Q.Acidic hydrogen is present
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Acidity Trend Order — From Intuition to Precision
Imagine you have two acids: HCl and HF. Both can donate a proton (H⁺). But which one does it more easily? That's the core question — acidity measures how readily an acid gives away its proton.
The stronger the acid, the more completely it dissociates in water. HCl dissociates almost completely; HF barely does. So HCl is a stronger acid than HF.
Now, if you look at the periodic table, you'll see a clear pattern in how acidity changes as you move across a row or down a column. That pattern is the acidity trend order.
The Intuition: What Makes a Proton Easy to Lose?
Think of the acid H–A, where A is some atom or group. The bond between H and A must break for the proton to leave. Two things matter:
- Bond strength — a weaker H–A bond breaks more easily.
- Stability of the conjugate base (A⁻ after losing H⁺) — a more stable A⁻ means the acid is more willing to give up its proton.
The second factor usually dominates. A stable conjugate base spreads out the negative charge, making it less eager to grab back the proton.
The Precise Statement: Acidity Trend on the Periodic Table
Across a period (left to right): Acidity increases.
Down a group (top to bottom): Acidity increases.
Let's see why.
Across a Period: Left to Right
Consider the hydrides of period 2: CH₄, NH₃, H₂O, HF.
| Acid | Conjugate base | Acidity |
|---|---|---|
| CH₄ | CH₃⁻ | weakest |
| NH₃ | NH₂⁻ | |
| H₂O | OH⁻ | |
| HF | F⁻ | strongest |
Why does acidity increase? As you move right, the atom A becomes more electronegative. It pulls electron density away from the H–A bond, weakening it. More importantly, the conjugate base A⁻ becomes more stable because the negative charge is held more tightly by the electronegative atom. A stable conjugate base means a stronger acid.
This trend is about binary acids (H–A). For oxyacids (H–O–X), the trend is different — we'll get to that.
Down a Group: Top to Bottom
Consider the hydrogen halides: HF, HCl, HBr, HI.
| Acid | Bond strength (kJ/mol) | Acidity |
|---|---|---|
| HF | 565 | weakest |
| HCl | 431 | |
| HBr | 366 | |
| HI | 299 | strongest |
Here, bond strength dominates. As you go down, the atom A gets larger. The H–A bond becomes longer and weaker, so the proton leaves more easily. Even though electronegativity decreases (F is most electronegative), the bond weakening effect wins.
| A common mistake: thinking that HF is the strongest acid among the hydrogen halides because F is most electronegative. But bond strength matters more here — HF has the strongest bond, so it's actually the weakest acid.
The Full Picture: Two Key Trends …
A hydrogen attached to an sp carbon (terminal alkyne) is weakly acidic because of the high s-character of the C–H bond. Ethyne (HC≡CH) has such acidic hydrogens. …
Terminal-alkyne (sp) C–H is acidic; ethyne has acidic hydrogen — option (a).
From NCERT Class 11 Chemistry (Hydrocarbons): acidity of a C–H bond increases with the s-character of the carbon: sp (50%) > sp² (33%) > sp³ (25%). The sp carbon holds the electron pair closer, stabilising the anion. …
- CBSE 2024Set ANNUAL1 markMCQQ.Which of the following compounds is most Acidic?(a) C2H4(b) C2H6(c) C2H2(d) C2H8 (as printed on the paper; note this molecular formula is not chemically possible for a saturated/unsaturated C2 hydrocarbon -- likely a printing artifact, transcribed exactly as shown)
›Reveal solutionSolution
Acidic character of a hydrocarbon C-H bond rises with s-character of the carbon: sp (50% s) > sp2 (33% s) > sp3 (25% s). Ethyne (C2H2), with sp-hybridised carbons, is the most acidic among C2H4 (sp2, ethylene), C2H6 (sp3, ethane), and C2H2 (sp, acetylene).
Greater s-character means the bonding orbital holds electron density closer to the carbon nucleus, so the C-H bonding pair is more tightly held and the C atom is more electronegative -- making it easier for the molecule to release H+ (more acidic).
- C2H6 (ethane): sp3 carbons, least acidic.
- C2H4 (ethylene): sp2 carbons, intermediate.
- C2H2 (acetylene/ethyne): sp carbons, most acidic among these -- terminal alkynes can even be deprotonated by strong bases like NaNH2. …
- CBSE 2023Set ANNUAL1 markQ.Fill in the blank: Acetylene, when it reacts with ammoniacal silver nitrate, shows ____________ property.
›Reveal solutionSolution
Acetylene forming a precipitate with ammoniacal AgNO3 demonstrates the acidic nature of its terminal (triple-bond) hydrogen atoms.
Terminal alkynes (like acetylene/ethyne, HC=CH) have hydrogen atoms directly attached to an sp-hybridised carbon of the triple bond. Because sp carbons hold their bonding electrons closer to the nucleus (higher effective electronegativity than sp2 or sp3 carbons), the C-H bond in a terminal alkyne is more polarized, and that hydrogen is weakly acidic and can be replaced by a metal.
…
- CBSE 2023Set ANNUAL1 markQ.Write the best reagent to distinguish between Propene and Propyne.
›Reveal solutionSolution
Ammoniacal silver nitrate distinguishes propyne (forms a white ppt of silver propynide) from propene (no reaction), because only propyne has an acidic terminal alkyne hydrogen.
Propyne (CH3-C=CH) is a terminal alkyne — it has a hydrogen atom directly attached to the sp-hybridised carbon of the triple bond, which is weakly acidic and can be replaced by a metal ion.
When propyne gas is passed through ammoniacal silver nitrate solution ([Ag(NH3)2]+ ), this acidic hydrogen is replaced by Ag+, giving a white/pale-yellow precipitate of silver propynide: CH3-C=CH + [Ag(NH3)2]+ -> CH3-C=C-Ag (ppt) + NH4+ + NH3.
…
- CBSE 2022Set ANNUAL1 markMCQQ.Acidic hydrogen is present(a) Ethyne(b) Ethene(c) Benzene(d) Ethane
›Reveal solutionSolution
Terminal-alkyne (sp) C–H is acidic; ethyne has acidic hydrogen — option (a).
From NCERT Class 11 Chemistry (Hydrocarbons): acidity of a C–H bond increases with the s-character of the carbon: sp (50%) > sp² (33%) > sp³ (25%). The sp carbon holds the electron pair closer, stabilising the anion. …
- CBSE 2022Set ANNUAL1 markMCQQ.Which of the following exhibits acidic property?(a) CH3-C≡CH(b) CH3-C≡C-CH3(c) CH3-CH3(d) CH2=CH2
›Reveal solutionSolution
Terminal alkyne CH₃–C≡CH shows acidic character — option (a).
From NCERT Class 11 Chemistry (Hydrocarbons): a hydrogen on an sp-hybridised terminal alkyne carbon is weakly acidic and can be removed by strong bases / reactive metals.
- CH₃–C≡CH (propyne): terminal ≡C–H → acidic (gives an acetylide with Na/ammoniacal AgNO₃). …
- CBSE 2021Set annual31 markQ.Out of ethene and ethyne, which is more acidic in nature.
›Reveal solutionSolution
Ethyne is more acidic than ethene because the sp-hybridised carbon of ethyne has a higher percentage of s-character than the sp2-hybridised carbon of ethene, making its C-H bond more polar and its acidic hydrogen easier to release as H+.
Acidity of a C-H bond in hydrocarbons depends on the hybridisation of the carbon atom bonded to that hydrogen. The greater the s-character of the hybrid orbital, the closer the electron density is held to the carbon nucleus, the more electronegative that carbon behaves, and the more easily the attached hydrogen is released as a proton.
- In ethyne (HC≡CH), the carbon atoms are sp hybridised (50% s-character).
- In ethene (H2C=CH2), the carbon atoms are sp2 hybridised (33% s-character).
- In ethane (H3C-CH3), the carbon atoms are sp3 hybridised (25% s-character). …
- CBSE 2020Set annual1 markQ.Out of ethylene and acetylene which is more acidic and why?
›Reveal solutionSolution
Acetylene is more acidic than ethylene because its sp-hybridised carbon (50% s-character) is more electronegative than ethylene's sp2 carbon (33% s-character), so the C-H bond in acetylene is more polarised and releases H+ more readily.
Acidity of a C-H bond depends on how tightly the carbon atom holds the bonding electron pair - the more electronegative (s-rich) the carbon, the more it pulls electron density towards itself, weakening/polarising the C-H bond and making it easier to lose H+.
- In ethylene (H2C=CH2), the carbons are sp2 hybridised: 33% s-character.
- In acetylene (HC≡CH), the carbons are sp hybridised: 50% s-character, making sp carbon more electronegative than sp2 carbon (which in turn is more electronegative than sp3 carbon). …
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