Nature of Hydroxides of s-Block Elements - Hydroxides of Alkali Metals and Alkaline Earth Metals, Practice problems and FAQ

Who hasn’t heard about antacids!

In fact, it is a household name, infamous for its stomach acid-reducing properties. Everyone uses antacid pills or liquid formulations to treat acid reflux, heartburn, or uncomfortable feelings after a heavy and rich meal. In fact, almost every home has it without even a prescription.

Originally, its composition reveals that it is a basic metal hydroxide (Milk of Magnesia, MgOH₂). As we all know, acids can be neutralised solely by bases!

So, to counter the acid attack, we need the weapon of bases! In fact, pH indicators are excellent for determining whether a substance is acidic or basic.

Hydroxides are commonly found on the more greenish blue to dark blue sides of this universal pH indicator! Because they are naturally weak – strongly basic or amphoteric. Let’s understand a little more about the hydroxides and their nature and spill the bean about hydroxides of s-block elements!

TABLE OF CONTENTS

• Hydroxides of Alkali Metals
• Nature of Alkali Metal Hydroxides
• Hydroxides of Alkaline Earth Metals
• Nature of Alkaline Earth Metal Hydroxides
• Practice Problems
• Frequently Asked Questions - FAQ

Hydroxides of Alkali Metals

The alkali metals react with water to form metal hydroxides and release hydrogen gas.

2M(s) + 2H₂O(l) ⟶ 2MOH(aq) + H₂(g)

Where M is an alkali metal.

The reaction of alkali metals with water is exothermic and the enthalpy of the reaction increases from Lithium to Caesium. Alkali metals float in the water during the reaction.

• Lithium is half as dense as water. The density of Sodium and Potassium is lower than of water. In heavier alkali metals, the reaction enthalpy is high such that the metal gets melted and rises to the surface. Hence, the reaction with water becomes kinetically faster, highly exothermic, and explosive leading to fire from lithium to caesium.

    Example: 2Na(s) + 2H₂O(l) --> 2NaOH(aq) +H₂(g)

• The melting point of hydroxides decreases down the group, and the reaction with water becomes more and more vigorous because as the metal is melted, the surface area exposed to water becomes more. Due to this, the reaction is kinetically faster. 

The oxides of the alkali metals react with moisture/water to form hydroxides and hydrogen gas. 

• Alkali metal peroxides react with water to form metal hydroxides and hydrogen peroxide.

M2O₂ + 2H₂O ⟶ 2MOH + H₂O₂

• Superoxides of alkali metals react with water to form hydroxides, and hydrogen peroxide and liberate oxygen gas.

2MO₂ + 2H₂O ⟶ 2MOH + H₂O₂ + O₂

• Alkali metal oxides, on hydrolysis, form hydroxides. 

M₂O +H₂O⟶ 2MOH

Example:

Li₂O (s) + 2H₂O (l) ⟶ 2LiOH (aq) + H₂(g)                                     [Oxide + Water]
Na₂O₂ (s) + 2H₂O (l) ⟶ 2NaOH (aq) + H₂O₂ (aq)                        [Peroxide+Water]
2KO₂ (s) + 2H₂O (l) ⟶ 2KOH (aq) + H₂O₂ (aq) + O₂(g)              [Superoxide + Water]
2RbO₂ (s) + 2H₂O (l) ⟶ 2RbOH (aq) + H₂O₂ (aq) + O₂(g)          [Superoxide + Water]
2CsO₂ (s) + 2H₂O (l) ⟶ 2CsOH (aq) + H₂O₂ (aq) + O₂(g)            [Superoxide + Water]

• Li₂O reacts with water to form LiOH.

Li₂O ⟶ 2Li+ +O^2-
O^2- + H₂O ⟶ 2OH^-
2Li⁺ + 2OH^- ⟶ 2LiOH

• Na2O2 reacts with water to form NaOH and H2O2.

Na₂O₂ ⟶ 2Na⁺ + O^2-
O^2- + 2H₂O ⟶ H₂O₂ + 2OH^-
2Na⁺ + 2OH^- + H₂O₂ ⟶ 2NaOH + H₂O₂

• KO₂ reacts with water to form KOH, H₂O₂, and O₂.

2KO₂ ⟶ 2K⁺ + 2O^-
2O^- + 2H₂O⟶ 2OH^- + H₂O₂ + O₂
2K⁺ + 2OH^- + H₂O₂ + O₂ ⟶ 2KOH + H₂O₂ + O₂

Nature of Hydroxides of Alkali Metals

• The hydroxides are all white crystalline solids.
• Of all bases, alkali metal hydroxides are the strongest.
• They dissolve readily in water with the evolution of much heat on account of intense hydration.
• LiOH is not as basic as other alkali metal hydroxides due to a certain amount of covalent character in LiOH. This is mainly due to the very small size of Li⁺ ion which makes it highly polarising in nature.
• The alkali metal hydroxides create white crystals that are hygroscopic and soluble in water, creating a lot of heat when they dissolve. As the alkali metal ions become larger and the lattice enthalpies fall, the solubility increases down the column.
• All alkali metal hydroxides are strong bases, which means they totally dissociate in solution to produce hydroxide ions.
• Alkali hydroxides are utilised in cleaning products because they are powerful bases that are highly corrosive.
• Sodium hydroxide can be found at most hardware stores in the form of drain cleaners. 
• Potassium hydroxide is also offered as a solution for cleaning terraces and other wooden structures. Both NaOH and KOH are utilised in soap and detergent manufacturing (saponification).
• Alkali hydroxides are utilised as desiccants because of their hygroscopic characteristics. They are also good at absorbing carbon dioxide, thus they're employed in carbon dioxide scrubbers.

Hydroxides of Alkaline Earth Metals

Alkali metals are more reactive as compared to alkaline earth metals. The reactivity of these elements increases down the group. This is because, as we move down the group, size increases. So, ionisation energy decreases and it becomes easier to remove the electron. Hence, their reactivity increases.

• Alkaline earth metals form compounds that are predominantly ionic, but less ionic than the corresponding compounds of alkali metals. This is due to the increased nuclear charge and smaller size (Fajan’s rule).
• Be and Mg are kinetically inert to water due to the formation of an oxide film on their surface.

2Be(s) + O₂(g) ⟶ 2BeO(s)

• Mg shows an insignificant reaction with water but burns vigorously with steam or water vapour to produce white magnesium oxide and hydrogen gas. Magnesium decomposes hot water.

Mg(s) + 2H₂O(g) ⟶ Mg(OH)₂(s) + H₂(g)

• Ca, Sr, Ba, and Ra react in cold water readily with the evolution of hydrogen.

M (s) + 2H₂O (l) ⟶ M(OH)₂(aq) + H₂(g)

Where M = Ca, Sr, Ba

• Another method is the treatment of alkaline metal oxides with water, which produces their corresponding hydroxides. This process is also proclaimed as ‘slaking’

MO (s)+ H₂O (l) --> M(OH)₂(aq)

Where M = Ca, Sr, Ba

Nature of Hydroxides of Alkaline Earth Metals

• Because of the insoluble nature of Be(OH)2 and Mg(OH)2, they are readily precipitated with hydroxide ions from appropriate metal cation solutions.

BeCl₂ (s) + 2NaOH (aq) → Be(OH)₂ (s) ↓ + 2NaCl (aq)
MgSO₂ (s) + 2NaOH (aq) → Be(OH)₂ (s) ↓ + 2Na₂SO4 (aq)

• The order of solubilities, thermal stabilities and basic character of Group 2 hydroxides follow the order Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂

Hence, all these three properties increase down the group.

• Down the group, the solubility of alkaline metal hydroxides increases. This is because, though both the lattice enthalpy and the hydration enthalpy decrease down the group as the size of the cation increases, lattice enthalpy decreases more rapidly than hydration enthalpy. Hence, solubility increases down the group.
• Down the group, the polarising power of cation decreases due to the increase in atomic radii of the cation. Thereby more will be the ionic character leading to high thermal stability of Group II metal hydroxides as we go down the group.

Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂

• The alkaline earth metal hydroxides are less basic and less stable than alkali metal hydroxides (As group 2 metal ions have more polarising power, the covalent character is more than group 1).
• As size increases down the group, the polarising power of cation decreases and hence ionic character increases. More ionic the bond, the more the basic character. Hence the basicity of Group II hydroxides also increases down the group.

Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂

• Beryllium hydroxide is amphoteric in nature since it reacts with both acids and alkalis.

Be(OH)₂ + 2OH^- ⟶[Be(OH)₄]^2-
Beryllate ion
Be(OH)₂ + 2HCl + 2H₂O ⟶ [Be(H₂O)₄]Cl₂

• Due to higher ionisation energies, smaller ionic sizes, and greater lattice energies, these hydroxides are less basic than the analogous alkali metal hydroxides.

Practice Problems

Q1. Which among the following is an amphoteric hydroxide?

1. CsOH
2. Be(OH)₂
3. Mg(OH)₂
4. KOH

Solution: Beryllium hydroxide is amphoteric in nature as it reacts with both acids and alkalis. 

Be(OH)₂ + 2OH^- ⟶[Be(OH)₄]^2-
Beryllate ion
Be(OH)₂ + 2HCI + 2H₂O ⟶[Be(H₂O)₄]Cl₂

So, option B) is the correct answer.

Q2. What is the major component of a suspension of antacid formulations?

1. Mg(OH)₂
2. Al(OH)3
3. Ba(OH)₂
4. Ca(OH)₂

Solution: Milk of Magnesia is the major component of antacid suspensions. Its formula is Mg(OH)₂. 

So, option A) is the correct answer.

Q3. What happens when sodium peroxide reacts with water?

1. Sodium hydroxide is formed
2. Hydrogen peroxide is formed
3. Sodium hydroxide, as well as hydrogen peroxide, is formed.
4. Only Oxygen is liberated.

Solution: Na₂O₂ reacts with water to form NaOH and H₂O₂. 

Na₂O₂ + 2H₂O --> 2NaOH + H₂O₂

So, option C) is the correct answer.

Q4. Which alkali metal hydroxide has the least ionic character?

1. NaOH
2. RbOH
3. CsOH
4. LiOH

Solution: Li+ has the smallest size and so is most polarising among all other alkali metals. So, LiOH is the least ionic. LiOH is not as basic as other alkali metal hydroxides due to a certain amount of covalent character, This is mainly due to the very small size of Li+ ion which makes it highly polarising in nature.

So, option D) is the correct answer.

Frequently Asked Questions - FAQ

Q1. What is the trend of thermal stability of alkali metal hydroxides?
Answer: The correct order of the thermal stabilities of alkali metal hydroxides is LiOH<NaOH<KOH<RbOH<CsOH.

The lattice enthalpy increases as the size of the alkali metal ions increases down the group. So, thermal stability also increases down the group.

Q2. Mg(OH)₂ is basic whereas Be(OH)₂ is amphoteric in nature. Why so?
Answer: The ionisation enthalpy of Mg is lower than the ionisation enthalpy of Be. So, the M-OH bond dissociation energy is lower in Mg(OH)₂ and hence, Mg-OH bond breaks more easily than Be-OH in Be(OH)₂. Hence, OH- ions are furnished by Mg(OH)₂ more readily, making it more basic.

Q3. Name a metal hydroxide used to make mortar.
Answer: Mortar is a rigid and strong deep bowl, which is used to crush or grind things inside it using a pestle. Calcium hydroxide (Ca(OH)₂) is used in preparing mortar.

Q4. Can hydroxides be acidic?
Answer: No. Hydroxides contain OH- ions. And they have a strong tendency to attract H+ ions from acidic sources so as to neutralise themselves. Therefore, they are basic in nature as they generally provide hydroxyl ions in aqueous solutions. 

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