Question

The hydroxides and carbonates of sodium and potassium are easily soluble in water while the corresponding salts of magnesium and calcium are sparingly soluble in water. Explain.

Short answer

Sodium and potassium compounds are more soluble due to larger ionic sizes and lower charges, whereas magnesium and calcium compounds form tighter ionic lattices, making them less soluble.

Step-by-step solution

Understand Solubility Basics

Solubility refers to the ability of a compound to dissolve in a solvent, forming a homogeneous solution at a specified temperature and pressure. Compounds that dissolve easily are termed 'soluble,' whereas those that do not are 'sparingly soluble' or 'insoluble.'

Examine Sodium and Potassium Compounds

Sodium (Na) and Potassium (K) are both alkali metals located in Group 1 of the periodic table. Their hydroxides (e.g., NaOH and KOH) and carbonates (e.g., Na2CO3 and K2CO3) form strong ionic bonds but still easily dissociate in water due to their high solubility, attributing to the alkali metals' low ionization energy and large ionic sizes which allow water molecules to solvately stabilize them effectively.

Examine Magnesium and Calcium Compounds

Magnesium (Mg) and Calcium (Ca) are alkaline earth metals located in Group 2 of the periodic table. Their hydroxides (e.g., Mg(OH)2 and Ca(OH)2) and carbonates (e.g., MgCO3 and CaCO3) are sparingly soluble in water because these metals form stronger and more stable ionic lattices due to higher cationic charges and smaller ionic sizes compared to alkali metals, making it difficult for water molecules to break these bonds.

Analyze the Impact of Ion Size and Charge

The solubility differences arise from the size and charge of the ions involved. Sodium and potassium ions are larger with a +1 charge, making them less compact and allowing water to solvate them more easily. Conversely, magnesium and calcium ions have a +2 charge and smaller sizes, which results in tighter ionic lattices that are less readily disrupted by water.

Conclusion of Solubility Differences

The easily soluble nature of sodium and potassium compounds and sparingly soluble nature of magnesium and calcium compounds is due to a combination of ionic structure, size, and charge differences. Group 1 compounds have larger ionic radii and lower charges, leading to easier solubility, while Group 2 compounds display the reverse properties.

Key concepts

Alkali Metals

Alkali metals are a fascinating group of elements found in Group 1 of the periodic table. These elements include lithium, sodium, potassium, rubidium, cesium, and francium. One of the defining features of alkali metals is their single electron in the outermost shell, which they tend to lose easily when reacting with other elements. This characteristic gives them their notable reactive nature.

Their reactivity increases as you move down the group from lithium to francium. This is because as you move down the group, the atomic size increases, and the outer electron is further away from the nucleus. Hence, it is less strongly attracted and easily removed. Alkali metals form ionic compounds, where they exist as positively charged ions, or cations (e.g., Na⁺, K⁺). These ions tend to form highly soluble salts in water, such as sodium chloride or potassium carbonate. Due to their low ionization energy and large atomic size, these salts can dissociate easily in water.

Alkaline Earth Metals

Alkaline earth metals are found in Group 2 of the periodic table and include elements such as beryllium, magnesium, calcium, strontium, barium, and radium. They are known for having two electrons in their outermost shell.

This makes them less reactive than their alkali metal counterparts but still sufficiently reactive to form compounds. Unlike alkali metals, alkaline earth metals form cations with a +2 charge (e.g., Mg²⁺, Ca²⁺) when they lose their two outer electrons.

While they also form ionic compounds, their salts, like magnesium hydroxide and calcium carbonate, are less soluble in water. This lower solubility is due to their higher ionic charge and smaller ionic radius, which create stronger ionic bonds that are tougher for water molecules to solvate and break apart.

Ionic Bonds

Ionic bonds are a type of chemical bond where atoms exchange electrons to achieve stable electron configurations. In essence, one atom donates an electron to another, resulting in the formation of ions. This bonding occurs typically between metals and non-metals, where metals lose electrons and non-metals gain them.

Alkali metals and alkaline earth metals both participate in ionic bonding. For example, when forming compounds like sodium chloride or calcium oxide, metals like sodium or calcium donate electrons to nonmetals like chlorine or oxygen, leading to the formation of ions (e.g., Na⁺, Cl⁻) that attract each other.

The strength of ionic bonds depends heavily on the size and charge of the ions formed. Larger ions with a lower charge create weaker ionic bonds, while smaller, more highly charged ions form stronger bonds. This is clear when comparing the solubility differences seen between alkali and alkaline earth metal compounds.

Periodic Table Group Trends

The periodic table displays recurring trends in properties of elements as you move across periods or down groups. These trends help to predict and explain elements' behaviors. Within a group, such as the alkali metals in Group 1 or alkaline earth metals in Group 2, elements exhibit similar chemical properties.

For alkali metals, reactivity increases down the group due to increasing atomic size and decreasing ionization energy. In alkaline earth metals, a similar trend is observed, although the increase in reactivity is not as pronounced, reflecting their relatively lower chemical activity.

Within a group, as the size of the atom increases, the outermost electrons are less tightly held, enhancing the element's ability to participate in reactions. Additionally, the solubility of their compounds is also impacted by these trends. For instance, larger alkali metal ions result in more soluble compounds compared to those formed by the smaller, highly charged alkaline earth metal ions.