chapter 18 acids bases and salts
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Chapter 18 – Acids, Bases, and Salts. Arrhenius (ah-ray-nee-uhs) definition . Acid – substance that dissociates in water to produce hydrogen ions (H + ) Base – a substance that dissociates in water to produce hydroxide ions (OH - ions). Bronsted – Lowry definition.

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arrhenius ah ray nee uhs definition
Arrhenius (ah-ray-nee-uhs) definition
  • Acid – substance that dissociates in water to produce hydrogen ions (H+)
  • Base – a substance that dissociates in water to produce hydroxide ions (OH- ions)
bronsted lowry definition
Bronsted – Lowry definition
  • Acid – is any substance that donates a hydrogen ion (a proton)
  • Base – any substance that accepts hydrogen ions
  • This is a broader definition
a hydrogen ion is a single proton
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)
  • HCl + H2O → H3O+ + Cl-
  • Acid base hydronium ion

Show the electron-dot of this process

slide5
NH3 + H2O NH4+ + OH-

base acid conjugate conjugate

acid base

Show electron-dot of this process

strong acid near complete dissociation
Strong acid – near complete dissociation

HCl + H2O → H3O+ + Cl-

Acid base conjugate conjugate

acid base

weak acid very little dissociation
Weak acid – very little dissociation

HC2H3O2 + H2O H3O+ + C2H3O2-

99.6% .4%

strong base lime cao
Strong Base (lime CaO)

CaO + H2O → Ca+2 + 2 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.

slide9
For strength of conjugate acid-base pairs see page 608
  • Note – H2O is a weak acid and a weak base.
diprotic acid two acidic protons per molecule
Diprotic Acid – two acidic protons per molecule

H2SO4 + H2O → H3O+ + HSO4-1

Acid base conj. acid conj. Base

HSO4-1 + H2O → H3O+ + SO4-2

Acid base conj. acid conj. Base

the acid dissociation constant
The Acid Dissociation Constant

HAaq + H2Ol H3O+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 (Keq is equilibrium constant)

[H3O] [A-]

Keq = --------------

[HA] [H2O]

slide12
[H3O] [A-]

Keq = --------------

[HA] [H2O]

There is so much more H2O than anything else, the water concentration does not change much (% change in [H2O])

[H3O] [A-]

Keq [H2O] = --------------

[HA]

Water can be included as part of the constant, now called acid dissociation constant Ka

slide13
[H3O] [A-]

Ka = --------------

[HA]

The greater the Ka, the stronger the acid

See Ka values on page 610.

base dissociation constant
Base dissociation constant

B + H2O HB+ + OH-

[HB+] [OH-]

Kb = --------------

[B]

slide15
NH3 + H2O NH4+ + OH-

[NH4+] [OH-]

Kb = -------------- = 1.8 x 10-5

[NH3] (from page 611)

See example problem page 612

Assign practice problems 1,2 page 613

acid base properties of salts
Acid-Base Properties of Salts
  • Salts from strong acids and strong bases – will be neutral

NaOH + HCl → NaCl + H2O

NaCl is neutral

  • Salts from strong acids and weak bases – will be slightly acidic

NH3 + HCl → NH4Cl

NH4Cl is slightly acidic

NH4Cl + H2O H3O + NH3 + Cl-)

(Ka = 5.6 x 10-10)

slide17
Salts from weak acids and strong bases – will be slightly basic

2NaOH + H2CO3→ Na2CO3 + 2H2O

Na2CO3 is slightly basic

CO3-2 + H2O HCO- + OH- Kb = 1.8 x 10-4

acidic hydrogen
Acidic Hydrogen
  • Not all hydrogen can dissociate in water.
  • HCl + H2O → H3O+ + Cl-
  • 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
Stabilization through resonance (the “spreading out” of a bond or charge)
  • 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.
  • Resonance helps stabilize the negative charge
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