigger the gap in reactivity, the larger the voltage.
Worked example: three pairs from one table
An invented class uses the same electrolyte and the same volume for each pair. Reactivity order, from most to least: magnesium, zinc, iron, copper.
| Pair | Voltage (V) | Negative terminal |
|---|---|---|
| Zinc and copper | 1.1 | Zinc |
| Iron and copper | 0.7 | Iron |
| Magnesium and copper | 2.0 | Magnesium |
Which pair gives the largest voltage? Magnesium and copper, at 2.0 V. Magnesium is furthest above copper in the series.
Why is iron the negative terminal in the second pair? Iron is more reactive than copper, so it loses electrons more readily.
What is the difference between the highest and lowest reading? 2.0 − 0.7 = 1.3 V.
What can the table not tell you?
The table only reports voltage at one moment, under the stated conditions. It says nothing about how long the cell keeps its voltage, how much the metals cost or whether the metals are safe to use.
A full comparison sentence therefore has two parts: “The magnesium and copper cell gives the largest voltage, 2.0 V, because magnesium is most reactive. The table does not show how long each cell lasts.”
The mistake that costs marks
The slip is to put the less reactive metal as the negative terminal, because the student assumes the “stronger-looking” metal, copper, gives the electrons.
| Step | Wrong | Right |
|---|---|---|
| Identify more reactive metal | Copper | Zinc |
| Negative terminal | Copper | Zinc |
| Electron flow in wire | Copper to zinc | Zinc to copper |
Check with the series. Reactive metals lose electrons. Metals nearer the bottom of the series hold on to them.
Check yourself
A table gives these readings with the same electrolyte: tin and copper, 0.5 V; zinc and tin, 0.6 V; zinc and copper, 1.1 V. Zinc is the most reactive of the three metals. Which is the least reactive, and what does the pattern of readings suggest?
Answer
Copper is the least reactive, because zinc is the negative terminal against every partner and tin becomes negative when paired with copper.
The readings also add up: 0.6 + 0.5 = 1.1 V. The voltage of the zinc and copper cell equals the two steps between them, which suggests tin sits between zinc and copper in reactivity.
What to study next
Continue to explaining applications without home experiment instructions, then attempt the electrochemistry practice set. The graph and evidence comparison lab gives more data to compare.
If you want a teacher to go through table questions with you, see online one-to-one Science tuition.