The method and the results are on the electrochemistry lab. Do the analysis yourself first, then check the answers below.
Quick answer: Electrochemistry links redox reactions to electrical energy. These answers go with the AP Chemistry lab on electrochemistry: the graph, the calculations, the uncertainties and the conclusion explained, for all 8 analysis questions.
Model answers use the sample data in the Results tables. Your own readings will differ a little, so judge your work by the pattern: the gradient near 0.030 V and the order of the metals.
Level: AP Chemistry · Curriculum: AP Chem Unit 9
Question 1 Identify the anodes
The metal is the negative electrode and the copper is the positive terminal in every pair, so each of Mg, Zn, Fe and Pb is oxidised at the anode. For magnesium: Mg → Mg²⁺ + 2e⁻.
Question 2 Experimental potentials
E°(M²⁺/M) = 0.34 − Emeasured. Mg: 0.34 − 2.61 = −2.27 V; Zn: 0.34 − 1.07 = −0.73 V; Fe: 0.34 − 0.75 = −0.41 V; Pb: 0.34 − 0.45 = −0.11 V. (Mg −2.27, Zn −0.73, Fe −0.41, Pb −0.11.)
Question 3 Percentage difference
Difference = −2.27 − (−2.37) = 0.10 V. Percentage = 0.10 / 2.37 × 100 = 4.2 %. The agreement is good for a school lab.
Question 4 Graph for the Nernst equation
The points are linear. Gradient = (1.09 − 1.00) / (0 − (−3)) = 0.09 / 3 = 0.030 V per unit of log [Cu²⁺].
Question 5 Compare with theory
n = 2, so the gradient is 0.0592 / 2 = 0.0296 V. The measured 0.030 V agrees within 2 %, confirming n = 2.
Question 6 Predict a new voltage
log [Cu²⁺] = −4, which is one decade beyond the data: 1.00 − 0.03 = 0.97 V.
Question 7 Why readings differ
An oxide film on magnesium or iron adds resistance and lowers the reading; the bridge adds a small junction potential; the solutions were 0.50 mol/dm³ rather than 1.0, so voltages differ from standard values; a temperature of 22 °C instead of 25 °C changes the constant slightly. Clean strips and 1.0 mol/dm³ solutions reduce these errors.
Question 8 Role of the bridge
The bridge completes the circuit and lets ions move so that charge does not build up and stop the reaction. A high-resistance meter draws almost no current, so the reading is the open-circuit voltage and the concentrations do not change during the measurement.
