Thermal Decomposition of Copper Sulfate Pentahydrate

Introduction 

When learning a new method, it is always a good idea to measure a system whose behavior is well known and documented under particular conditions. The thermal decomposition of CuSO₄ ⋅ 5 H₂O is a good example of this, and the interpretation of the TGA curve poses no problems for the beginner.

Experimental Details 

The measurements were performed with a TGA/SDTA851e (range: 5 g; resolution 1 µg), coupled to an Inficon Themostar QMS mass spectrometer (mass range 1-300).

Evaluation 

The first three steps show a total relative weight loss of 35.58% (Figure 1). This corresponds to the loss of all five water molecules. In the last step, a further loss of 32.87% occurs so that finally only CuO remains. Table 1 summarizes the relative weight losses and the reactions involved in each step.

Confirmation of the Proposed Reaction Steps by Measuring the Relevant MS Ion Currents

If the ion currents of the corresponding m/z values for water and SO₃ are continuously measured, it should be possible to check whether the proposed reaction scheme is in fact correct. As can be seen in Figure 2, water is indeed eliminated in the first three steps. However, rather unexpectedly, no SO₃ is formed in the last step. The derivative curve shows that this last step is in fact a two-step process.

Interpretation of the Experimental Results

The fact that SO₃ was not detected can have two possible reasons. It could be that O₂ and SO₂ are eliminated from the crystal lattice of the CuSO₄ and not SO₃. Alternatively, it is also possible that SO₃ is initially formed, but is unstable under the conditions of temperature and pressure in the instrument. The question of the stability of SO3 can be answered with the help of thermochemical calculations. For the decomposition reaction, the following thermochemical data is available for 25 °C:

At room temperature, the equilibrium is completely on the SO₃ side. The following four equations can be used to determine the temperature at which the equilibrium is on the SO₂ and O₂ side.

At temperatures below 780 °C, the Gibbs’ free reaction enthalpy for the decomposition of SO₃ is positive, then the sign changes and the equilibrium is on the side of SO₂ and O₂. This explains the observation that the ion current for SO₃ as a function of time remains constant. The question of the initially formed decomposition products cannot therefore be answered. The fact that O₂ and SO₂ are observed in the last two steps is evidence against a step-wise elimination of O₂ and SO_2.

What Practical Educational Value Does This Experiment Have?

The decomposition reaction of CuSO₄ ⋅ 5 H₂O is an ideal example for demonstrating the principles of TGA-MS to students. The initially unexpected observation that SO₃ is not formed under the conditions used makes students realize the advantages that such an online coupling of the two instruments offers. The experiment is also useful because students can apply the basic principles of chemical thermodynamics that they covered in the theoretical course to explain the experimental observations.

^{Thermal Decomposition of Copper Sulfate Pentahydrate | Thermal Analysis Application No. UC 156 | Application published in METTLER TOLEDO Thermal Analysis UserCom 15}