Problem 172 - A Complete Resource Book in Chemistry for JEE Main

Question

The mainoxides formed on combusting of \(\mathrm{Li}, \mathrm{Na}\) and \(\mathrm{K}\) in excess of air are respectively? (a) \(\mathrm{LiO}_{2} \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{K}_{2} \mathrm{O}\) (b) \(\mathrm{Li}_{2} \mathrm{O}_{2} \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{KO}_{2}\) (c) \(\mathrm{Li}_{2} \mathrm{O} \mathrm{Na}_{2} \mathrm{O}_{2}\) and \(\mathrm{KO}_{2}\) (d) \(\mathrm{Li}_{2} \mathrm{O} \mathrm{Na}_{2} \mathrm{O}\) and \(\mathrm{KO}_{2}\)

Step-by-Step Solution

Answer

Option (c): 702, 002, and 02.

Step 1: Understanding Lithium Combustion

Lithium (7) reacts with oxygen in the simplest oxide formation due to its position in the periodic table and its tendency to form stable compounds. When lithium combusts in excess oxygen, it forms lithium monoxide (702). Therefore, the oxide formed is 702.

Step 2: Understanding Sodium Combustion

Sodium (0) reacts with oxygen to form peroxide when it combusts in excess oxygen. This is due to its ability to stabilize a larger oxide ion structure. Therefore, the oxide formed by sodium is sodium peroxide (002).

Step 3: Understanding Potassium Combustion

Potassium (1) reacts more vigorous with oxygen, resulting in the formation of superoxide when burned in excess air. Potassium forms potassium superoxide (02) due to its higher reactivity compared to lithium and sodium.

Step 4: Compiling the Reaction Outcomes

From the steps above, we establish the oxides for each element: lithium forms 702, sodium forms 002, and potassium forms 02 upon combustion in excess oxygen. Hence, the sequence of the compounds is 702, 002, and 02.

Step 5: Selecting the Correct Option

From the compiled reaction results, it corresponds to option (c): 702, 002, 02. This matches the sequence that was determined for the oxides formed during combustion of Li, Na, and K.

Key Concepts

Lithium Oxide

Lithium is a unique alkali metal primarily because of its place at the top of the group. When lithium combusts, it tends to form the simplest oxide due to its characteristics. Specifically, lithium reacts with oxygen to form lithium oxide, represented by the formula \(\mathrm{Li_2O}\).
Lithium oxide is produced due to lithium's strong affinity to bonding with oxygen. Oxygen molecules separate, and each lithium atom pairs up, leading to the formation of \(\mathrm{Li_2O}\). Here are a few things to remember about lithium oxide:

• Lithium atoms don’t react as vigorously as other alkali metals with oxygen.
• Combustion of lithium is less violent compared to sodium or potassium.
• Lithium oxide forms as a simple binary compound with only two types of atoms—lithium and oxygen.

Understanding this simple and direct reaction provides insights into the nature of lithium and its position in the periodic table, where it interacts with other elements.

Sodium Peroxide

Sodium, located below lithium in the periodic table, reacts differently with oxygen, leading to the formation of sodium peroxide, represented by \(\mathrm{Na_2O_2}\). The difference in its oxide formation arises from sodium’s ability to stabilize larger oxide ion structures.
When sodium burns in excess oxygen, it forms this compound rather than a simple oxide. Here’s why:

• Sodium atoms are larger and can accommodate peroxide ions more easily.
• The reaction is more energetic, resulting in the formation of sodium peroxide.
• Peroxide compounds like \(\mathrm{Na_2O_2}\) contain oxygen-oxygen bonds, which is distinctive compared to lithium’s simple oxide formation.

Sodium peroxide is crucial as it reflects sodium’s intermediate position in reactivity among the alkali metals, more reactive than lithium but less so than potassium.

Potassium Superoxide

Further down the group, potassium is even more reactive with oxygen. Upon burning in air, potassium forms potassium superoxide, denoted as \(\mathrm{KO_2}\). The formation of the superoxide is indicative of potassium's strong reactivity and its interaction with oxygen.
Potassium’s reaction is vigorous, and here is why the superoxide is its main oxide:

• Potassium can stabilize the larger \(\mathrm{O_2^-}\) superoxide ion due to its larger atomic size.
• The reactivity of potassium is higher than both lithium and sodium, leading to the formation of an oxide with more oxygen content.
• Superoxide compounds are unique because they incorporate the whole \(\mathrm{O_2}\) molecule as a superoxide ion.

Potassium superoxide is significant, showing how potassium, a more reactive element, efficiently forms a compound indicative of its energetic interactions with non-metals like oxygen.