Chapter 3: Acids & Bases

What is a Bronsted-Lowry acid?

A substance that donates a proton (H⁺)

What is a Bronsted-Lowry base?

A substance that accepts a proton

In a Bronsted-Lowry reaction, what always moves?

A proton (H⁺) moves from the acid to the base

What is a conjugate acid?

The species formed when a base gains a proton (H⁺)

What is the conjugate base?

The species formed when an acid loses a proton (H⁺)

How can you quickly identify the acid and base in a reaction?

• Find the H⁺

• Who loses H⁺ = acid

• Who gains H⁺ = base

An example of acid and base

Label the acid, base, and the conjugates in the reaction below

base

What do curved arrows represent in reaction mechanisms?

Curved arrows show the movement of electron pairs that result in bond breaking and bond forming during a chemical reaction

base, acid

How do strong acids differ from weak acids?

Strong acids fully dissociate in solution, while weak acids partially dissociate

What does pk_a measure?

• measures acid strength

• Lower pka= stronger acid

What is the relationship between Ka and pKa?

They are inversely related:

• Large Ka → small pKa → strong acid

How much stronger is an acid with a pKa that is 1 unit lower?

It is 10x stronger

• if it goes down by 2 its acidity goes up 100X

• if it goes down by 3 it s acidity goes up 1000x

In an acid base reaction, which direction is favored?

Reactions favor formation of the weaker acid and more stable conjugate base

What is quantitative acid strength analysis?

Comparing pKa values to determine which acid or base is stronger

What is qualitative acid strength analysis?

Comparing structural stability (resonance, induction, hybridization, atom size) when pKa values are unavailable

What makes a conjugate base more stable?

• Resonance delocalization

• Higher electronegativity

• Larger atom size

• Greater s-character (sp > sp² > sp³)

How is base strength related to conjugate acid pKa?

Stronger base ↔ conjugate acid has higher pKₐ.

Name the strong acids

• HCl, HBr, HI

• H₂SO₄, HNO₃, HClO₄

The conjugate base of a strong acid is

weak

The conjugate base of a weak acid is?

strong

e

ask Erick

c

What determines the strength of an acid?

The stability of its conjugate base

More stable conjugate base → stronger acid

What does ARIO stand for in acidity analysis?

Atom, Resonance, Induction, Orbital

When should ARIO be used instead of pka values?

When pKa values are not given or not easily known

What does A (Atom) mean in ARIO?

• The type of atom holding the negative charge

• More electronegative or larger atoms stabilize charge better

When do you use electronegatively in acidity comparisons?

When the negative charge is on different atoms in the conjugate bases

What does R (Resonance) mean in ARIO?

The ability to delocalize negative charge through resonance structures

Why does resonance increase acidity?

Resonance spreads out negative charge, stabilizing the conjugate base

What does I (Induction) mean in ARIO?

Stabilization of charge through electron-withdrawing effects of nearby electronegative atoms via sigma bonds

How is induction different from electronegativity?

• Electronegativity applies when the atom holds the charge

• Induction applies when nearby atoms influence the charge

What does O (Orbital) mean in ARIO?

The hybridization of the atom holding the charge

More s- character = more stable negative charge

Rank the stability for negative charge

sp> sp²>sp⁵

Which ARIO factor usually has the greatest effect

Resonance

What is the correct order to apply ARIO?

Atom → Resonance → Induction → Orbital

Explain this

Ethanol has the more conjugate base because the negative charge resides on oxygen rather than carbon

Why must a solvent not react in an acid/base reaction?

Because if the solvent reacts, it interferes with the intended acid/base chemistry.

Why is water a special case as a solvent in acid/base reactions?

Because water can act as both an acid and a base.

What is the leveling effect of water?

Water “levels” strong acids to H₃O⁺ and strong bases to OH⁻, making them appear equally strong.

Why can acids stronger than H₃O⁺ not be used in water?

Because water immediately converts them into H₃O⁺.

Why can bases stronger than OH⁻ not be used in water?

Because water immediately converts them into OH⁻.

When should water NOT be used as a solvent?

When studying or using very strong acids or bases, due to the leveling effect.

What is a Lewis acid?

A substance that accepts a pair of electrons

What is a Lewis base?

A substance that donates a pair of electrons

What do Lewis acids and bases share when they react?

They share an electron pair

Are all Brønsted–Lowry acids also Lewis acids?

Yes.

Are all Brønsted–Lowry bases also Lewis bases?

Yes.

Why is the Lewis definition broader than Brønsted–Lowry?

Because it includes reactions without proton transfer, only electron sharing.