Corrosion of copper in nitric acid

Problem statement

1) In a materials laboratory, the corrosion of copper in the presence of dilute nitric acid is being studied, because this reaction is responsible both for the dissolution of metallic copper in industrial processes and for the generation of polluting nitrogen oxides. In the corrosion process, metallic copper reacts with nitric acid to give copper(II) nitrate, nitrogen(II) oxide and water.

a) (1 mark) Balance the molecular reaction using the ion-electron (half-equation) method, stating the oxidation and reduction half-equations.

b) (0.5 marks) Starting from the reaction in the previous part, a student proposes reproducing the corrosion of copper in an electrochemical cell made of a Cu electrode and an inert platinum electrode, both immersed in a 1 M HNO$_3$ solution. Identify the anode and the cathode, stating at which electrode oxidation takes place and at which reduction, and calculate the standard potential of the cell.

c) (1 mark) For environmental reasons, it is important to reduce the amount of Cu$^{2+}$ ions present in aqueous solutions. It is known that adding a NaOH solution of pH = 9.0 can form a precipitate of Cu(OH)$_2$, which allows the copper to be removed by filtration. Write the chemical equation for the solubility equilibrium of Cu(OH)$_2$, indicating the physical state of each species. Determine the molar concentration of Cu$^{2+}$ ions above which Cu(OH)$_2$ starts to precipitate when the NaOH solution of pH = 9.0 is added at 25 ºC.

Data. $E^\circ$(V): (Cu$^{2+}$/Cu) = 0.34; (NO$_3^-$/NO) = 0.96. At 25 ºC, $K_s$(Cu(OH)$_2$) = $2.2\cdot10^{-20}$.

Source: Madrid university entrance exam (PAU, Spain) · 2025-2026 · Ordinary session · Chemistry · Question 1. Official exam set by the public universities of the Community of Madrid, to whom it belongs; translated from Spanish and reproduced with its source in order to explain it for teaching purposes.