Comparing Acid/Base Strength

This is part of the HSC Chemistry course under the topic Brønsted-Lowry Theory.

HSC Chemistry Syllabus

  • Conduct an investigation to demonstrate the use of pH to indicate the differences between the strengths of acids and bases

How to compare strength of acids and bases

The pH and electrical conductivity of acid/base solutions can be used to compare their relative strengths, but only under certain conditions.

 


Using pH to Compare Strength of Acids

Ionisation of acids in water produces hydrogen (hydronium) ions. The strength of an acid refers to its degree of ionisation. 

The pH of acid solutions should only be used to compare the strength of acids if they are equal in concentration and monoprotic. 

For equimolar (equal concentration) and monoprotic acid solutions, the stronger acid will produce a lower pH.

 

Consider two solutions:

Solution 1: 

  • 1.0 mol L-1 HCl (strong acid)
    • Completely dissociates
    • [H+] = 1.0 mol L-1 
    • pH = 0.00

 

Solution 2: 

  • 1.0 mol L-1 CH3COOH (weak acid)
    • Partially dissociates
    • [H+] = 0.020 mol L-1 
    • pH = 1.70
    • Since solution 1 is a stronger acid and produces a higher [H+], it results in a lower pH
    • Comparison is only fair when both acids have equal starting concentration and are monoprotic

     

    Consider two solutions:

    Solution 1: 

    • 1.0 mol L-1 HCl (strong acid)
      • Completely dissociates
      • [H+] = 1.0 mol L-1 
      • pH = 0.00

     

    Solution 2: 

    • 1.0 mol L-1 H2SO4 (strong acid)
      • First proton completely dissociates
      • Second proton partially dissociates
      • [H+] = 1.2 mol L-1 
      • pH = –0.79
    • The diprotic strong acid produces a higher hydrogen ion concentration and results in an overall lower pH
    • It is invalid to conclude that H2SO4 has a greater strength than HCl because the lower pH is attributed to its diprotic nature.

     

    Consider two solutions:

    Solution 1: 

    • 0.0010 mol L-1 HCl (strong acid)
      • Completely dissociates
      • [H+] = 0.001 mol L-1 
      • pH = 3.00

     

    Solution 2: 

    • 1.0 mol L-1 CH3COOH3 (acetic acid, weak acid)
      • Partially dissociates
      • [H+] = 0.0042 mol L-1 
      • pH = 2.37
    • Despite being a weak acid, the acetic acid solution produces a lower pH than hydrochloric acid. This is due to its much greater starting concentration. 
    • It is invalid to conclude that acetic acid has a greater strength than hydrochloric acid. 

    Using Electrical Conductivity to compare [H+]

    Electrical conductivity measures how well electrons can move through a particular substance. Ionic solutions are more conductive than pure water because the ions present can act as charge carriers, promoting electron movement.
     
    Acid solutions produce hydrogen ions and conjugate bases which are also charged.
     
    $$HCl(aq) \rightarrow H^+(aq) + Cl^-(aq)$$
     
    The electrical conductivity of an acid solution increases with its concentration of ions, which is in turn dependent on the acid solution's starting concentration and strength. 
    Similar to pH, the electrical conductivity of acid solutions can be used to compare their relative strengths. This comparison is only valid when the acid solutions are equal in concentration and monoprotic.
     
    For equimolar monoprotic acid solutions, the stronger acid will cause the solution to have a greater electrical conductivity. 

     

    Comparing Base Strength

    Similar to acid solutions, the relative strength of bases can also be compared using their pH and electrical conductivity.
    • pH: for equimolar base solutions, the stronger base will have a higher pH or lower pOH.
    • Electrical conductance: stronger bases have greater electrical conductivity due to a higher [OH-].

     

     

    BACK TO MODULE 6: ACID/BASE REACTIONS