# 7.2 Simple Chemical Cells

F.5 Chemistry · Topic VII Redox Reactions, Chemical Cells and Electrolysis · Lesson 2

Two metals and an electrolyte make electricity; the E.C.S. of metals; salt bridge and porous pot.

## Figure

Two half cells joined by a salt bridge: electrons go through the wire from Mg to Cu; ions move through the solutions and the bridge.

## Key points

- **Chemical cell ／ 化學電池**: **chemical → electrical** energy; the electrons lost by one metal flow through the **external circuit** to the other electrode.
- **Negative electrode (anode ／ 陽極)** = the metal **higher** in the E.C.S.: it **loses electrons** (oxidation) and dissolves. **Positive electrode (cathode ／ 陰極)** = the lower metal: ions **gain electrons** (reduction) on its surface.
- **E.C.S. ／ 電化序**: metals in order of their tendency to **lose electrons in aqueous solution**: K, Ca, Na, Mg, Al, Zn, Fe, Pb, Cu, Ag, Au. **Ca above Na**, unlike the reactivity series.
- **Voltage** rises with the **E.C.S. gap** between the two metals; a **negative** reading means the metal is **below copper**, so electrons flow from copper to it.
- **Half cells ／ 半電池** (each metal in a solution of its own ions) joined by a **salt bridge** stop the reactive metal reacting directly with Cu^2+^(aq), which coats it and makes the voltage drop quickly.
- **Salt bridge ／ 鹽橋**: filter paper soaked in a saturated electrolyte that does **not react** with the half cells (KNO~3~(aq)); it **completes the circuit** and **supplies ions to balance the charges** (Mg^2+^ into the bridge or NO~3~^−^ out of it; SO~4~^2−^ in or K^+^ out).
- **Daniell cell**: Zn | ZnSO~4~(aq) and Cu | CuSO~4~(aq) with a **porous pot ／ 多孔瓶**, about **1.1 V**; the pot keeps the solutions apart and lets ions through but supplies **no** ions of its own.
- **Electrons** move only in the wires and electrodes; **ions** move in the electrolytes and the salt bridge. A **non-electrolyte** (ethanol, sugar solution) gives almost no voltage.

## Must know

| Idea | Key point | Example |
|---|---|---|
| **Chemical cell** | Chemical → electrical energy; electrons from one metal flow through the external circuit to the other electrode | Zn and Cu in CuSO~4~(aq) lighting an LED |
| **Negative electrode (anode)** | The metal higher in the E.C.S.: loses electrons (oxidation), dissolves, becomes lighter | Mg(s) → Mg^2+^(aq) + 2e^−^ |
| **Positive electrode (cathode)** | The metal lower in the E.C.S.: ions in solution gain electrons (reduction) on its surface | Cu^2+^(aq) + 2e^−^ → Cu(s); 2H^+^(aq) + 2e^−^ → H~2~(g) |
| **Voltage** | Farther apart in the E.C.S. = higher; negative reading = metal below copper | Mg/Cu +1.41 V, Zn/Cu +0.79 V, Ag/Cu −0.17 V |
| **Salt bridge** | Saturated electrolyte that does not react with the half cells; completes the circuit and supplies ions to balance charges | KNO~3~(aq) |
| **Porous pot** | Keeps the solutions apart, lets ions pass, supplies no ions | Daniell cell, about 1.1 V |

## Common mistakes

- ✗ "The salt bridge lets electrons flow from one half cell to the other." ✓ Electrons flow only through the **wires and electrodes**; **ions** carry the charge through the solutions and the salt bridge.
- ✗ A NaCl(aq) salt bridge for a Pb/Pb^2+^ half cell. ✓ Cl^−^ gives a white precipitate, Pb^2+^(aq) + 2Cl^−^(aq) → PbCl~2~(s), so the bridge cannot supply ions. No Cl^−^ with Ag^+^ or Pb^2+^, no SO~4~^2−^ with Pb^2+^; **KNO~3~(aq)** is the safe choice.
- ✗ "Ethanol or sugar solution can be the electrolyte." ✓ A **non-electrolyte** has almost no mobile ions: no current, almost no voltage.
- ✗ "Sodium is above calcium in the E.C.S.: it reacts more vigorously with water." ✓ In **cell reactions** calcium loses electrons more readily, so **Ca is above Na** in the E.C.S.
- ✗ "In a Mg/Cu cell, oxidation takes place at the copper electrode." ✓ Copper is the lower metal here: Cu^2+^ ions are **reduced** on it. **Oxidation** is at the more reactive metal, the **negative electrode (anode)**.

## Quick check

1. A zinc strip and a copper strip in copper(II) sulphate solution light an LED. Write the half equation at each strip and state which strip loses mass.
2. In lemons, Mg/Cu gives +1.41 V and Zn/Cu +0.79 V. Which of Mg and Zn loses electrons more readily? What sign would an Ag/Cu lemon cell show, and why?
3. An iron strip in FeSO~4~(aq) and a silver strip in AgNO~3~(aq) are joined by a salt bridge. Why must the bridge not be soaked in NaCl(aq)? Suggest a suitable solution and give the direction of electron flow.
4. State the two functions of a salt bridge. What does a porous pot do, and what does it not do?
5. In a Daniell cell, what happens to the mass of each electrode and to the colour of the copper(II) sulphate solution?

## Answers

1. Zn(s) → Zn^2+^(aq) + 2e^−^; Cu^2+^(aq) + 2e^−^ → Cu(s). The zinc strip loses mass (copper is deposited on the copper strip).
2. Mg: the Mg/Cu voltage is higher, so Mg and Cu are farther apart in the E.C.S. Ag/Cu is negative (−0.17 V): copper loses electrons more readily than silver, so electrons flow from Cu to Ag.
3. Cl^−^ ions form a precipitate with Ag^+^: Ag^+^(aq) + Cl^−^(aq) → AgCl(s). Use KNO~3~(aq). Electrons flow from iron to silver.
4. A salt bridge completes the circuit by letting ions move between the half cells, and supplies ions to balance the charges. A porous pot keeps the solutions apart and lets the cell's ions pass through, but supplies no ions of its own.
5. The zinc electrode becomes smaller (Zn → Zn^2+^ + 2e^−^); the copper electrode becomes larger, and the blue solution becomes paler as Cu^2+^ ions are used up.

## Did you know?

The first electric battery was built by the Italian scientist Alessandro Volta in 1800: a pile of zinc and copper (or silver) discs separated by cloth soaked in salt water. The unit of voltage, the volt, is named after him.