The high natural abundance and high concentrations of copper and copper ores make copper an economic — Physical Chemistry — Thermodynamics Chemistry Question

The high natural abundance and high concentrations of copper and copper ores make copper an economical choice for many industrial applications. Copper may exist in three oxidation states, I, II, and III. However, most copper compounds are commonly encountered as salts of Cu²⁺, while Cu³⁺ is the least stable form. Copper, being a transition metal, also forms coordination compounds.

In this experiment, potassium bisoxalatocuprate(II) dihydrate will be prepared by reaction of copper(II) sulfate pentahydrate with potassium oxalate.
CuSO₄ · 5 H₂O(aq) + 2 K₂C₂O₄(aq) → K₂Cu(C₂O₄)₂ · 2 H₂O(s) + K₂SO₄(aq) + 3 H₂O(l)
The number of oxalato ligands in the complex ion will be determined by titration with standard permanganate solution. The copper content of the complex ion will be determined by iodine-thiosulfate titration.

Chemicals and reagents
* Copper(II) sulfate pentahydrate, CuSO₄ . 5 H₂O
* Potassium oxalate monohydrate, K₂C₂O₄ . H₂O
* Sulfuric acid, H₂SO₄(aq), c = 2.5 mol dm–3
* Potassium permanganate, KMnO₄, (0.020 mol dm–3)
* Sodium carbonate, Na₂CO₃
* Acetic acid, CH₃COOH, dilute
* Potassium iodide, KI
* Sodium thiosulfate, Na₂S₂O₃, (0.020 mol dm–3)
* Starch indicator (freshly prepared, w = 0.05)
* Potassium thiocyanate, KSCN
* Ethanol, C₂H₅OH
* Acetone, CH₃COCH₃

Apparatus and glassware
* Erlenmeyers, 250 cm₃ (2)
* Beakers, 50 cm₃ (2)
* Funnel
* Pipettes, 5 cm₃, 10 cm₃
* Graduated cylinder, 50 cm₃
* Burettes (2)
* Stirring rod
* Stirring bar
* Ice-bath
* Washing bottle
* Filter paper
* Heater-stirrer

Procedures
A. Preparation of potassium bisoxalatocuprate(lI) dihydrate
1. Dissolve 4.1 g copper(II) sulfate pentahydrate, CuSO₄ . 5 H₂O in 8.0 cm₃ of water and heat the solution to 90 °C.
2. Gradually add the hot solution, while stirring, to a solution (also at 90 °C) of 12.3 g K₂C₂O₄ . H₂O in 35 cm₃ of water.
3. Allow the solution to cool down to room temperature and then cool in an ice-bath to 10°C. Filter off the solid, wash with ice cold water followed by ethanol and then acetone. Dry at 40 °C in air for 1 h.
4. Weigh the dried sample.

B. Determination of oxalate in the prepared compound
1. Transfer an accurately known amount (0.16 – 0.18 g) of potassium bis-oxalatocuprate(lI) dihydrate prepared in Part A into a 250 cm₃ Erlenmeyer flask, and dissolve the complex by adding about 25 cm₃ of water.
2. Add 20 cm₃ of the solution of sulfuric acid (2.5 mol dm–3) and heat the solution to about 80 °C.
H₂SO₄(aq) + K₂Cu(C₂O₄)₂ (aq) → K₂C₂O₄(aq) + CuSO₄(aq) + H₂C₂O₄(aq)
3. Titrate the solution with standardized 0.020 mol dm–3 potassium permanganate solution, until the color of the solution becomes pink persistent for 1-2 minutes (end-point). Record the volume of standard KMnO₄ solution used.
16 H⁺(aq) + 2 MnO₄ –(aq) + 5 C₂O₄ 2–(aq) → 10 CO₂(g) + 2 Mn²⁺(aq) + 8 H₂O(l)

C. Determination of copper in the prepared compound
1. Add solid Na₂CO₃ to the solution obtained in Part B^-2 until a precipitate first appears. Then add dilute acetic acid (10 % w/v) until the pH is about 5. Finally, add about 1 g of solid potassium iodide into the solution.
Na₂CO₃(aq) + CuSO₄(aq) → Na₂SO₄(aq) + CuCO₃(s)
2 Cu²⁺(aq) + 5 I–(aq) → 2 CuI(s) + I₃ –(aq)
2. Titrate the liberated iodine which forms the I₃ – ion with standardized sodium thiosulfate solution (0.020 mol dm–3) using freshly prepared 5% (w/v) starch solution as indicator. A sharper end-point is obtained by the addition of 1 - 2 g potassium thiocyanate as the end-point is approached.