Making copper sulfate from copper oxide and sulfuric acid is a common experiment that demonstrates an acid-base reaction, producing a salt (copper sulfate) and water.
Process Overview
The fundamental chemical reaction involves mixing copper(II) oxide (CuO), a black solid, with dilute sulfuric acid (H₂SO₄). This reaction is typically exothermic and relatively fast, especially when slightly warmed. The product is a solution of copper(II) sulfate (CuSO₄), which is blue, and water (H₂O). To obtain solid copper sulfate crystals, the water is then evaporated from the solution.
Step-by-Step Method
Here's a typical laboratory method for preparing copper sulfate from copper oxide and sulfuric acid:
- Prepare the Acid: Carefully measure out a quantity of dilute sulfuric acid into a beaker.
- Warm the Acid: Place the beaker in a water bath to warm the sulfuric acid. Warming increases the rate of the reaction.
- Add Copper Oxide: Begin adding copper(II) oxide to the warm acid. Add copper oxide to the acid, 1 spatula at a time, stirring with a glass rod between additions.
- Continue Adding Until Excess: Continue adding copper oxide until it is in excess and the solid doesn't disappear when stirred. This ensures all the sulfuric acid has reacted. You will see black solid settling at the bottom of the beaker. The solution should turn blue as copper sulfate is formed.
- Filter the Mixture: Once the reaction is complete and you have excess copper oxide, filter the mixture. This removes the unreacted copper oxide and any other insoluble impurities, leaving a clear blue solution of copper sulfate.
- Evaporate the Solution: Heat the blue copper sulfate solution gently, usually in an evaporating dish, to evaporate most of the water. Stop heating when small crystals start to appear around the edge of the dish or when a sample cooled on a glass rod crystallizes. This indicates the solution is saturated.
- Cool to Crystallize: Allow the hot, saturated copper sulfate solution to cool slowly, ideally at room temperature, without disturbing it. As it cools, copper sulfate crystals (CuSO₄·5H₂O, copper(II) sulfate pentahydrate) will form.
- Dry the Crystals: Filter or decant the remaining liquid (mother liquor) from the crystals. Wash the crystals briefly with a small amount of cold distilled water to remove any surface impurities, and then dry them. This can be done by leaving them on filter paper in a warm, dry place or using a desiccator.
Key Observations
- The black copper oxide solid disappears as it reacts.
- The clear sulfuric acid solution turns blue.
- Adding excess copper oxide leaves black solid unreacted.
- Filtering removes the excess black solid.
- Evaporating the water concentrates the blue solution.
- Cooling the saturated solution forms blue crystals.
The Chemical Reaction
The reaction is a straightforward acid-base neutralization:
Copper(II) Oxide + Sulfuric Acid → Copper(II) Sulfate + Water
In chemical symbols:
CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l)
This reaction produces copper(II) sulfate dissolved in water. The solid copper sulfate obtained after crystallization is usually the pentahydrate form, containing 5 molecules of water of crystallization, CuSO₄·5H₂O.
Reaction Components
| Reactant 1 | Reactant 2 | Product 1 | Product 2 |
|---|---|---|---|
| Copper(II) Oxide | Sulfuric Acid | Copper(II) Sulfate | Water |
| Black Solid (CuO) | Clear Solution (H₂SO₄) | Blue Solution (CuSO₄) | Liquid (H₂O) |
What Happens Next? Obtaining Crystals
The process described above doesn't just produce a solution; it leads to pure, solid copper sulfate crystals. The steps after the initial reaction are crucial for isolating and purifying the salt.
- Filtration: Removes any unreacted starting material or insoluble by-products.
- Evaporation & Crystallization: Allows the soluble copper sulfate to come out of solution as solid crystals when the water is removed and the solution cools. Slow cooling promotes the formation of larger, more regular crystals.
- Drying: Removes any remaining surface moisture.
By following these steps, starting with warming the acid and adding copper oxide until in excess as specified, you can successfully synthesize copper sulfate crystals.
