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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Chapter 18 – Acids, Bases, and Salts

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Chapter 18 acids bases and salts

Chapter 18 – Acids, Bases, and Salts


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


Chapter 18 acids bases and salts

NH3 + H2O NH4+ + OH-

base acid conjugate conjugate

acidbase

Show electron-dot of this process


Strong acid near complete dissociation

Strong acid – near complete dissociation

HCl + H2O → H3O+ + Cl-

Acid base conjugate conjugate

acidbase


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.


Chapter 18 acids bases and salts

  • 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]


Chapter 18 acids bases and salts

[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


Chapter 18 acids bases and salts

[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]


Chapter 18 acids bases and salts

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)


Chapter 18 acids bases and salts

  • 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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