Chapter 18 – Acids, Bases, and Salts

Arrhenius (ah-ray-nee- uhs) definition Acid – substance that dissociates in water to produce hydrogen ions (H + ) Acid – substance that dissociates in water to produce hydrogen ions (H + ) Base – a substance that dissociates in water to produce hydroxide ions (OH - ions) Base – a substance that dissociates in water to produce hydroxide ions (OH - ions)

Bronsted – Lowry definition Acid – is any substance that donates a hydrogen ion (a proton) Acid – is any substance that donates a hydrogen ion (a proton) Base – any substance that accepts hydrogen ions Base – any substance that accepts hydrogen ions This is a broader definition This is a broader definition

A hydrogen ion is a single proton Substances that can accept a proton (H + ion) have a nonbonding pair of electrons that are used by the H + ion. (think electron-dot structures) Substances that can accept a proton (H + ion) have a nonbonding pair of electrons that are used by the H + ion. (think electron-dot structures) HCl + H 2 O → H 3 O + + Cl - HCl + H 2 O → H 3 O + + Cl - Acid base hydronium ion Acid base hydronium ion Show the electron-dot of this process

NH 3 + H 2 O NH OH - base acid conjugate conjugate acidbase Show electron-dot of this process

Strong acid – near complete dissociation HCl + H 2 O → H 3 O + + Cl - Acid base conjugate conjugate acidbase

Weak acid – very little dissociation HC 2 H 3 O 2 + H 2 O H 3 O + + C 2 H 3 O %.4% 99.6%.4%

Strong Base (lime CaO) CaO + H 2 O → Ca OH - Show electron – dot of this process Ca +2 is called a spectator ion because it does not participate in the reaction (it just “watches” the water and oxygen ion react.

For strength of conjugate acid-base pairs see page 608 For strength of conjugate acid-base pairs see page 608 Note – H 2 O is a weak acid and a weak base. Note – H 2 O is a weak acid and a weak base.

Diprotic Acid – two acidic protons per molecule H 2 SO 4 + H 2 O → H 3 O + + HSO 4 -1 Acid base conj. acid conj. Base HSO H 2 O → H 3 O + + SO 4 -2 Acid base conj. acid conj. Base

The Acid Dissociation Constant HA aq + H 2 O l H 3 O + aq + A - aq This is an equilibrium – The forward reaction equals the rate of the backward reaction so the concentration of all ions remains constant (K eq is equilibrium constant) [H 3 O] [A - ] [H 3 O] [A - ] K eq = [HA] [H 2 O] [HA] [H 2 O]

[H 3 O] [A - ] [H 3 O] [A - ] K eq = [HA] [H 2 O] [HA] [H 2 O] There is so much more H2O than anything else, the water concentration does not change much (% change in [H 2 O]) [H 3 O] [A - ] [H 3 O] [A - ] K eq [H 2 O] = [HA] [HA] Water can be included as part of the constant, now called acid dissociation constant K a

[H 3 O] [A - ] [H 3 O] [A - ] K a = [HA] [HA] The greater the Ka, the stronger the acid See Ka values on page 610.

Base dissociation constant B + H 2 O HB + + OH - [HB + ] [OH - ] [HB + ] [OH - ] K b = [B] [B]

NH 3 + H 2 O NH OH - [NH 4 + ] [OH - ] [NH 4 + ] [OH - ] K b = = 1.8 x [NH 3 ](from page 611) [NH 3 ](from page 611) See example problem page 612 Assign practice problems 1,2 page 613

Acid-Base Properties of Salts Salts from strong acids and strong bases – will be neutral Salts from strong acids and strong bases – will be neutral NaOH + HCl → NaCl + H 2 O NaCl is neutral NaCl is neutral Salts from strong acids and weak bases – will be slightly acidic Salts from strong acids and weak bases – will be slightly acidic NH 3 + HCl → NH 4 Cl NH 3 + HCl → NH 4 Cl NH 4 Cl is slightly acidic NH 4 Cl is slightly acidic NH 4 Cl + H 2 O H 3 O + NH 3 + Cl - ) (Ka = 5.6 x ) (Ka = 5.6 x )

Salts from weak acids and strong bases – will be slightly basic Salts from weak acids and strong bases – will be slightly basic 2NaOH + H 2 CO 3 → Na 2 CO 3 + 2H 2 O 2NaOH + H 2 CO 3 → Na 2 CO 3 + 2H 2 O Na 2 CO 3 is slightly basic Na 2 CO 3 is slightly basic CO H 2 O HCO - + OH - K b = 1.8 x CO H 2 O HCO - + OH - K b = 1.8 x 10 -4

Acidic Hydrogen Not all hydrogen can dissociate in water. Not all hydrogen can dissociate in water. HCl + H 2 O → H 3 O + + Cl - HCl + H 2 O → H 3 O + + Cl - The negative ion (anion) left behind when H + leaves must be stable with the negative charge. (high electronegativity) Halogens work well The negative ion (anion) left behind when H + leaves must be stable with the negative charge. (high electronegativity) Halogens work well

Stabilization through resonance (the “spreading out” of a bond or charge) Alcohols are slightly acidic; but organic acids are more so (because of resonance) Alcohols are slightly acidic; but organic acids are more so (because of resonance) Compare electron dot of an alcohol to the electron dot showing resonance of a carboxylic acid. Compare electron dot of an alcohol to the electron dot showing resonance of a carboxylic acid. Resonance helps stabilize the negative charge Resonance helps stabilize the negative charge