Name: what makes one acid stronger?
Uploaded: 2025-05-22T09:05:33.000Z
Duration: 2 min 34 s
Channel: JoVE

The strength of an acid depends on the stability of its conjugate anion. A stable anion is a weak base, while its corresponding acid is strong.
Several factors influence the stability of the anions.
Consider two anions containing atoms from the same column. The bigger atom spreads the negative charge over a larger space volume, which makes its anion more stable, and the corresponding acid stronger.&#160;&#160;&#160;&#160;
When anions containing atoms in the same row are compared, the electronegativity of the atom carrying the charge dominates the anion&#8217;s stability.
The more electronegative atom stabilizes the negative charge to give a stable base and hence, a strong acid.
If anions with the negative charge on the same atom are compared, their stability depends on the resonating structures.&#160; &#160;
For example, the ethoxide ion has no resonance structure, but the methanesulfonate ion has three in which the charge is delocalized over three atoms.&#160;
As such, methanesulfonic acid &#8212; with a resonance-stabilized conjugate base &#8212; is a stronger acid than ethanol.
An electronegative substituent, when placed adjacent to the negatively charged region, withdraws electron density from that region via induction, thereby s

molecular structure and acidity

Molecular Structure and Acidity

An acid can be deprotonated to form a conjugate base or an anion. If the produced anion is more stable, then the acid is stronger. On the contrary, if the anion is unstable, then the acid is weaker. Hence, to determine the acidity of the compound, the stability of its conjugate base is studied using various factors.
The size effect explains the change in atomic size on acidity. When comparing the acids formed from elements that belong to the same column in the periodic table, their atomic sizes are compared. Compounds, which contain an element with a larger atomic size can stabilize a negative charge better by spreading it over a larger space volume. Hence, these compounds correspond to the stronger acid.
Further, consider compounds formed with elements that belong to the same row in a periodic table. In such cases, the electronegativity of the element dictates the acidity. The more electronegative element forms the stable anion that corresponds to the stronger acid. This is called the charge effect.
If the compounds having the negative charge on a similar atom are compared, another factor, called resonance, determines the acidity. The compound with more stabilizing resonance structures is more acidic. To determine the acidity of compounds having an equal amount of resonance, the induction effect is analyzed. In this effect, the presence of electronegative elements at varying distances is used to identify the strength of the acid. The compound with an electronegative element closer to the acidic hydrogen is determined more acidic.
Hybridization is used to help analyze the acidity in compounds with no resonance, no electronegative elements, and with the negative charge on a similar atom, as, for example, an alkane, alkene, and alkyne. In an alkyne, the carbon atom has an sp orbital with a 50 % s character. In an alkene, the carbon atom has a 33.3 % s character in its sp2 orbital, and the carbon with a single bond has a 25 % s character in its sp3 orbital. A higher s character means that electrons are closer to the nucleus. Therefore the negative charge of the conjugate base can be more stabilized, making alkynes the most acidic compare to an alkene or alkane.

An acid can be deprotonated to form a conjugate base or an anion. If the produced anion is more stable, then the acid is stronger. On the contrary, if the ...

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Organic Chemistry

Unit 1

Acids and Bases

KEY TERMS AND CONCEPTS

what makes one acid stronger?

Acid strength depends on how stable the conjugate base is after the acid loses a proton. When deprotonation forms a more stable anion, the acid is stronger. If the anion is unstable, the acid is weaker. Chemists compare several structure-based factors to judge that stability.

One factor is size. When acids come from elements in the same column of the periodic table, atomic size is compared. A larger atom can spread out a negative charge over a bigger space. That makes the conjugate base more stable and the acid stronger.

Another factor is electronegativity, also called the charge effect in this context. When compounds are formed from elements in the same row of the periodic table, the more electronegative element holds negative charge better. That produces a more stable anion and a stronger acid.

Resonance also matters when the negative charge is on a similar atom in different compounds. The compound with more stabilizing resonance structures is more acidic. If two compounds have equal resonance, induction is used next. In induction, electronegative atoms at different distances affect acidity, and the atom closer to the acidic hydrogen usually makes the acid stronger.

Hybridization helps compare compounds with no resonance and no electronegative elements, such as an alkane, alkene, and alkyne. The carbon in an alkyne has an sp orbital with 50% s character. The carbon in an alkene has 33.3% s character in an sp2 orbital, and the carbon with a single bond has 25% s character in an sp3 orbital. Higher s character keeps electrons closer to the nucleus, which stabilizes the conjugate base. For that reason, alkynes are more acidic than alkenes or alkanes.

What Makes One Acid Stronger?

br&#248;nsted-lowry acids and bases

Proton Transfer in Brønsted-Lowry Reactions

Brønsted-Lowry acids and bases are defined by proton transfer in a chemical reaction. In this model, a Brønsted acid donates a proton and becomes its conjugate base. A Brønsted base accepts a proton and becomes its conjugate acid. This theory was proposed in 1923 by Johannes Brønsted and Thomas Lowry. It helps explain acid-base reactions by tracking how protons move between species. The electron shifts that drive these transfers are shown with curved arrows. A reaction between acetic acid and...

lewis acids and bases

Electron-Pair Reactions in Lewis Theory

Lewis theory explains acid-base reactions through electron pairs, especially for electron-deficient compounds that do not fit the Brønsted definition. It is often used to describe group 13 elements such as boron and aluminium. These elements have three valence electrons, so their trivalent compounds can have a sextet of electrons and an empty orbital at the central atom. Because of that electron shortage, these compounds can accept electrons from another species to complete their octet.

acid and bases: ka, pka, and relative strengths

acid-and-bases-ka-pka-and-relative-strengths

Comparing Acid and Base Strength with pKa

Acid and base strength can be compared using Ka and pKa. These values help show how much an acid dissociates in water and how strong a base is through its conjugate acid. Acid strength depends on dissociation into a conjugate base and hydronium ion in water. A strong acid dissociates almost completely, so the hydronium ion concentration is very high. A weak acid dissociates only partly, so more acid remains undissociated and the equilibrium stays farther back. For dilute weak acids, the...

position of equilibrium in acid-base reactions

position-of-equilibrium-in-acid-base-reactions

Predicting Acid-Base Equilibrium Direction

Acid-base equilibrium direction depends on acid strength and pKa values. A lower pKa means a stronger acid, while a higher pKa means a weaker acid. A negative pKa shows a stronger acid, and a positive pKa shows a weaker acid. In the reaction between ammonia and an ethoxide ion, ethanol has a pKa of 15.9 and ammonia has a pKa of 38. That means ethanol is the stronger acid in this pair. A strong acid forms a weak conjugate base, and a weak acid forms a strong conjugate base, so the ethoxide ion...

Acid strength depends on how stable the conjugate base is after the acid loses a proton. When deprotonation forms a more stable anion, the acid is stronger. If the anion is unstable, the acid is weaker. Chemists compare several structure-based factors to judge that stability. One factor is size. When acids come from elements in the same column of the periodic table, atomic size is compared. A larger atom can spread out a negative charge over a bigger space. That makes the conjugate base more...

leveling effect and non-aqueous acid-base solutions

leveling-effect-and-non-aqueous-acid-base-solutions

Solvent Effects on Strong Acids and Bases

Solvent effects can change how strong acids and bases react in a chemical system. The leveling effect is one important example. It shows that a solvent can take part in the reaction before the acid and base react with each other. For a generic acid-base reaction, a generic acid (HA) reacts with a generic base (B−) to form the conjugate base (A−) and the conjugate acid (HB). In a solvent called HX, the solvent may also react if it is stronger than one of the reactants. If HA is weaker than the...

solvating effects

How Solvation Changes Acidity

Solvation changes acidity by affecting how stable a conjugate base is. The strength of an acid is commonly compared with its pKa value. When an ion dissolves, solvent molecules surround it and help stabilize it through solvation. Solvation of ions can happen in three main ways. In donor interaction, the solvent donates unshared electron pairs to the ion. The solvent acts as a Lewis base, and the ion acts as a Lewis acid. In charge-dipole interaction, polar solvent molecules align their partial...

key terms and concepts

Learn how conjugate base stability, size, electronegativity, resonance, induction, and hybridization help compare acid strength in high school chemistry.

What Makes One Acid Stronger? | JoVE K12

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What Makes One Acid Stronger?

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