Doc Brown's Chemistry  Theoretical Chemistry - Equilibria - Chemical Equilibrium 5.3

5.3 Definition, theory, examples and pH calculations of strong acids

Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry revising courses for pre-university students (equal to US grade 11 and grade 12 and Honours/honors level courses) KS4 Science GCSE/IGCSE Chemistry reversible reactions-equilibrium * KS4 Science GCSE/IGCSE notes acids and bases * KS4 Science GCSE/IGCSE notes acid-base theory

Equilibria Part 5 sub-index:  5.1 Lewis and Bronsted-Lowry acid-base theories * 5.2 self-ionisation of water and pH scale * 5.3 strong acids-examples-calculations * 5.4 weak acids-examples & pH-K_a-pK_a calculations * 5.5 strong bases-examples-pH calculations * 5.6 weak bases- examples & pH-K_b-pK_b calculations * 5.7 A level notes on Acids, Bases, Salts, uses of acid-base titrations - upgrade from GCSE! * EMAIL query?comment

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle-rules * Part 2. K_c and K_p equilibrium expressions and calculations * Part 3. Equilibria and industrial processes * 4. Partition, solubility product and ion-exchange * Part 5. pH, weak-strong acid-base theory and calculations * Part 6. Salt hydrolysis, Acid-base titrations-indicators, pH curves and buffers * Part 7. Redox equilibria, half-cell electrode potentials, electrolysis and electrochemical series * Part 8. Phase equilibria-vapour pressure, boiling point and intermolecular forces

5.3 Definition, examples and pH calculations of strong acids

• 5.3.1 Definition and examples of STRONG ACIDS

  • Strong acids are highly ionised in water.

  • This means for the general reaction of an acid HX interacting in an acid-base manner with water ...

    • HX_(aq) + H₂O_(l) ==> H₃O⁺_(aq) + X^-_(aq)

    • K_a = [H₃O⁺_(aq)] [X^-_(aq)]/[HX_(aq)] (units are mol dm^-3)

    • K_a the equilibrium constant for this reaction is called the acid dissociation/ionisation constant.

    • [H₂O_(l)] is considered constant and incorporated into K_a.

    • The high % of ionisation gives the maximum concentration of hydrogen ions and therefore the most acidic solution of lowest possible pH.

  • In 'strong acid' pH calculations, it is assumed to be 100% and in reality they have very large K_a values and very negative pK_a values (pKa = -log(K_a/mol dm^-3), compare K_a for weak acids e.g.

    • HCl K_a  ~10⁷ pK_a ~-7, HBr K_a  ~10⁹ pK_a ~-9, HI K_a  ~10¹⁰ pK_a ~-10

    • The Ka is so high it is virtually 100% ionised and so the equilibrium sign is pointless and omitted.

    • Note the acid gets stronger down group 7 as the H-X bond enthalpy/strength decreases as the halogen atom gets bigger and the bond length increases.

    • These are all monobasic/monoprotic acids, meaning only one proton is available for transfer to a base, see HCl below, or the conjugate base of the acid can only accept one proton).

  • 5.3.1a: HCl_(g) + H₂O_(l) ==> H₃O⁺_(aq) + Cl^-_(aq)

    • dissolving hydrogen chloride in water to form hydrochloric acid,

    • or more simply HCl_(aq) ==> H⁺_(aq) + Cl^-_(aq)

    • This has very high equilibrium constant K_a i.e. virtually 100% to the right.

    • The other gases, hydrogen bromide and hydrogen iodide similarly dissolve to form the very strong hydrobromic acid and hydriodic acid respectively.

    • However, hydrogen fluoride gas dissolves in water to form the relatively weak hydrofluoric acid (see 5.4.2d) in dilute solution.

  • 5.3.1b: HNO_3(aq) + H₂O_(l) ==> H₃O⁺_(aq) + NO₃^-_(aq)

    • or the dissociation of dilute nitric acid (monobasic) can be simply shown as

    • HNO_3(aq) ==> H⁺_(aq) + NO₃^-_(aq)

    • K_a = 40, pK_a = -1.4

  • 5.3.1c: H₂SO_4(l) + 2H₂O_(l) ==> 2H₃O⁺_(aq) + SO₄^2-_(aq)

    • dissolving concentrated sulphuric acid in water to make dilute sulphuric acid (dibasic).

    • or more simply H₂SO_4(aq) ==> 2H⁺_(aq) + SO₄^2-_(aq)

    • Strictly speaking the ionization occurs in two stages since it is a dibasic/diprotic^* acid

      1. H₂SO_4(aq) ==> H⁺_(aq) + HSO₄^-_(aq)

         • K_a1 is very high, pK_a1 very negative, when 1st conjugate base formed.

      2. HSO₄^-_(aq) H⁺_(aq) + SO₄^2-_(aq)

         • K_a2 =  1 x 10^-2 mol dm^-3, pK_a2 = 2 positive, when 2nd conjugate base formed.

         • Strictly speaking, ionisation 2. is incomplete, but is often ignored at AS-A2 level.

         • H₂SO₄ is a strong acid but HSO₄^- is a weak acid!

    • ^*Dibasic means a maximum of two protons per molecule are available for transfer to a base, or the 2nd conjugate base of the acid can accept two protons. (See section 5.1.3 for more examples)

  • See 5.3 for a brief comparison of selected weak/strong acid properties

• 5.3.2 Calculating the pH of a strong acid

  • Calculation example 5.3.2a

    • (a) Calculate the hydrogen ion concentration and pH of a 0.25 mol dm^-3 solution of hydrochloric acid.

      • HCl is monobasic/monoprotic acid, so [H⁺_(aq)] = 0.25 mol dm^-3

      • pH = -log(0.25) = 0.602

  • Calculation example 5.3.2b

    • (a) Calculate the hydrogen ion concentration and pH of a 1.5 mol dm^-3 solution of sulphuric acid.

      • H₂SO₄ is dibasic/diprotic acid, so [H⁺_(aq)] = 2 x 1.5 = 3.0 mol dm^-3

        • This isn't strictly true, the 1st ionisation is 100%, but the ionisation of the hydrogensulphate ion to release the 2nd proton is not complete, but 100% ionisation is assumed at this academic level.

      • pH = -log(3.0) = -0.477

      • but in reality it will be a little higher (see above comment).

  • Calculation example 5.3.2c

    • Calculate the hydrogen ion concentration solution of hydrochloric acid of pH 1.2

    • [H⁺_(aq)] = 10^-pH = 10^-1.2 = 0.0631 mol dm^-3

Copyright © Dr W P Brown 2000-2010 All rights reserved on the revision notes pages, quizzes, worksheets, x-words etc.