IB Chemistry SL

Revision Notes

8.1.12 Comparing Strong & Weak Acids

Comparing Strong & Weak Acids

  • Strong and weak acids can be distinguished from each other by their:
    • pH value (using a pH meter or universal indicator)
    • Electrical conductivity
    • Reactivity

pH value

  • An acid dissociates into H+ in solution according to:

HA → H+ + A

  • The stronger the acid, the greater the concentration of H+ and therefore the lower the pH

pH value of a Strong Acid & Weak Acid Table

Equilibria Table 1_Strong & Weak Acids & Bases, downloadable AS & A Level Chemistry revision notes

Electrical conductivity

  • Since a stronger acid has a higher concentration of H+ it conducts electricity better
  • Stronger acids therefore have a greater electrical conductivity
  • The electrical conductivity can be determined by using a conductivity meter
  • Like the pH meter, the conductivity meter is connected to an electrode
  • The conductivity of the solution can be read off the meter

Equilibria Conductivity Meter, downloadable AS & A Level Chemistry revision notes

The diagram shows a digital conductivity meter that measures the electrical conductivity of a solution using an electrode

Reactivity

  • Strong and weak acids of the same concentrations react differently with reactive metals
  • This is because the concentration of H+ is greater in strong acids compared to weak acids
  • The greater H+ concentration means that more H2 gas is produced in a shorter time

Equilibria Strong Acid with Reactive Metal, downloadable AS & A Level Chemistry revision notes

The diagram shows the reaction of 0.1 mol dm-3 of a strong acid (HCl) with Mg. The reaction produces a lot of bubbles and hydrogen gas due to the high concentration of H+ present in solution

Equilibria Weak Acid with Reactive Metal, downloadable AS & A Level Chemistry revision notes

The diagram shows the reaction of 0.1 mol dm-3 of a weak acid (CH3COOH) with Mg. The reaction produces fewer bubbles of hydrogen gas due to the lower concentration of H+ present in solution

  • Similar observations would be made in the reaction between strong and weak acids with carbonates and hydrogencarbonates, although the gas given off this time is carbon dioxide
  • With oxides and hydroxides, there may not be a lot of visible changes although it is likely that they would dissolve faster in a strong acid than in a weak acid
  • These reactions are also likely to produce larger enthalpy changes which could be seen in higher temperature rises

Exam Tip

The above-mentioned properties of strong and weak acids depend on their ability to dissociate and form H+ ions.

Stronger acids dissociate more, producing a greater concentration of H+ ions and therefore showing lower pH values, greater electrical conductivity and more vigorous reactions with reactive metals.

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