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Ionic equilibrium

Ionic equilibrium . Lec.9. Introduction . Ionization : a process in which unionized molecules are changed into ions. Types of electrolytes:. Where: K a = acid ionization constant K b = base ionization constant. Ostwald Law(1888):

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Ionic equilibrium

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  1. Ionic equilibrium

    Lec.9
  2. Introduction Ionization: a process in which unionized molecules are changed into ions. Types of electrolytes:
  3. Where: Ka = acid ionization constant Kb = base ionization constant Ostwald Law(1888): Described the relation between the extent of ionization (α) and concentration (C) mole/ liter for solutions. Weak electrolyte dissociates in water to ions . No.of moles Where : [A-], [B+] : concentrations of the ions produced [AB] :the undissociated form at equilibrium state. K : ionization or dissociation constant Suppose V = volume Concentration = number of moles/ volume (L)
  4. In case of very weak electrolyte α = very small value and can be neglected i.e. (1- α) = 1 Where : C =1/V weak electrolyte Very weak electrolyte Degree of Ionization
  5. Determination of (α) Experimentally: Determine the value of( Ʌ v ) : equivalent conductance with different weak electrolyte concentrations and calculated K in each case. K was always constant and independent of the dilution only in case of weak electrolytes. Determine the value of(Ʌ o )= equivalent conductance at infinite dilution.   If λ v ↑ α ↑ ---------- λ v α (α) α= Ʌ v / Ʌ o
  6. Example (1):
  7. Concentration of hydrogen ion: The acid can be considered very weak, [α ‹‹‹‹ 1]
  8. The Ion Product of Water Water is very slightly ionized ( Very weak electrolyte) H2O is very weak electrolyte [H2O] = constant value [H2O]*K=constant value [H2O]*K=KW= ion product of water [H+] * [OH-]= KW= 10-14 at constant temperature
  9. For example: at samples of H2O: 1- When [H+] = 10-5> 10-7 (acidic solution) 2-When [H+] = 10-9< 10-7 ( alkaline solution) 4- When [OH-] =10-11<10-7 (acidic solution)
  10. The Hydrogen Ion Exponent:( pH) pH is defined as the negative exponent of 10 which gives the hydrogen ion concentration , pH= - log 10[H+] pOH = - log [OH-] [H+][OH-] = Kw =10-14 -log [H+]-log[OH-]=-log10-14 p H +p OH =14
  11. Example (1): What is the pH value of: Pure water. HCl (0.01 mole/L). NaOH (0.001 mole/L).
  12. Example (2): The ionization constant of acetic acid at 25 0C is 1.82x10-5. Calculate the pH of 0.1 mole/ L acid. Another method of calculating [H+]: According to Ostwald’s dilution law, pH= -log [H+]= 2.87
  13. Example (3): If the pH value of 0.1 mole/L acetic acid is 2.872, calculate the ionization constant of the acid. Answer C=0.1 pH= 2.872 K=? pH= -log [H+] 2.872 = -log [H+] [H+]= 1.353x10-3
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