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
• • Strong acids dissociate completely in water
  • This produces a solution with a high concentration of H+ ions 
  • This means that strong acids have a low pH, typically pH 1 - 3 
  • Examples of strong acids include:
  • Hydrochloric acid, HCl

  HCl (aq) → H+ (aq) + Cl– (aq) 

  • Nitric acid, HNO3 

  HNO3 (aq) → H+ (aq) + NO3– (aq)

  • Sulfuric acid, H2SO4 

  H2SO4 (aq) → H+ (aq) + SO42– (aq)
• • Weak acids partially dissociate (or ionise) in water
  • This produces a solution with a low concentration of H+ ions
  • This means that weak acids have pH values that are closer to the middle of the pH scale, whilst still being below 7, i.e. pH 4 - 6
• • Another example of a weak acid is ethanoic acid which will react with alkalis such as sodium hydroxide to form ethanoate salts

  ethanoic acid + sodium hydroxide → sodium ethanoate + water
• The terms strong and weak refer to the ability to dissociate whereas the term concentration refers to the amount of acid present in solution.
• • Weak acids such as ethanoic acid, CH3COOH, and hydrofluoric acid, HF, only partially ionise in water, producing solutions of pH values between 4 – 6
• • A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution so would produce pH values below 4 and above 10
• • A dilute acid or base solution is therefore one that has much fewer acid or base molecules per dm3 of solution, hence the pH value would lie between 5 and 9
• Remember concentration describes the total number of acid molecules added to the solution but does not consider those that dissociated. This is measured using the pH scale.
• • The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
  • Therefore an acid with a pH of 3 has ten times the concentration of H+ions than an acid of pH 4
• An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ions than an acid with a pH of 
• that for two acids of equal concentration, where one is strong and the other is weak, then the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H+ ions into solution than the weak acid