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Describing Acids & Bases Quantitatively

Describing Acids & Bases Quantitatively. Acids & Bases Can be Strong or Weak. Strong acids ionise almost completely in water –nearly all the H + ions will be released. Strong Acid, HCl Strong Base, NaOH. Dissociation Reactions. Strong Acid: HCl (g) → H + (aq) + Cl - (aq)

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Describing Acids & Bases Quantitatively

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  1. Describing Acids & Bases Quantitatively

  2. Acids & Bases Can be Strong or Weak • Strong acids ionise almost completely in water –nearly all the H+ ions will be released. • Strong Acid, HCl • Strong Base, NaOH

  3. Dissociation Reactions Strong Acid: HCl(g) → H+(aq) + Cl-(aq) Strong Base: NaOH(aq) → Na+(aq) + OH-(aq)

  4. Weak Acids • Such as ethanoic acid or citric acid. • Ionise only slightly in water • So only small amounts of H+ ions are formed CH3COOH ↔ CH3COO-(aq) + H+(aq)

  5. Weak Bases • Such as ammonia only slightly ionise in water too. NH3(aq) + H2O(l) ↔ NH4+(aq) + OH-(aq)

  6. Water is Amphoteric • We know that water can act as an acid (donates a proton) or as a base (accepts a proton). • So in water, there will always be present hydronium ions and hydroxide ions. H2O(l) + H2O(l) ↔ H3O+(aq) + OH-(aq)

  7. Ion Product Constant for Water, Kw • Or more simply H2O(l) ↔ H+(aq) + OH-(aq) Kw = [H+] [OH- ] = 1.00 X 10-14 The Kwis simply the equilibrium constant for this reaction.

  8. Neutral Solutions • Have equal concentrations of H+and OH-. • [H+] = [OH-] = 1.00 X 10-7 mol/L • If [H+] is greater than the [OH-], then the solution is acidic. • If [H+] is less than the [OH-], then the solution is basic.

  9. Determining the Concentrations of [H+] and [OH-] • For strong acids, the concentration of [H+] can be found by using the stoichiometric ratio of your dissociation equation. • For strong bases, the concentration of [OH-] can be found by using the stoichiometric ratio of your dissociation equation. • This is due to the fact that they fully ionise in water.

  10. Example 1: • Calculate the concentration of the hydrogen ion and the hydroxide ion in a 2.5 mol/L solution of nitric acid. • First, write the formula of the acid.

  11. Strategy: • Nitric acid, HNO3, is a strong acid • Will fully ionise in water • Write the dissociation equation HNO3(aq) → H+(aq) + NO3-(aq) • [HNO3+]= [H+] = 2.5 mol/L

  12. How to determine the concentration of [OH-]? • Kw=1.00 X 10-14 • 1.00 X 10-14 = [H+][OH-] [OH-] = 1.00 X 10-14 = 1.00 X 10-14 [H+] 2.5 mol/L = 4.00 X 10-15 mol/L

  13. Example 2: • Calculate the concentration of the hydrogen ion and the hydroxide ion in a 0.16 mol/L solution of barium hydroxide. (First, write the formula.)

  14. Strategy: • Barium hydroxide, Ba(OH)2, is a strong base • Will fully ionise in water • Write the dissociation equation Ba(OH)2(aq) → Ba2+ (aq) + 2OH-(aq) • [OH-] = 2 [Ba(OH)2]= 0.32 mol/L

  15. How to determine the concentration of [H+]? • Kw=1.00 X 10-14 • 1.00 X 10-14 = [H+][OH-] [H+] = 1.00 X 10-14 = 1.00 X 10-14 [OH-] 0.32 mol/L = 3.1 X 10-14 mol/L

  16. pH Calculations • The pH scale is a measure of the hydrogen (or hydronium) ion concentration. • It is measured using the logarithmic scale. • A logarithmic scale is used since there is an enormous variation in concentrations, so this helps “simplify” the numbers.

  17. pH Scale

  18. pH Calculations • We will be using this formula: pH = - log10[H+] Example: If the concentration of [H+] = 1.00 X 10-12, what is the pH?

  19. pH Calculations pH= - log10[H+] = - log10 (1.00 X 10-12) = 12

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