Strength of Acids and bases

A substance that ionizes completely in water to form H₃O+ ions (H+ ions).

E.g. HNO3(aq) + H2O(l) → H3O+ (aq) + NO− 3 (aq)

⇒ A one way arrow is used since this reaction is complete, all of the HNO3 molecules break apart to form H3O+ and NO3 - ions. (Thus, all of the reactants are converted to form products.)

KNOW 7 Strong acids! Memorize these! HCl, HBr, HI, HNO3 , H2SO4 , HClO4 , HCl

An acid that ionizes only to a small extent in water.

E.g. HC₂H₃O₂(aq) + H₂O(l) ↔H₃O⁺ (aq) + C₂H₃O₂ ^- (aq)

⇒ A two-way arrow is used since this reaction is not complete but instead consists of an equilibrium mixture of HC₂H₃O_2,, H₃O+ and C₂H₃O₂ ^-

⇒ Equilibrium lies to the left since most of the acid molecules have not ionized.

examples: ethanoic acid (vinegar), citric acid (citrus fruits)

HF, HNO2 , HCN, H3PO4 , H2CO3 , carboxylic acids like CH3COOH and HCOOH

This is a substance that ionizes completely in water to form OH- ions.

_H2O

E.g. NaOH(s) → Na+ (aq) + OH- (aq)

⇒The reaction goes to completion – all of the Na+ and OH- ions have dissociated from the solid lattice and no undissociated NaOH(s) remains.

These Group 1A & 2A hydroxides (except Be): LiOH, NaOH, KOH, RbOH, CsOH, Ca(OH)2 , Ba(OH)2 , Sr(OH)2

Memorize the 8 SB’s! 

This is a base that ionizes only to a small extent in water.

NH₃(aq) + H₂O(l) ↔ NH₄⁺ (aq) + OH^- (aq)

⇒ Equilibrium lies to the left since most of the ammonia molecules have not accepted protons to form NH + 4 and OH- ions.

Examples include: ammonia, bleach

This can be determined based on the reaction involved or can be measured on a pH scale.

A strong acid will have a pH nearer to 1 while a weak acid has a pH closer to 7

A strong alkali (base in solution) has a pH of closer to 14 while a weak alkali has a pH closer to 7

Remember, pH 7 is neutral! pH less than 7 is acidic and greater than 7 is alkaline!

Weak acids and bases can be used to make buffers.

A buffer is a solution that is resistant to large changes in pH and so are added to solutions (such as medicines) to make them more stable.

Buffers can be prepared by mixing a weak acid and its salt or a weak base and its salt. .

Because a buffer can react with both an acid and a base, its pH remains relatively constant

It is important to understand the difference between concentration and strength.

Concentration is the amount of solute dissolved in a given amount of solution. Strength refers to the solute’s tendency to form ions in water.

You cannot assume that a strong acid has a low pH, because its concentration also affects pH.

For instance, a dilute solution of HCl (a strong acid) can have a pH of 6. But a concentrated solution of acetic acid (a weak acid) can have a pH of 3.