Question

When an alkaline earth carbonate is heated, it releases \(\mathrm{CO}_{2},\) leaving the metal oxide. The temperature at which each Group \(2 \mathrm{~A}(2)\) carbonate yields a \(\mathrm{CO}_{2}\) partial pressure of 1 atm is listed below: $$ \begin{array}{lc} \text { Carbonate } & \text { Temperature }\left({ }^{\circ} \mathrm{C}\right) \\\ \hline \mathrm{Mg} \mathrm{CO}_{3} & 542 \\\\\mathrm{CaCO}_{3} & 882 \\\\\mathrm{SrCO}_{3} & 1155 \\\\\mathrm{BaCO}_{3} & 1360\end{array}$$ (a) Suggest a reason for this trend. (b) Mixtures of \(\mathrm{CaCO}_{3}\) and \(\mathrm{MgO}\) are used to absorb dissolved silicates from boiler water. How would you prepare a mixture of \(\mathrm{CaCO}_{3}\) and \(\mathrm{MgO}\) from dolomite, which contains \(\mathrm{CaCO}_{3}\) and \(\mathrm{MgCO}_{3} ?\)

Short answer

The trend in decomposition temperature is due to the decrease in lattice energy down the group. To prepare the mixture, heat dolomite between 542°C and 882°C to decompose MgCO₃ into MgO.

Step-by-step solution

Understand the Trend in Decomposition Temperatures

The decomposition temperature of Group 2A carbonates increases as you move down the group. This means that MgCO₃ decomposes at 542°C, CaCO₃ at 882°C, SrCO₃ at 1155°C, and BaCO₃ at 1360°C.

Explain the Trend

The trend can be explained by considering the lattice energy of the metal carbonates. As the atomic size of the metal ion increases down the group, the lattice energy decreases. Lower lattice energy means it requires more energy (higher temperature) to decompose the carbonate.

Identify Components in Dolomite

Dolomite contains both CaCO₃ and MgCO₃. These carbonates decompose at different temperatures; CaCO₃ at 882°C and MgCO₃ at 542°C.

Heat the Dolomite

To prepare a mixture of CaCO₃ and MgO, heat dolomite to a temperature above 542°C but below 882°C. At this temperature, MgCO₃ decomposes into MgO and CO₂, while CaCO₃ remains unchanged.

Isolate the Components

After heating, the resultant mixture will contain MgO and CaCO₃. You now have the desired mixture.

Key concepts

alkaline earth metals

Alkaline earth metals are elements found in Group 2 of the periodic table. They include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These metals have similar properties and reactivities because they all have two electrons in their outer shell.

When it comes to their carbonates, these react differently under heat. For example, MgCO₃ decomposes at a lower temperature (542°C), whereas BaCO₃ decomposes at a significantly higher temperature (1360°C). This trend is due to the atomic structure and size of each metal.

As you move down the group, the atomic size increases. This change affects the lattice energy of the compounds they form.

lattice energy

Lattice energy is the energy required to break apart an ionic solid and convert its constituents into gaseous ions. It basically measures the strength of the bonds in an ionic compound.

For carbonates of alkaline earth metals, the lattice energy decreases as you move down the group. This happens because, as atomic size increases, the distance between the ions in the lattice also increases. Larger distances mean weaker attractions, which results in lower lattice energy.

The lower the lattice energy, the less stable the carbonate and the more heat (higher temperature) it requires to decompose. This explains why MgCO₃ breaks down at a much lower temperature compared to BaCO₃. In simpler terms, a more stable lattice holds together longer under heat.

dolomite

Dolomite is a naturally occurring mineral composed of calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃). It's often used in industrial processes, such as in the treatment of boiler water to remove dissolved silicates.

When preparing a mixture of CaCO₃ and MgO from dolomite, you should heat the dolomite to a temperature that selectively decomposes MgCO₃. CaCO₃ decomposes at 882°C, but MgCO₃ decomposes at 542°C.

By heating dolomite to a temperature between 542°C and 882°C, MgCO₃ decomposes to form MgO and CO₂, while CaCO₃ remains unchanged. The resultant mixture will contain CaCO₃ and MgO, which is often the desired outcome for industrial applications.