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Electrolytic dissociation of water

H 2 O + H 2 O. H 3 O + + HO -. Electrolytic dissociation of water. The water molecules dissociates according to the reaction:.

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Electrolytic dissociation of water

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  1. H2O + H2O H3O+ + HO- Electrolytic dissociation of water The water molecules dissociates according to the reaction: The equilibrium is shifted far to the left. Experimentally, it was determined that at 25°C, only one molecule of water, out of 556,000,000is dissociated, which means the dissociation degree of water is α = 18·10-10. The equilibrium constant for the dissociation reaction of water is:

  2. mol L-1 Kw is called the ionic product of water The ionic product of water depends on the temperature. At 25°C, the value of KW is 10-14 mol L-1. In pure water the concentration of the H3O+ ions is equal to that of the HO-, which means that at 25°C:

  3. To express the concentration of the hydrogen ions in aqueous solutions, the notion of pH was introduced by Sörensen (1909): The relation was modified by Bates by replacing the concentration of the hydronium ions with their activity: or For diluted solutions, the activity can be considered equal to the concentration

  4. Similar to the pH notion the term of pOH was introduced, that is a measure of the concentration of the hydroxyl ions: It is easily demonstrated that, at the temperature of 25°C: pH + pOH = -lg KW = 14

  5. NaOH Na+ + HO- HCl H+ + Cl- Acid – base equilibrium Acids are substances that, in aqueous solutions, release hydrogen ions H+. For example, the hydrochloric acid dissociates in H+ and Cl- ions: Bases are substances that, in aqueous solutions, produce hydroxyl ions, like the case of sodium hydroxide, that dissociates in Na+ and HO- ions:

  6. Strong acidsare completely dissociated, the dissociation degree α = 1 Ex: HCl, H2SO4, HNO3 ACIDS Weak acidsare partially dissociated, the dissociation degree α << 1 Ex: CH3COOH, HCN, H2S Strong basesare completely dissociated, the dissociation degree α = 1 Ex: NaOH, KOH BASES Weak basesare partially dissociated, the dissociation degree α << 1 Ex: NH3, organic amines R-NH2

  7. The dissociation degree is defined as the ratio between the number of dissociated molecules and the total number of dissolved molecules:

  8. BOH B+ + HO- H+ + A- HA Acidity constant the acidity constant is: Base constant the base constant is:

  9. Very weak acids: the first acidity constant lower than 10-7. Weak acids: the first acidity constant between 10-7 and 10-2. Strong acids are completely dissociated in aqueous solution; one can not distinguish between their acidity constants.

  10. Weak bases, like ammonia, aniline, have the base constant below 10-3: Strong bases, like sodium hydroxide, calcium hydroxide, are completely dissociated in water, like in case of strong acids, one can not distinguish between their base constants.

  11. Calculation of pH for acid and base solutions Strong acids are completely dissociated in aqueous solutions so the hydronium ions concentration is equal to the concentration of the acid. For example, for a 10-3 mol L-1 solution of HCl, the concentration of the hydronium ions is [H3O+] = 10-3 mol L-1. The pH of the solution is: pH = -lg[H+] = -lg 10-3 = 3 For a strong base, for example 10-3 mol L-1 KOH the concentration of hydroxyl ions is [HO-] = 10-3 mol L-1. It results that the pOH of the solution is: pOH = -lg[HO-] = -lg 10-3 = 3 Considering the relation between pOH and pH one obtains: pH = 14 – pOH = 14 – 3 = 11

  12. For concentrations higher than 10-3 mol L-1 the pH is calculated using the Bates relation because the activity differs from the concentration At very low acid concentrations to calculate pH it is necessary to consider the hydronium ions coming from the dissociation of both acid as well as water molecule.

  13. For example, the pH of a 10-7 mol L-1 solution of HCl is not 7 because the hydronium ions result not only from the dissociation of the acid, but from the dissociation of water as well: HCl H+ + Cl- H+ + HO- H2O Considering that the concentration of hydrochloric acid in the solution is c = 10-7 and the concentration of hydronium, respectively hydroxyl ions is x, the total hydronium ions concentration will be c+x. The ionic product of water, at the temperature of 25°C, will be: (x + c)x = 10-14

  14. One obtains a second degree equation: x2 + cx - 10-14 = 0 =6.1810-8 Solving the equation one obtains: The solution with “minus” in front of the square root has no meaning, since it is negative. The concentration of the hydronium ions will be: [H+] = 10-7 + 6,1810-8 = 1.61810-7 Thus, the pH of a 10-7 mol L-1 solution of HCl, will be: pH = -lg[H3O+] = -lg 1.61810-7 = 6.79

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