Edexcel Chemistry IGCSE Practical 2.42: Preparation of Hydrated Copper(II) Sulfate Crystals

Overview and Objectives of Practical 2.42

The primary focus of Edexcel Chemistry IGCSE Practical 2.42 is to successfully prepare a pure and dry sample of hydrated copper(II) sulfate crystals, utilizing copper(II) oxide as the initial reactant. This experiment is a demonstration of synthesizing a soluble salt from an insoluble base and an acid, specifically requiring the application of multiple separation and purification techniques such as filtration, evaporation, and crystallization to achieve a high-quality final product.

Required Equipment and Chemical Reagents

To conduct this experiment effectively, the following equipment is required: a $250\,cm^3$ beaker, a boiling tube, a weighing boat, a $100\,cm^3$ conical flask, and a measuring cylinder for volume accuracy. The setup for heating and separation includes a crystallizing dish, a Bunsen burner, a funnel with filter paper, a tripod supported by a gauze, and a heat proof mat for safety. Additional tools for measurement and handling include a digital balance, a kettle for boiling water, and tongs for moving hot glassware. The chemicals strictly required for this reaction are sulfuric acid and copper(II) oxide.

Detailed Experimental Methodology

The procedure begins with the preparation of a water bath. Using a kettle, the $250\,cm^3$ beaker is half-filled with boiling water. Next, precisely $15\,cm^3$ of sulfuric acid is added to a boiling tube, which is then submerged in the hot water bath. On a digital balance, $2.0\,g$ of copper(II) oxide is weighed into a weighing boat. The reaction is initiated by adding approximately one-quarter of the copper(II) oxide to the boiling tube containing the sulfuric acid. After each addition, the tube must be lifted and agitated—specifically shaken—to ensure the reactants mix thoroughly before being returned to the beaker of hot water. This addition and agitation process is repeated three additional times until the total $2.0\,g$ of copper(II) oxide has been introduced. To ensure the chemical reaction reaches completion, the water in the beaker should be replaced with fresh boiling water, and the boiling tube should be left to react for another $5\,\text{minutes}$, with the mixture being agitated every minute.

Purification and Initial Crystallization

Once the reaction phase is complete, the mixture must be purified. A filter paper is placed into a funnel which is then positioned over a $100\,cm^3$ conical flask. The mixture from the boiling tube is poured into the funnel and left undisturbed until all the liquid (the copper(II) sulfate solution) has passed through into the conical flask, leaving behind any unreacted copper(II) oxide. For the production of the hydrated crystals, the mass of a clean crystallizing dish is first recorded. The solution within the conical flask is then heated to a boil using the Bunsen burner and tripod for exactly $3\,\text{minutes}$. Following this concentration step, tongs must be used to carefully pour the hot solution from the conical flask into the crystallizing dish. The solution is then allowed to sit and cool for a period of $5\,\text{minutes}$.

Final Measurement of Formed Crystals

The crystallization process continues as the dish is left undisturbed for at least $24\,\text{hours}$ to allow the crystals to grow and dry thoroughly. After this time, the mass of the crystals and the dish together is measured. The final mass of the produced hydrated copper(II) sulfate crystals is determined by subtracting the previously recorded mass of the empty crystallizing dish from the total measured mass.

Chemical Equations and Technical Principles

The chemical reaction for this preparation is defined by the following equation:

$CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O$

a process in which copper(II) oxide reacts with sulfuric acid to produce copper(II) sulfate and water. A key technical point in this method is that the reaction between the acid and the oxide is conducted within a bath of boiling water; this is done specifically to increase the temperature and thus speed up the rate of reaction. When performing step 9, it is vital to ensure the copper sulfate solution is not allowed to boil until dry, as this can negatively affect crystal quality.

Identification and Mitigation of Experimental Errors

There are two primary potential errors associated with this procedure. First, the copper sulfate solution may not be fully transferred from the conical flask to the crystallizing dish, leading to product loss. This can be improved by washing the conical flask with deionised water and adding those washings to the crystallizing dish. Second, some anhydrous copper sulfate may be produced if the solvent is evaporated too aggressively. This is avoided by reducing the amount of time the Bunsen burner is used to heat the solution. The experimental setup for these phases is often referred to in diagrams, such as Figure 1 for the Filtration Process Setup and Figure 2 for the Initial Crystallisation Process Setup.

Safety Precautions and Risk Management

Strict safety protocols must be followed during this practical. Any chemical spillages must be cleared immediately to prevent contact. Sulfuric acid is an irritant, requiring the user to wear safety glasses and wash their hands thoroughly after use. Copper(II) oxide is harmful if ingested, so hand-washing is mandatory. The resulting copper(II) sulfate crystals are also irritants to the skin and eyes; direct contact should be avoided, and if skin contact occurs, hands must be washed immediately. Regarding heat safety, long hair must be tied back when using the Bunsen burner, and the burner should be kept on the orange safety flame or turned off when not in use. Since the conical flask becomes hot during the $3\,\text{minutes}$ of boiling, it must only be moved using tongs. If a burn occurs, the affected area should be held under cold running water for up to $10\,\text{minutes}$, depending on severity.

Analysis of Results and Yield Calculations

The final analysis involves calculating the mass of the copper sulfate crystals recovered. If the theoretical yield is known from prior stoichiometric calculations, the efficiency of the experiment can be determined by calculating the percentage yield using the following formula:

$\text{Percentage yield} = \frac{\text{Yield}}{\text{Theoretical yield}} \times 100$