Quick answer: Electrochemistry links redox reactions to electrical energy. This AP Chemistry lab on electrochemistry gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Can the voltage of a cell be predicted from a table of potentials? In this lab you build simple two-beaker cells, compare their voltages with data values and then test how dilution changes the voltage of a zinc-copper cell.
Level: AP Chemistry · Curriculum: AP Chem Unit 9
Aim
Rank four metals by the voltage they give against copper, then test how the voltage of a zinc-copper cell changes as the copper(II) ion concentration is diluted.
Equipment
- Strips of zinc, iron, magnesium, lead and copper, cleaned with sandpaper
- 0.50 mol/dm³ solutions of the metal sulfates or nitrates
- 0.50 mol/dm³ copper(II) sulfate and four dilutions (0.10, 0.010, 0.0010 mol/dm³)
- Small beakers, filter paper strips, potassium nitrate solution
- Digital multimeter with two leads and crocodile clips
Safety
Wear safety goggles. Lead compounds are toxic: wear gloves and wash hands after use. Magnesium strip should not be near a flame. Collect all metal salt solutions for disposal; never pour them down the sink.
Method
- Pour 50 cm³ of the metal solution into one beaker and 50 cm³ of copper sulfate into another.
- Soak a filter-paper strip in potassium nitrate and fold it over both beaker rims as a bridge.
- Clip the metal strip and the copper strip to the multimeter and record the steady reading, noting which lead is positive.
- Repeat for each metal with a fresh bridge.
- Keep zinc in 0.50 mol/dm³ zinc sulfate and swap the copper half-cell for each dilution; record the voltage.
Results
Table 1: each metal against copper, solutions 0.50 mol/dm³, 22 °C.
| Metal M | Measured voltage (V) | Positive terminal |
|---|---|---|
| Mg | 2.61 | Cu |
| Zn | 1.07 | Cu |
| Fe | 0.75 | Cu |
| Pb | 0.45 | Cu |
Table 2: zinc-copper cell, zinc sulfate fixed at 0.50 mol/dm³, copper(II) sulfate diluted.
| [Cu²⁺] (mol/dm³) | log [Cu²⁺] | Voltage (V) |
|---|---|---|
| 1.0 | 0 | 1.09 |
| 0.10 | −1 | 1.06 |
| 0.010 | −2 | 1.03 |
| 0.0010 | −3 | 1.00 |
Analysis questions
Question 1 Identify the anodes
Use Table 1 to state which electrode is the anode in each pair and write the anode half-reaction for magnesium.Question 2 Experimental potentials
Use E°(Cu²⁺/Cu) = +0.34 V to calculate E° for each M²⁺/M couple from the measured voltages.Question 3 Percentage difference
Compare the experimental E°(Mg²⁺/Mg) with the data value −2.37 V and calculate the percentage difference.Question 4 Graph for the Nernst equation
Plot voltage (y) against log [Cu²⁺] (x) from Table 2, draw the best line and find its gradient.Question 5 Compare with theory
The Nernst equation predicts a gradient of 0.0592 / n for this cell. Calculate the predicted value and compare.Question 6 Predict a new voltage
Use your graph to predict the voltage for [Cu²⁺] = 1.0 × 10⁻⁴ mol/dm³.Question 7 Why readings differ
Name two reasons your measured voltages could differ from the data values, and say whether each raises or lowers the reading.Question 8 Role of the bridge
Explain why the filter-paper bridge is needed and why a high-resistance multimeter is used.
