Nitric acid is an oxidising agent and reacts with PbO but it does not react with PbO2. Explain why?

NCERT Exemplar · Q20

Q. Nitric acid is an oxidising agent and reacts with PbO but it does not react with PbO2. Explain why?

Rajasthan RbseShort· 2mImportance★★★★★est

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Concept understanding — Oxidation State Stability

Oxidation State Stability: The First Meeting

Imagine you have a pile of Lego bricks. Some colours click together easily; others keep falling apart. An atom's oxidation state is like the colour of a Lego brick — it tells you how many electrons the atom has lost (positive state) or gained (negative state) compared to its neutral form. But not all oxidation states are equally happy. Some are rock-solid; others are fragile and want to change.

That's oxidation state stability: why some oxidation states of an element are comfortable and long-lasting, while others are restless and reactive.

The Intuition: Why would an atom "prefer" one state over another?

An atom wants to be stable. In chemistry, stability usually means one of two things:

A full outer shell — like noble gases (8 electrons in the outermost shell, or 2 for helium). Atoms will gain, lose, or share electrons to get there.
A half-filled or fully-filled subshell — this is a special bonus stability. For example, chromium loves having a $3d^5$ configuration (half-filled d-subshell) even if it means sacrificing a 4s electron.

So when an atom takes on an oxidation state, it's essentially deciding how many electrons to give away or accept. The "best" oxidation states are those that leave the atom with a stable electronic configuration.

Note

This is why sodium (Na) almost always shows +1 — losing one electron gives it the same electron configuration as neon (a noble gas). It's not trying to be fancy; it's trying to be comfortable.

The Precise Statement

Oxidation state stability refers to the tendency of an element to exist in a particular oxidation state under given conditions (temperature, pH, presence of other substances). An oxidation state is stable if:

The atom's electronic configuration after gaining/losing electrons is noble-gas-like (octet) or has a half-filled/full-filled d or f subshell.
The energy required to reach that state is low compared to other possible states.
The state does not spontaneously change into another state (e.g., by reacting with air, water, or itself).

Stability order (general trend): For main-group elements: oxidation states that give a noble gas configuration are most stable. For transition metals: +2 and +3 are common, but stability varies with the element and the environment.

Examples to cement the idea

Element	Common stable states	Why?
Sodium (Na)	+1	Losing 1 electron → Ne configuration (2,8)
Chlorine (Cl)	-1	Gaining 1 electron → Ar configuration (2,8,8)
Iron (Fe)	+2, +3	Both leave d-subshell partially filled; +3 is more stable in acidic conditions
Manganese (Mn)	+2, +7	+2 is stable (half-filled d⁵); +7 is stable in permanganate ion (MnO₄⁻) due to strong bonding with oxygen
Carbon (C)	+4, -4	Both give noble gas configuration (He for +4? No — careful: carbon's +4 means losing 4 electrons, leaving 1s², which is He-like. -4 means gaining 4 electrons, giving Ne-like 2,8)

Watch out

Don't confuse "common" with "stable". Carbon's +4 is common but not always stable — CO₂ is stable, but CCl₄ is not very stable in water. Stability depends on the compound, not just the element.

The deeper reason: Why do some states "disproportionate"?

Some oxidation states are so unstable that the atom reacts with itself. This is called disproportionation — one atom in an intermediate state splits into two atoms: one higher, one lower.

Example: Copper(I) in water. Cu⁺ (oxidation state +1) is unstable in aqueous solution. It spontaneously does this:

2\text{Cu}^+ \rightarrow \text{Cu}^{2+} + \text{Cu}

One Cu⁺ gets oxidised to Cu²⁺ (more stable in water), the other gets reduced to Cu metal (even more stable). The +1 state was disproportionation-unstable.

Tip

A quick check: If an element has three consecutive oxidation states (say +1, +2, +3), the middle one (+2) is often prone to disproportionation if the +1 and +3 states are both stable. This is common for elements like copper, mercury, and thallium.

What determines stability in practice? …

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PbO is a basic oxide with lead in its stable +2 state, so it undergoes a simple

acid–base reaction with nitric acid. In PbO₂ lead is already at +4 — its highest

state — so an oxidising acid like HNO₃ has nothing to oxidise and no reaction

occurs: PbO₂ stays passive.

The two oxides are chemically different animals

Lead forms two common oxides. In PbO lead is in the $+2$ state — the stable state

for lead (the inert pair effect makes +2 more stable than +4 down Group 14). In

PbO₂ lead is in the $+4$ state, which for lead is both the highest state it

reaches and a strongly oxidising one (it wants to gain electrons and fall back to

+2).

Why HNO₃ reacts with PbO

PbO is a basic oxide. Nitric acid is an acid, so the two meet in a plain

acid–base (neutralisation) reaction — no oxidation states change at all:

2\text{PbO} + 4\text{HNO}_3 \rightarrow 2\text{Pb(NO}_3)_2 + 2\text{H}_2\text{O} \quad \text{(acid–base)}

Lead enters as +2 and leaves as +2 (in lead nitrate). The fact that HNO₃ can act as

an oxidising agent is irrelevant here — there is nothing in PbO that needs oxidising

for salt formation, and nothing HNO₃ can reduce.

Why HNO₃ does not react with PbO₂

For a reaction with PbO₂, one of two things would have to happen:

HNO₃ oxidises PbO₂ — impossible: lead in PbO₂ is already at +4, the maximum oxidation state lead attains. There is no higher state to be oxidised into.
PbO₂ oxidises something in the mixture — but nitric acid is itself an oxidising agent (nitrogen is at +5, its own maximum). Unlike HCl, it offers no oxidisable species (chloride, say) for PbO₂ to attack.

With neither direction available, no redox happens; and since PbO₂ is not a basic

oxide (it does not behave as a base towards HNO₃), no acid–base reaction happens

either. PbO₂ remains passive towards nitric acid. …

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Q.Nitric acid is an oxidising agent and reacts with PbO but it does not react with PbO2. Explain why?

The two oxides are chemically different animals

Why HNO₃ reacts with PbO

Why HNO₃ does not react with PbO₂

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