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Chapter 18 – Acids, Bases, and Salts

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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  1. Chapter 18 – Acids, Bases, and Salts

  2. 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)

  3. 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

  4. 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

  5. NH3 + H2O NH4+ + OH- base acid conjugate conjugate acid base Show electron-dot of this process

  6. Strong acid – near complete dissociation HCl + H2O → H3O+ + Cl- Acid base conjugate conjugate acid base

  7. Weak acid – very little dissociation HC2H3O2 + H2O H3O+ + C2H3O2- 99.6% .4%

  8. 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.

  9. For strength of conjugate acid-base pairs see page 608 • Note – H2O is a weak acid and a weak base.

  10. 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

  11. 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]

  12. [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

  13. [H3O] [A-] Ka = -------------- [HA] The greater the Ka, the stronger the acid See Ka values on page 610.

  14. Base dissociation constant B + H2O HB+ + OH- [HB+] [OH-] Kb = -------------- [B]

  15. 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

  16. 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)

  17. 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

  18. 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

  19. 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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