An electrochemical cell produces an electrical potential difference (measured as electromotive force, EMF) from chemical redox reactions. This guide walks you through the construction of metal-metal ion half-cells and measuring cell potentials under standard conditions.
Core Specification Link
This practical supports 3.1.11 Electrode potentials and electrochemical cells, emphasizing redox half-equations, cell notation, salt bridge function, and EMF calculations.
Schematic Diagram of an Electrochemical Cell
A typical zinc-copper cell (Daniel cell) uses a salt bridge to connect the two liquid half-cells electrically:
Aim
To measure the cell potential of electrochemical cells using zinc, copper, iron, and magnesium half-cells, and to compare the experimental values with calculated standard cell potentials (\(E^\circ_{\text{cell}}\)).
Equipment List
- High-resistance voltmeter (digital multimeter)
- Metal strips (electrodes): copper, zinc, iron, magnesium
- Sandpaper
- 1.0 mol dm⁻³ metal sulfate solutions: CuSO₄, ZnSO₄, FeSO₄, MgSO₄
- Beakers (100 cm³)
- Connecting leads with crocodile clips
- Filter paper strips
- Saturated potassium nitrate (KNO₃) solution (or sodium chloride, NaCl)
Experimental Method
- Clean the metal electrodes thoroughly with sandpaper to remove any oxide layer, which acts as an insulator.
- Set up the Cu²⁺/Cu half-cell: Pour 50 cm³ of 1.0 mol dm⁻³ copper(II) sulfate solution into a beaker and insert the copper electrode.
- Set up the Zn²⁺/Zn half-cell: Pour 50 cm³ of 1.0 mol dm⁻³ zinc sulfate solution into a separate beaker and insert the zinc electrode.
- Connect the copper electrode to the positive terminal of the voltmeter, and the zinc electrode to the negative terminal using the crocodile clips and wires.
- Prepare the salt bridge: Dip a strip of filter paper into saturated potassium nitrate solution until it is completely soaked.
- Place the salt bridge so that one end dips into the zinc sulfate solution and the other dips into the copper sulfate solution.
- Record the reading on the voltmeter as soon as the value stabilises . The reading should be close to +1.10 V.
- Repeat this procedure for other half-cell combinations, always preparing a fresh salt bridge for each run to prevent cross-contamination of ions.
The Role of the Salt Bridge
The salt bridge serves two vital functions in an electrochemical cell:
- Completes the electrical circuit: It allows the flow of charge (carried by ions) between the two half-cells.
- Maintains electrical neutrality: At the anode, metal atoms are oxidised, generating excess positive ions in solution. In contrast, at the cathode, metal ions are reduced, leaving an excess of negative spectator anions in solution. The salt bridge allows anions (e.g. NO₃⁻) to migrate to the anode beaker and cations (e.g. K⁺) to migrate to the cathode beaker to neutralise these charges. Without a salt bridge, charge build-up would instantly halt electron flow.
Predicted EMF vs Standard Electrode Potentials
The theoretical standard cell potential is calculated using standard electrode potentials (\(E^\circ\)):
\[ E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}} \]Alternatively: \(E^\circ_{\text{cell}} = E^\circ_{\text{more positive}} - E^\circ_{\text{more negative}}\)
| Cell System | Anode Half-Reaction | Cathode Half-Reaction | Theoretical \(E^\circ_{\text{cell}}\) / V |
|---|---|---|---|
| Zn / Cu | Zn(s) → Zn²⁺(aq) + 2e⁻ (\(E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\text{ V}\)) |
Cu²⁺(aq) + 2e⁻ → Cu(s) (\(E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\)) |
+0.34 − (−0.76) = +1.10 V |
| Fe / Cu | Fe(s) → Fe²⁺(aq) + 2e⁻ (\(E^\circ(\text{Fe}^{2+}/\text{Fe}) = -0.44\text{ V}\)) |
Cu²⁺(aq) + 2e⁻ → Cu(s) (\(E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\)) |
+0.34 − (−0.44) = +0.78 V |
| Mg / Cu | Mg(s) → Mg²⁺(aq) + 2e⁻ (\(E^\circ(\text{Mg}^{2+}/\text{Mg}) = -2.37\text{ V}\)) |
Cu²⁺(aq) + 2e⁻ → Cu(s) (\(E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\)) |
+0.34 − (−2.37) = +2.71 V |
- \( \text{Zn}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Zn}(\text{s}) \quad E^\circ = -0.76\text{ V} \)
- \( \text{Fe}^{2+}(\text{aq}) + 2\text{e}^- \rightleftharpoons \text{Fe}(\text{s}) \quad E^\circ = -0.44\text{ V} \)
2. Calculate the standard cell potential (\(E^\circ_{\text{cell}}\)).
3. Predict and explain the effect on the measured cell potential (EMF) if the concentration of zinc sulfate solution is decreased.
Step 1: Write the cell representation
The more negative half-cell (Zn²⁺/Zn) undergoes oxidation (anode) and is written on the left. The more positive half-cell (Fe²⁺/Fe) undergoes reduction (cathode) and is written on the right: \[ \text{Zn(s)} \mid \text{Zn}^{2+}(\text{aq}) \parallel \text{Fe}^{2+}(\text{aq}) \mid \text{Fe(s)} \]
Step 2: Calculate the standard cell potential
\[ E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}} \] \[ E^\circ_{\text{cell}} = -0.44\text{ V} - (-0.76\text{ V}) = +0.32\text{ V} \]Step 3: Analyse the effect of non-standard concentrations
The overall cell reaction is: \[ \text{Zn(s)} + \text{Fe}^{2+}(\text{aq}) \rightleftharpoons \text{Zn}^{2+}(\text{aq}) + \text{Fe(s)} \]
If the concentration of Zn²⁺(aq) is decreased, by Le Chatelier's principle, the position of equilibrium will shift to the right to increase the concentration of Zn²⁺(aq) ions. This makes the oxidation reaction at the anode more energetically favourable, increasing the potential difference across the cell. Therefore, the measured cell potential (EMF) will increase (become more positive than +0.32 V).
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Copper sulfate (CuSO₄) | Harmful if swallowed; causes skin and serious eye irritation. Toxic to aquatic life. | Wear safety goggles and gloves. Dispose of waste in a designated heavy metal collection bottle. |
| Zinc sulfate (ZnSO₄) | Corrosive; causes serious eye damage. | Wear safety goggles and handle with care. Wash skin immediately if contact occurs. |
| Potassium nitrate (KNO₃) | Oxidising agent, skin irritant. | Keep saturated solution away from heat and flammable substances. Wear gloves. |
Sources of Error & Improvements
- High internal resistance/voltmeter choice: A standard low-resistance voltmeter draws current, which leads to a potential drop across the internal resistance of the cell, giving an EMF value lower than the true value. Improvement: Always use a high-resistance digital voltmeter (typically >10 MΩ), which draws negligible current.
- Oxide layers on metal surfaces: Oxidation products (e.g. magnesium oxide) insulate the electrode, disrupting standard electron transfer. Improvement: Clean all metal strips with emery paper or sandpaper immediately before inserting them into the solution.
- Non-standard conditions: The measured EMF will deviate from the standard electrode potentials if the temperature is not 298 K (25 °C) or if the metal ion solutions are not exactly 1.0 mol dm⁻³. Improvement: Control the room temperature using a water bath and use precisely prepared standard solutions.
Exam questions, marked instantly
Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.
Exam-style question
2 marksExplain why potassium chloride should not be used in the salt bridge when one half-cell contains silver ions.
Show Mark Scheme
- Chloride ions react with silver ions to form a precipitate of silver chloride [1]
- This changes the silver ion concentration (and so the electrode potential) or blocks the salt bridge; potassium nitrate is used because its ions do not react [1]
Exam-style question
3 marksWater is added to the zinc half-cell of a zinc–copper cell. Predict the effect on the cell EMF and explain your answer.
Show Mark Scheme
- The concentration of Zn²⁺ ions decreases [1]
- The equilibrium Zn²⁺(aq) + 2e⁻ ⇌ Zn(s) shifts to the left, so the zinc electrode potential becomes more negative [1]
- So the EMF of the cell increases [1]
CPAC Skills Assessed
- CPAC 2: Safely constructs and compiles electrochemical systems from discrete components.
- CPAC 3: Safely uses toxic heavy metal salt solutions.
- CPAC 4: Accurately records steady-state potential readings from digital multimeters.
In cell notation, state symbols must always be included for every component. Make sure you place the oxidised species closest to the salt bridge: Metal | Metal Ion || Metal Ion | Metal.