1. Principles of Electrochemical Cells
Required Practical 8 explores how chemical energy is converted into electrical energy in galvanic cells. When two different redox half-cells are coupled, a potential difference (electromotive force, EMF) arises from the difference in electron-releasing ability (standard electrode potential, E°) between the two couples.
- Anode (Negative Electrode): Zinc undergoes oxidation →
Zn(s) → Zn2+(aq) + 2e- (E° = -0.76 V).- Cathode (Positive Electrode): Copper(II) undergoes reduction →
Cu2+(aq) + 2e- → Cu(s) (E° = +0.34 V).- Standard EMF:
E°_cell = E°(cathode) - E°(anode) = +0.34 - (-0.76) = +1.10 V.
2. Half-Cell Construction & Electrode Preparation
Accurate EMF measurement requires careful physical preparation of metal electrodes and standard solutions:
- Cleaning Metal Strips: Clean strips of zinc, copper, and iron with emery paper or sandpaper to scour off the oxide coating (e.g. ZnO, CuO). Rinse with deionised water and dry with a paper towel.
- Half-Cell Solutions: Measure 50 cm3 of 1.00 mol dm^-3 ZnSO4(aq) into one beaker, and 50 cm3 of 1.00 mol dm^-3 CuSO4(aq) into a second beaker.
- Immerse Electrodes: Place the zinc strip into the ZnSO4 solution, and the copper strip into the CuSO4 solution.
- Inert Electrodes for Solutions: For half-cells consisting of two ions in the same phase (e.g. Fe3+(aq) / Fe2+(aq)), an inert platinum (Pt) electrode must be used to conduct electrons without reacting chemically.
3. The Salt Bridge: Function, Preparation & Rules
The salt bridge is a vital electrical link between the two separate half-cells:
Function of the Salt Bridge
- Completes the Circuit: Allows the free movement of ions between beakers so current can flow without transferring the bulk electrolyte solutions.
- Maintains Electrical Neutrality: As Zn2+ ions enter the left beaker, anions (NO3-) migrate out of the bridge into the zinc beaker. As Cu2+ ions discharge at the right electrode, cations (K+) migrate into the copper beaker.
Preparation & Chemical Compatibility Rules
- Preparation: Soak a strip of clean filter paper in saturated potassium nitrate (KNO3) or potassium chloride (KCl) solution.
- Crucial Exclusion Rule: Never use a potassium chloride (KCl) salt bridge if either half-cell contains silver (Ag+) or lead (Pb2+) ions. Insoluble AgCl or PbCl2 precipitates would crystallize inside the porous paper, blocking ion pores and breaking the electrical circuit. Always use KNO3 as the universal bridge.
4. Why a HIGH-RESISTANCE Voltmeter Must Be Used
A classic Paper 5 examination question challenges candidates on the electrical properties of the measuring instrument:
Why High Resistance is Essential:
1. By Ohm's law (I = V / R), a massive internal resistance ensures that virtually zero electric current flows through the external circuit.
2. If current flowed, electrons would be consumed, driving the cell reaction forward. Reactant concentrations would drop, products would accumulate, and the system would shift away from equilibrium.
3. Drawing zero current measures the true maximum potential difference (electromotive force) under reversible, non-polarized equilibrium conditions.
5. Calculating Standard Cell Potential (E°_cell)
The electromotive force is calculated from standard electrode potentials listed in the data booklet:
Standard Conditions:
- Temperature: 298 K (25 deg C)
- Pressure: 100 kPa (1 bar) for gaseous half-cells (e.g. H2 gas in the standard hydrogen electrode)
- Concentration: 1.00 mol dm^-3 for all aqueous ions
6. Le Chatelier Concentration Shifts on Cell EMF
When ion concentrations deviate from 1.00 mol dm^-3, the equilibrium position of the half-cell reaction shifts, altering the electrode potential and overall cell voltage:
| Concentration Change | Equilibrium Shift (Le Chatelier) | Effect on Half-Cell Potential | Effect on Overall Cell EMF |
|---|---|---|---|
| Diluting Zn2+ at Anode (e.g. from 1.0 M down to 0.10 M) | Zn2+ + 2e- ⇔ Zn shifts left to replace lost ions. |
Zinc releases electrons more readily; E(anode) becomes more negative. | Cell EMF INCREASES (e.g. rises from 1.10 V to ~1.13 V). |
| Diluting Cu2+ at Cathode (e.g. from 1.0 M down to 0.10 M) | Cu2+ + 2e- ⇔ Cu shifts left. |
Copper accepts electrons less readily; E(cathode) becomes less positive. | Cell EMF DECREASES (falls from 1.10 V to ~1.07 V). |
7. Worked Electrochemical Cell Problem
Problem: An electrochemical cell is constructed using an iron half-cell and a silver half-cell under standard conditions:
Fe2+(aq) + 2e- ⇔ Fe(s) E° = -0.44 VAg+(aq) + e- ⇔ Ag(s) E° = +0.80 V
1. Identify the negative electrode (anode) and positive electrode (cathode).
2. Write the standard conventional cell representation.
3. Calculate the standard EMF (E°_cell).
4. Write the overall spontaneous cell equation.
Step 1: Identify Electrodes
Fe2+/Fe has more negative E° (-0.44 V) → Oxidation → Anode (Negative).
Ag+/Ag has more positive E° (+0.80 V) → Reduction → Cathode (Positive).
Step 2: Conventional Cell Representation
Fe(s) | Fe2+(aq) || Ag+(aq) | Ag(s)
Step 3: Calculate Standard EMF
E°_cell = E°(cathode) - E°(anode)
E°_cell = +0.80 - (-0.44) = +1.24 V
Step 4: Overall Balanced Equation
Fe(s) + 2Ag+(aq) → Fe2+(aq) + 2Ag(s)
Final Answer: E°_cell = +1.24 V
8. Practice Exam Questions
Question 1: Why is potassium nitrate (KNO3) preferred over potassium chloride (KCl) for preparing a salt bridge in an electrochemical cell that includes a silver half-cell (Ag+/Ag)?
Show Answer & Explanation
Correct Answer: B
Chloride ions diffusing out of a KCl bridge react with Ag+ in solution to precipitate insoluble AgCl(s), clogging the filter paper and depleting free silver ions. All nitrates are completely soluble, making KNO3 non-interfering.
Question 2: What would happen to the measured potential of a Daniell cell (Zn | Zn2+ || Cu2+ | Cu) if deionised water is added to the zinc sulfate half-cell beaker?
Show Answer & Explanation
Correct Answer: B
Lowering [Zn2+] shifts the equilibrium Zn2+ + 2e- ⇔ Zn to the left, releasing more electrons and making the zinc electrode potential more negative. This widens the gap between the two electrode potentials, increasing overall cell EMF.