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Understanding Acids and Bases: Strong vs Weak and Their Quantitative Properties

This guide explores the quantitative aspects of acids and bases, detailing the distinctions between strong and weak acids and bases. Strong acids, like HCl, ionize completely in water, releasing nearly all H+ ions, while weak acids, such as ethanoic acid, only slightly ionize. The document also covers the amphoteric nature of water, the ion product constant (Kw), and methods for calculating ion concentrations and pH levels using logarithmic scales. This foundational knowledge is essential for understanding acid-base chemistry.

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Understanding Acids and Bases: Strong vs Weak and Their Quantitative Properties

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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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