Problem 154 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the compounds in which the sulphur atoms are in different oxidation states? (a) \(\mathrm{K}_{2} \mathrm{~S}_{2} \mathrm{O}_{7}\) (b) \(\mathrm{Na}_{2} \mathrm{~S}_{2} \mathrm{O}_{3}\) (c) \(\mathrm{Na}_{2} \mathrm{~S}_{4} \mathrm{O}_{6}\) (d) \(\mathrm{K}_{2} \mathrm{~S}_{2} \mathrm{O}_{8}\)

Step-by-Step Solution

Answer

The compound \( \mathrm{Na}_2 \mathrm{~S}_4 \mathrm{O}_6 \) has sulfurs in different oxidation states.

Step 1: Determine Oxidation States Rules

To find the oxidation state, we use the rule that the sum of the oxidation states in a neutral compound is 0 and in an ion equals the ion charge. Typically, oxygen is -2 and alkali metals like K and Na are +1.

Step 2: Calculate Oxidation States in K2S2O7

In the compound \( \mathrm{K}_2\mathrm{S}_2\mathrm{O}_7 \), each potassium (K) is +1 (total +2 for two). Oxygen is generally -2, so for seven oxygens, it gives -14. Let the oxidation state of each sulfur be \(x\). Then, the equation is: \[ 2(+1) + 2x + 7(-2) = 0 \]This simplifies to \( 2 + 2x - 14 = 0 \), so \( 2x = 12 \) which gives \( x = +6 \). Therefore, both sulfurs are in the +6 state.

Step 3: Calculate Oxidation States in Na2S2O3

In \( \mathrm{Na}_2\mathrm{S}_2\mathrm{O}_3 \), each sodium (Na) is +1 (total +2). Oxygen is -2, contributing -6. Let the oxidation state of each sulfur be \(x\). The equation is: \[ 2(+1) + 2x + 3(-2) = 0 \]This simplifies to \( 2 + 2x - 6 = 0 \), or \( 2x = 4 \) giving \( x = +2 \). Therefore, both sulfurs are in the +2 state.

Step 4: Calculate Oxidation States in Na2S4O6

For \( \mathrm{Na}_2\mathrm{S}_4\mathrm{O}_6 \), sodium is +1 (total +2). Oxygen is -2, therefore six oxygens give -12. Let the oxidation states of sulfurs be \(x_1\) and \(x_2\). Assume two sulfurs are \(x_1\) and the other two are \(x_2\):\[ 2(+1) + 2x_1 + 2x_2 + 6(-2) = 0 \]\( 2 + 2x_1 + 2x_2 - 12 = 0 \) yields \( x_1 + x_2 = +5 \). Typically, in polythionates like this, x-values are +2 and +5. Consequently, this compound contains sulfurs in different oxidation states.

Step 5: Calculate Oxidation States in K2S2O8

In \( \mathrm{K}_2\mathrm{S}_2\mathrm{O}_8 \), potassium is +1 (total +2). Oxygen gives -16 because each is -2. The equation for sulfurs is: \[ 2(+1) + 2x + 8(-2) = 0 \]This simplifies to \( 2 + 2x - 16 = 0 \) which results in \( 2x = 14 \), yielding \( x = +7 \). Therefore, both sulfurs are in the +7 state.

Key Concepts

Sulphur Compounds

Sulphur compounds are chemical substances that contain the element sulphur. This element is known for its versatility since it can form a wide variety of compounds with different oxidation states. In sulphur compounds, the oxidation state of the sulphur atom can vary, which impacts the chemical behavior and properties of the compound. Understanding the oxidation states is crucial for predicting the interaction of the compounds with other substances.

• Sulphur can exhibit a range of oxidation states from -2 to +6. Common oxidation states include -2, 0, +2, +4, and +6.
• The compounds discussed here include combinations with oxygen and alkali metals, where oxygen typically has an oxidation state of -2 and metals like potassium (K) and sodium (Na) have an oxidation state of +1.
• In neutral compounds like K extsubscript{2}S extsubscript{2}O extsubscript{7} and K extsubscript{2}S extsubscript{2}O extsubscript{8}, the total charge is zero, guiding us to find the specific oxidation states of sulphur.

Polythionates

Polythionates are a class of sulphur compounds that contain more than two sulphur atoms in their structure. They are characterized by the presence of multiple oxidation states of sulphur. The characteristic feature of these compounds is their chain-like structure, which can lead to a combination of different sulphur oxidation states that confer unique properties to the compound.

• For instance, in the compound Na extsubscript{2}S extsubscript{4}O extsubscript{6}, there are four sulphur atoms that can have different oxidation states.
• This compound typically consists of two sulphur atoms having one oxidation state, and the other two sulphur atoms having a different oxidation state.
• In the provided example, the oxidation states +2 and +5 are seen, showcasing sulphur's ability to maintain different oxidation levels within the same molecule.

This unique property of accommodating multiple oxidation states allows polythionates to engage in complex chemical reactions and makes them subject to study in various chemical applications. The variance in oxidation states directly impacts the reactivity and stability of these compounds.

Neutral Compounds

Neutral compounds are chemical species that have no overall charge. In these compounds, the sum of the oxidation states of all atoms equals zero. This condition is critical for balancing chemical equations and deducing the correct oxidation states of the elements within the compound.

• An important rule in deducing oxidation states in neutral compounds is knowing that the algebraic sum of all the oxidation states should balance to zero.
• In compounds like Na extsubscript{2}S extsubscript{2}O extsubscript{3}, K extsubscript{2}S extsubscript{2}O extsubscript{8}, and others, we leverage the known oxidation states of commonly occurring elements like oxygen (-2) and alkali metals (+1) to calculate those of sulphur.
• Understanding the neutral charge condition helps ensure the correct calculation and characterization of each elemental component's oxidation state.

By mastering these concepts, students can effectively analyze various neutral compounds, predict their chemical behavior, and ascertain the roles of different atoms within the structure. This understanding is fundamental in fields such as chemistry and material sciences.