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Chemistry 122

Chemistry 122. Hydrogen Ions and Acidity. Hydrogen Ions from Water. Water is highly polar – what does that mean? Water particles are in continuous motion If they possess enough energy, a H + can be transferred from one water molecule to another

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Chemistry 122

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  1. Chemistry 122 Hydrogen Ions and Acidity

  2. Hydrogen Ions from Water • Water is highly polar – what does that mean? • Water particles are in continuous motion • If they possess enough energy, a H+ can be transferred from one water molecule to another • The particle that remains once the H+ leaves is OH- • The water molecule gaining the H+ becomes hydronium (H3O+) H2O(l) + H2O(l)→ H3O+(aq) + OH-(aq)

  3. Self-ionization of water • Another way of writing the ionization of water is: H2O(l)↔ H+(aq) + OH-(aq) • Hydronium and hydrogen ions can be written interchangeably • Both examples show water forming ions • The equilibrium concentration for both hydrogen and hydroxide ions is very small at 25°C • Each is only 1 x 10-7 M • When both [H+] and [OH-] are equal in concentration, the solution is said to be neutral

  4. Ion Product Constant for Water • In any aqueous solution, as hydrogen ion concentration increases, hydroxide ion concentration decreases • If hydrogen ions were to be added, the equilibrium would shift and the hydroxide ion concentration would decrease • The product of the hydrogen-hydroxide concentration is 1 x 10-14 • In other words, the Kw = [H+][OH-] = 1 x 10-14

  5. Sample Problem, p. 596 Complete 9, 10, p. 596

  6. Acidic and Basic Solutions • When acids form aqueous solutions, there is more H+ ions than OH- (coming from the ionization of water) • The [H+] is greater than 1 x 10-7 M • Basic solutions are the opposite • The hydroxide ion concentration is greater than the hydrogen ion concentration • The [OH-] is greater than 1 x 10-7 M • They are otherwise known as alkalinesolutions

  7. Calculating pH (power of hydrogen) • Ranges from 0 – 14 • Neutral solutions have a pH of 7 • The closer the pH is to zero, the more acidic it is; the closer it is to 14, the more basic it is • To calculate pH from the hydrogen ion concentration, pH = -log[H+] Figure 19.9, p. 597

  8. Calculating pOH pH + pOH = 14 pOH = -log[OH-] • Express concentrations in scientific notation • When writing the pH or pOH, the concentration of the solution is equivalent to the same number of digits after the decimal of the value itself • Ex. 1.0 x 10-5 = 5.00 (two digits in the base of the scientific notation – two digits after the decimal)

  9. When is pH not a whole number? • Most of the time… As a result, it is not easy to make a simple mental calculation. Instead, use the logarithmic equation to convert from concentration to pH or pOH. Ex. [H+] = 4.2 x 10-10M pH = -log[[4.2 x 10-10] = -9.37675 = 9.38 Questions 11-12, p. 599

  10. Calculating [H+] from pH • Rearrange the equation pH = -log[H+] [H+] = - antilog pH Using Kw, in addition to pH/pOH, [H+]/[OH-], you can solve for any unknown

  11. Sample Problem 19.3, p. 600

  12. Sample Problem 19.4, p. 601 Questions 15-16, p. 601

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