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3.3.4 Strong & Weak Acids

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Strong & Weak Acids

  • Acids can be either strong or weak, depending on how many ions they produce when they dissolve in water
  • When added to water, acids ionise or dissociate to produce H+ ions:

Hydrochloric acid: HCl ⟶ H+ + Cl

Nitric acid: HNO3 ⟶ H+ + NO3

  • Strong acids such as HCl and H2SO4 dissociate completely in water, producing solutions with a high concentration of H+ ions and thus a very low pH
  • Weak acids such as ethanoic acid, CH3COOH and hydrofluoric acid, HF, only partially ionise in water, producing solutions of pH values between 4 – 6
  • This data is summarized in the table below:

Strong & Weak Acids Table

Strong & Weak Acids Table, downloadable IGCSE & GCSE Chemistry revision notes

  • For weak acids there is an equilibrium set-up between the molecules and their ions once they have been added to water
  • Propanoic acid for example dissociates as follows:

CH3CH2COOH ⇌ H+ + CH3CH2COO

  • The ⇌ symbol indicates that the process is reversible, as the products can react together forming the original reactants
  • The equilibrium lies to the left, meaning there is a high concentration of intact acid molecules and therefore a low concentration of ions in solution, hence the pH is that of a weak acid and closer to 7 than a strong acid

Concentrated & Dilute Acids 

  • A strong acid is not the same as a concentrated one, just like a dilute acid is not the same as a weak one
  • Strong and weak refer to the ability of an acid to dissociate 
  • Concentration refers to how many acid particles there are in a certain volume
  • A concentrated solution will have more acid particles than a dilute one per dm3
  • Solutions can be both concentrated and strong or weak and dilute 
  • A dilute solution of a strong acid can have a lower pH than a concentrated solution of a weak acid, due to the stronger acid undergoing complete dissociation

3-3-4-comparison-of-acids-

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