Which of the followings are Lewis acids? H2O, BF3, H+, and NH4 +

Exercises · 6.36

Q. Which of the followings are Lewis acids? H2O, BF3, H+, and NH4 +

Puducherry CbseNCERTSubjective· 2mImportance★★★★★est

41% · 64/155 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Brønsted Lowry Conjugate Pairs

From Intuition to Precision: Brønsted–Lowry Conjugate Pairs

Imagine you're at a party where people are passing around a single, very special coin. The coin represents a proton ( $\text{H}^+$ ). The game is simple: someone can give the coin to someone else, but only if the other person is willing to take it. You can't just throw the coin; you need a willing receiver.

In chemistry, acids and bases play exactly this game. An acid is the person who gives away the proton. A base is the person who accepts it. But here's the twist: the moment the acid gives away its proton, it transforms into something new — something that can now accept a proton back. That transformed form is called the conjugate base. Similarly, the base, after accepting the proton, becomes something that can donate it back — the conjugate acid.

This is the core of the Brønsted–Lowry theory: every acid-base reaction involves a pair of substances that are linked by the gain or loss of a single proton.

The Precise Statement

\text{Acid} \rightleftharpoons \text{Conjugate Base} + \text{H}^+

A conjugate acid-base pair consists of two species that differ by exactly one proton ( $\text{H}^+$ ). The acid has the proton; the conjugate base does not.

When an acid donates a proton, it becomes its conjugate base.

When a base accepts a proton, it becomes its conjugate acid.

Important

In any Brønsted–Lowry reaction, there are two conjugate pairs: one on the reactant side and one on the product side. They always appear together.

Seeing It in Action

Take the classic reaction between hydrochloric acid and water:

\text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+

Let's identify the pairs:

Pair 1: $\text{HCl}$ (acid) and $\text{Cl}^-$ (conjugate base). They differ by one $\text{H}^+$ .
Pair 2: $\text{H}_2\text{O}$ (base) and $\text{H}_3\text{O}^+$ (conjugate acid). They also differ by one $\text{H}^+$ .

Notice: water acted as a base here — it accepted the proton from HCl. But water can also act as an acid in other reactions. That's the beauty of the Brønsted–Lowry theory: a substance's role depends on the reaction, not on a fixed label.

Tip

To find the conjugate base of any acid, simply remove one $\text{H}^+$ and reduce the charge by +1.

To find the conjugate acid of any base, add one $\text{H}^+$ and increase the charge by +1.

A Quick Reference Table

Acid	Conjugate Base	Base	Conjugate Acid
$\text{HCl}$	$\text{Cl}^-$	$\text{NH}_3$	$\text{NH}_4^+$
$\text{H}_2\text{SO}_4$	$\text{HSO}_4^-$	$\text{H}_2\text{O}$	$\text{H}_3\text{O}^+$
$\text{NH}_4^+$	$\text{NH}_3$	$\text{OH}^-$	$\text{H}_2\text{O}$
$\text{H}_2\text{O}$	$\text{OH}^-$	$\text{CO}_3^{2-}$	$\text{HCO}_3^-$

Notice how water appears in both columns — it's amphoteric, meaning it can act as either an acid or a base depending on its partner.

Why This Matters …

Start your 14-day free trial to unlock the full solution →

A Lewis acid accepts an electron pair. Of the four species, BF₃ and H⁺ are the Lewis acids — BF₃ through boron's empty p-orbital and H⁺ through its empty 1s orbital. H₂O is a Lewis base (electron-pair donor), and NH₄⁺ — with nitrogen's octet complete and no vacant low-energy orbital — cannot accept an electron pair.

The Lewis Acid Concept

The Lewis definition broadens our view of acids beyond proton transfer. A Lewis acid is any species that accepts an electron pair, while a Lewis base donates an electron pair. This framework explains reactions that don't involve H⁺ at all—like the formation of complex ions or coordination compounds.

The key question for each species: Does it have room to accept electrons, either through an empty orbital or by breaking a bond to make space?

Let's examine each candidate:

Analysis of Each Species

H₂O (Water)

Water has two lone pairs on oxygen. In nearly every reaction, water donates one of these pairs—acting as a Lewis base. For example, when water coordinates to a metal ion like Cu²⁺, it donates electrons into the metal's empty orbitals.

Could water ever accept electrons? Only in extremely unusual circumstances where the oxygen would need to accommodate more than an octet, which violates its normal bonding behavior. In standard chemistry, water is a Lewis base.
BF₃ (Boron trifluoride)

Boron in BF₃ has only six valence electrons (three bonds to fluorine). It's electron-deficient with an empty p-orbital. This makes BF₃ a textbook Lewis acid. The classic example:

\ce{BF3 + :NH3 -> F3B-NH3}

Ammonia donates its lone pair into boron's empty orbital, forming a coordinate covalent bond.

H⁺ (Proton)

A bare proton has no electrons at all—just an empty 1s orbital. It's the ultimate electron acceptor. Every Brønsted acid-base reaction is also a Lewis acid-base reaction:

\ce{H+ + :NH3 -> NH4+}

The proton accepts ammonia's electron pair. H⁺ is always a Lewis acid.

NH₄⁺ (Ammonium ion) …

Start your 14-day free trial to unlock the full solution →

←Previous question · EXERCISES6.35 — What is meant by the conjugate acid-base pair? Find the conjugate acid…→Next question · EXERCISES6.37 — What will be the conjugate bases for the Brönsted acids: HF, H2SO4 and…

Q.Which of the followings are Lewis acids? H2O, BF3, H+, and NH4 +

Concept understanding — Brønsted Lowry Conjugate Pairs

From Intuition to Precision: Brønsted–Lowry Conjugate Pairs

The Precise Statement

Seeing It in Action

A Quick Reference Table

Why This Matters …

Concept: Lewis Acid Definition

The Lewis Acid Concept

Analysis of Each Species

Q.Which of the followings are Lewis acids? H2O, BF3, H+, and NH4 +

The Lewis Acid Concept

Analysis of Each Species

🎓Unlock everything free for 14 days

Unlock everything free for 14 days