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Strong and Weak Acids

Chemistry: AQA GCSE Higher, topic 4.3.13

Strong vs Weak Acids

All acids ionise (become ions) in aqueous solutions. The strength of an acid is determined by how much it ionises in water. The strength of an acid is NOT the same as the concentration of an acid!

Strong acids

Strong acids

  • Fully ionise in water, meaning that all of the acid molecules break up to form H+ ions.
  • E.g. hydrochloric acid:
  • HCl → H+ + Cl-
  • Sulfuric and nitric acid are also strong acids.
Weak acids

Weak acids

  • Only partially ionise in water, meaning that not all of the acid molecules break up to form H+ ions.
  • E.g. ethanoic acid (the main ingredient in vinegar):
  • CH3COOH ⇌ CH3COO- + H+
  • Most acids are weak acids.

Test yourself on Strong and Weak Acids

  • Which is the stronger acid?

  • Why does the equation for ethanoic acid use the symbol ⇌ instead of →?

  • An acid completely breaks up into ions when it is dissolved in water. What type of acid is this?

  • What determines the strength of an acid?

  • A newly discovered acid has not yet been tested. Based on the fact that most acids are weak acids, what is most likely true about it?

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