Problem 7 - Chemistry: Introducing Inorganic, Organic and Physical Chemistry

Question

Which of the following pairs of Group 1 compounds would you expect to be more soluble in water? (Section 26.3 ) (a) \(\mathrm{LiNO}_{3}\) or \(\mathrm{CsNO}_{3}\) (b) \(\mathrm{LOH}\) or \(\mathrm{RbOH}\) (c) \(\mathrm{NaBr}\) or \(\mathrm{CsBr}\)

Step-by-Step Solution

Answer

(a) cCsNO39, (b) cRbOHe, (c) cCsBre are more soluble.

Step 1: Review Solubility Rules

In general, most alkali metal compounds (Group 1 compounds) are highly soluble in water. However, the solubility can increase with more massive alkali metals due to increased ionic radius, which enhances dissolution.

Step 2: Analyze (a) cLiNO39 or cCsNO39

Lithium nitrate (cLiNO39) and cesium nitrate (cCsNO39) are both Group 1 compounds. According to the solubility trends, cesium compounds are typically more soluble than lithium compounds because cesium has a larger ionic radius. Thus, cCsNO39 would be more soluble in water than cLiNO39.

Step 3: Evaluate (b) cLOHe or cRbOHe

For hydroxides of Group 1 elements, solubility increases down the group. Since rubidium is below lithium in the periodic table, cRbOHe is expected to be more soluble in water than cLOHe.

Step 4: Compare (c) cNaBre or cCsBre

Both sodium bromide (cNaBre) and cesium bromide (cCsBre) are alkali metal halides. Following the solubility trend of Group 1 halides, cesium compounds, which have a larger ionic radius, tend to be more soluble. Therefore, cCsBre is likely more soluble in water than cNaBre.

Key Concepts

Alkali Metals

Alkali metals form a key group of elements in the periodic table, known as Group 1. These consist of lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). Located in the leftmost column, they are known for their highly reactive nature. This high reactivity is due to having a single electron in their outermost shell, which they readily lose to form positive ions.

• They are soft, shiny metals.
• They have low melting points compared to most other elements.
• These metals tarnish quickly when exposed to air.

Their reactivity increases down the group, meaning cesium is more reactive than lithium. The differences in reactivity and other properties also lead to differences in solubility when these metals form compounds.

Ionic Radius

The ionic radius is a measure of the size of an ion. For alkali metals, the ionic radius increases as you move down the group on the periodic table. This is because each subsequent element has an additional electron shell, making the ions larger.

• Lithium ions ( Li^+ ) are the smallest among the alkali metals.
• Cesium ions ( Cs^+ ) have a much larger ionic radius compared to lithium.

This increase in ionic radius has significant effects on properties such as solubility. Larger ions tend to have weaker lattice energies, meaning the forces holding the ionic crystal together are lower. Therefore, the larger ionic radius seen in heavier alkali metals often enhances their solubility in water.

Solubility Trends

Solubility trends for alkali metal compounds show a noticeable pattern. As you go down Group 1, the solubility of most of their compounds in water increases. This trend is closely related to ionic radius and lattice energy concepts.

• Solubility generally increases for nitrates, hydroxides, and halides as the ionic radius increases.
• Weaker lattice energy due to larger ions enhances water solubility.

For instance, cesium compounds like cesium nitrate ( CsNO_3 ) and cesium bromide ( CsBr ) are more soluble in water than their smaller counterparts, such as lithium nitrate ( LiNO_3 ) and sodium bromide ( NaBr ). This trend is pivotal when predicting solubility for homework exercises involving Group 1 elements.

Water Solubility

Water solubility is the ability of a compound to dissolve in water, and this trait varies among different compounds of alkali metals. The increased size of ions down Group 1 affects how easily they dissolve in water. These larger ions break the lattice more readily, allowing water molecules to interact with them more effectively.

• Compounds with larger cations like Cs^+ show enhanced solubility.
• Hydroxides and halides of bigger alkali metals dissolve more completely in water.

When thinking about solubility, it's important to remember that while most alkali metal compounds are soluble, the extent to which they dissolve can vary significantly, as seen in the example of rubidium hydroxide ( RbOH ) being more soluble than lithium hydroxide ( LOH ).

Periodic Table

The periodic table is an organized chart of elements arranged according to increasing atomic number. It helps us understand the properties and trends of chemical elements, including alkali metals in Group 1.

• Elements are grouped by similar properties, such as alkali metals having one valence electron.
• Trends like increasing ionic radius down a group are evident.

By examining the table, one can observe the predictable patterns like solubility trends and reactivity changes. These patterns help us make informed predictions about the behavior of compounds, such as enhanced solubility of cesium compounds versus lithium ones, drawing from their respective positions in the periodic table.