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Acid-Base Equilibrium. Strong Acids & Bases – - Autoionization of Water - . Strong Acids. a strong acid ionizes completely in water to form hydrogen ions, percent ionization is greater than 99% (we assume 100% in calculations ) Example: HCl completely ionizes in water 100%

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acid base equilibrium

Acid-Base Equilibrium

Strong Acids & Bases –

- Autoionization of Water -

strong acids
Strong Acids
  • a strong acid ionizes completely in water to form hydrogen ions, percent ionization is greater than 99% (we assume 100% in calculations)
  • Example: HCl completely ionizes in water

100%

HCl(g) H+(aq) + Cl-(aq)

H2O(l)

slide3

Common strong acids: hydrochloric acid, hydrobromic acid, sulfuric acid, nitric acid, phosphoric acid

  • monoprotic acid is an acid that possesses only one ionizable (acidic) proton
  • diprotic acid possesses two or more ionizableprotons
strong bases
Strong Bases
  • a strong base (Arrhenius definition) dissociates completely in water to release hydroxide ions
  • all hydroxides of Group 1 elements are strong bases (only one mole of hydroxide ions are formed when one mole of Group 1 hydroxides are dissolved in water)

100%

NaOH(s) Na+(aq) + OH-(aq)

H2O(l)

slide5

these metal hydroxides are all highly soluble in water

  • all hydroxides of Group 2 elements are also bases when dissolved in water (two moles of hydroxide ions are formed when one mole of Group 2 hydroxides are dissolved in water)

100%

Ba(OH)2(s) Ba+(aq) + 2OH-(aq)

H2O(l)

  • these metal hydroxides are only slightly soluble in water (main reason used in medicine – milk of magnesia, Mg(OH)2(aq))
the autoionization of water
The Autoionization of Water
  • Experiments have revealed that some water molecules react with each other to produce hydronium, H3O+(aq), and hydroxide, OH-(aq), ions according to the following equation

H2O(l)+H2O(l) H3O+(aq) + OH-(aq)

  • conductivity is slight, an equilibrium is formed between hydronium and hydroxide ions, and water molecules are greatly favoured
slide7

the collision that forms hydronium and hydroxide ions are very rare (need the right energy and orientation)

  • when this reaction does occur, it is called the autoionization of water (since water molecules ionize one another).
slide8

chemists often omit the water molecule that carries the H+ ion for convenience

H2O(l) H+(aq) + OH-(aq)

  • water molecules do NOT spontaneously dissociate into H+ and OH- ions, but rather, they are formed by the ionization process in which a proton is transferred from one molecule to another
the water equilibrium
The Water Equilibrium
  • water equilibrium also obeys equilibrium law

Keq = [H+(aq)][OH-(aq)]

[H2O(l)]

slide10

[H+(aq)][OH-(aq)] = K

[H2O(l)]

[H+(aq)][OH-(aq)] = K[H2O(l)]

[H+(aq)][OH-(aq)] = Kw

  • the concentration of water molecules in pure water and in dilute aqueous solutions is essentially CONSTANT and equal to 55.6 mol/L
  • therefore, a new constant, which incorporates both the constant value of [H2O(l)] and the equilibrium constant, can be calculated called the ion product constant for water, Kw
slide11

note the 1:1 ratio of hydrogen ions and hydroxide ions, therefore the concentration of hydrogen and hydroxide ions in pure water must be EQUAL

[H+(aq)] = [OH-(aq)] = 1.0 x 10-7 mol/L

  • At SATP,

Kw = [H+(aq)][OH-(aq)]

Kw= (1.0 x 10-7 mol/L) (1.0 x 10-7 mol/L)

Kw = 1.0 x 10-14

slide12

autoionization occurs in all aqueous solutions

  • In all aqueous solutions at SATP,

[H+(aq)][OH-(aq)] = Kw =1.0 x 10-14

  • What makes a solution neutral, acidic or basic?

In neutral solutions [H+(aq)] = [OH-(aq)]

In acidic solutions [H+(aq)] > [OH-(aq)]

In basic solutions [H+(aq)] < [OH-(aq)]

slide13

Kw is valid at SATP, however at different temperatures it will change (equilibrium constant is temperature dependent)

  • the ion product constant for water, Kw, can be manipulated to solve for either hydrogen ion or the hydroxide ion concentration

Since Kw = [H+(aq)] [OH-(aq)]

then [H+(aq)] = Kw__

[OH-(aq)]

and [OH-(aq)] = Kw__

[H+(aq)]

learning checkpoint
Learning Checkpoint

p. 537 Practice

UC # 4, 5, 6

p. 540 Practice

UC # 8, 9, 10