Which one of the following elements does not form solid hydrogen carbonate?
Lithium
Understanding Alkali Metal Hydrogen Carbonates
The question asks which of the given alkali metal elements does not form a solid hydrogen carbonate. The elements provided are Sodium, Potassium, Caesium, and Lithium. These are all members of Group 1 of the periodic table, known as alkali metals.
Hydrogen carbonates, also known as bicarbonates, are compounds containing the hydrogen carbonate ion (\(\text{HCO}_3^{-}\)). When alkali metals react or are involved in reactions forming hydrogen carbonates, they typically form salts with the general formula \(\text{MHCO}_3\), where M represents the alkali metal.
Formation and Stability of Alkali Metal Hydrogen Carbonates
Most alkali metal hydrogen carbonates are known to exist as solid compounds under standard conditions. For example:
- Sodium forms solid sodium hydrogen carbonate (\(\text{NaHCO}_3\)), commonly known as baking soda.
- Potassium forms solid potassium hydrogen carbonate (\(\text{KHCO}_3\)).
- Caesium forms solid caesium hydrogen carbonate (\(\text{CsHCO}_3\)).
The stability of hydrogen carbonates often depends on the size and charge density of the cation. Larger, less polarizing cations tend to stabilize the hydrogen carbonate anion more effectively, allowing for the formation of stable solid salts.
Lithium and its Hydrogen Carbonate
Lithium (Li) is the first element in the alkali metal group and has the smallest ionic radius and highest charge density among the alkali metals. Due to its small size and high polarizing power, lithium behaves differently in many respects compared to the other alkali metals. Lithium carbonate (\(\text{Li}_2\text{CO}_3\)) is less stable to heat than other alkali metal carbonates, decomposing more easily.
Lithium hydrogen carbonate (\(\text{LiHCO}_3\)) exists in aqueous solution, but attempts to isolate it as a solid under normal conditions are generally unsuccessful. When solutions containing lithium hydrogen carbonate are evaporated or cooled, lithium carbonate (\(\text{Li}_2\text{CO}_3\)) is usually precipitated instead of lithium hydrogen carbonate. This indicates that solid lithium hydrogen carbonate is unstable and readily decomposes or converts to lithium carbonate and carbon dioxide (\(\text{CO}_2\)) and water (\(\text{H}_2\text{O}\)).
The equilibrium reaction for the decomposition of hydrogen carbonates is:
\(\text{2MHCO}_3\text{(s)} \rightarrow \text{M}_2\text{CO}_3\text{(s)} + \text{H}_2\text{O}\text{(l)} + \text{CO}_2\text{(g)}\)
For Lithium, this equilibrium lies far to the right, making the solid \(\text{LiHCO}_3\) unstable relative to \(\text{Li}_2\text{CO}_3\).
Comparing Alkali Metal Hydrogen Carbonate Formation
Let's summarize the typical behavior:
| Alkali Metal | Symbol | Forms Solid Hydrogen Carbonate? | Notes |
|---|---|---|---|
| Lithium | Li | No (unstable solid) | Forms \(\text{LiHCO}_3\) in solution, but precipitates \(\text{Li}_2\text{CO}_3\) upon drying. |
| Sodium | Na | Yes | \(\text{NaHCO}_3\) (Baking Soda) is a common solid. |
| Potassium | K | Yes | \(\text{KHCO}_3\) is a stable solid. |
| Caesium | Cs | Yes | \(\text{CsHCO}_3\) is a stable solid. |
Based on this comparison, Lithium is the alkali metal among the options that does not readily form or maintain a stable solid hydrogen carbonate under typical conditions.
Conclusion
The element among the given options that does not form solid hydrogen carbonate is Lithium. This is due to the relative instability of solid lithium hydrogen carbonate, which tends to decompose into lithium carbonate, water, and carbon dioxide.
Revision Table: Alkali Metals and Hydrogen Carbonates
| Alkali Metal | Solid Hydrogen Carbonate Formed? |
|---|---|
| Lithium | No (unstable solid) |
| Sodium | Yes |
| Potassium | Yes |
| Caesium | Yes |
Additional Information: Diagonal Relationship
Lithium exhibits a diagonal relationship with Magnesium (Mg) in the periodic table. This means Lithium shares some properties with Magnesium, which is in Group 2. Like Lithium hydrogen carbonate, Magnesium hydrogen carbonate (\(\text{Mg(HCO}_3\text{)}_2\)) exists only in aqueous solution and decomposes to magnesium carbonate (\(\text{MgCO}_3\)), water, and carbon dioxide upon heating or evaporation of the solution. This diagonal relationship helps explain some of Lithium's anomalous behaviors compared to other alkali metals, including the instability of its hydrogen carbonate as a solid.
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