Specified Practical: Stability of Carbonates
Introduction
- Metal carbonates decompose to the metal oxide and carbon dioxide
MCO3 (s) → MO (s) + CO2 (g)
- When carbon dioxide is bubbled through limewater a milky solution is observed, so when this change is observed the metal carbonate has decomposed
- To determine which metal carbonate has the highest thermal stability the following steps can be carried out
Apparatus
- 3 boiling tubes
- Test tube holder
- Bunsen burner
- Heat proof mat
- Rack for boiling tubes
- Samples of calcium carbonate, copper(II) carbonate and sodium carbonate
- Spatula
- Electronic balance (resolution of ± 0.01 g)
Diagram
Apparatus required for the thermal decomposition of metal carbonates
Method
- Record the mass of one empty boiling tubes
- Weigh 2 g of copper(II) carbonate and add to the tube
- Record the mass of the boiling tube and copper(II) carbonate
- Time how long it takes for the limewater to turn milky
- Record this in the results table
- Repeat using sodium carbonate and calcium carbonate
| Time taken for limewater to turn milky (s) | |
| copper(II) carbonate | |
| sodium carbonate | |
| calcium carbonate |
Practical Tip
- This investigation can be carried out without using limewater and recording the change in mass of the boiling tube. The greater the change in mass the more carbon dioxide has been released leaving the metal oxide behind.
Analysis of results
Results
Results table
| Time taken for limewater to turn milky (s) | |
| copper(II) carbonate | 22 |
| sodium carbonate | no change |
| calcium carbonate | 140 |
Evaluation
- The metal carbonate that takes the longest to decompose into the metal oxide and carbon dioxide is the most stable metal carbonate
- The most stable metal carbonate is sodium carbonate as the colour of the limewater did not change, so the carbonate did not decompose
- The Bunsen burner flame does not reach a high enough temperature to decompose sodium carbonate
Conclusion
- Calcium carbonate is the second most stable and then copper(II) carbonate is the least stable
- This is because sodium is the most reactive metal, followed by calcium and then copper is the least reactive
- The more reactive the metal, the more stable the metal carbonate
Examiner Tip
The most stable metal carbonate will have the least change in mass and the least stable metal carbonate will have the greatest change in mass.
Worked example
A student investigated the order of stability of three different carbonates, copper carbonate, calcium carbonate and sodium carbonate.
i) Predict the order of stability
| most stable | ................................................ |
| ................................................ | |
| least stable | ............................................... |
ii) Explain the order of stability of the carbonate
Answer
i) The order of stability is
| most stable | sodium carbonate |
| calcium carbonate | |
| least stable | copper(II) carbonate |
ii) The order of stability is due to the reactivity of the metal. The more reactive the metal, the more stable the metal carbonate.
