Strong & Weak Acids (DP IB Chemistry): Revision Note

Strong & weak acids

Strong acids

• A strong acid is an acid that dissociates almost completely in aqueous solutions

  • Examples include HCl (hydrochloric acid), HNO₃ (nitric acid) and H₂SO₄ (sulfuric acid)

  • The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

Diagram to show the dissociation of a strong acid

• The solution formed is highly acidic due to the high concentration of the H⁺/H₃O⁺ ions

• The pH depends on the concentration of H⁺/H₃O⁺ ions so the pH can be calculated if the concentration of the strong acid is known

pH = -log₁₀[H⁺ (aq)]

• [H⁺ (aq)] = concentration of H⁺ / H₃O⁺ ions

• pH is the negative log of the concentration of H⁺ / H₃O⁺ ions and can be calculated if the concentration of the strong acid is known using the stoichiometry of the reaction

Weak acids

• A weak acid is an acid that partially (or incompletely) dissociates in aqueous solutions

  • E.g. most organic acids (ethanoic acid), HCN (hydrocyanic acid), H₂S (hydrogen sulfide) and H₂CO₃ (carbonic acid)

  • The position of the equilibrium is more towards the left and an equilibrium is established

 Diagram to show the dissociation of a weak acid

• The solution is less acidic due to the lower concentration of H⁺ / H₃O⁺ ions

Acid & equilibrium position table

• The strength of a Brønsted–Lowry acid depends on how easily it donates H⁺ ions:

  • This is influenced by the strength of the bond between hydrogen and the rest of the molecule

• For hydrogen halides (HX), the halogen atom gets larger as you go down Group 17

• This increases the H–X bond length

• Longer bonds are weaker and easier to break, making it easier to release H⁺

• Therefore, acid strength increases down the group:

  • HF < HCl < HBr < HI

Strong bases

• A strong base is a base that dissociates almost completely in aqueous solutions

  • E.g. Group 1 metal hydroxides such as NaOH (sodium hydroxide)

  • The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

 Diagram to show the dissociation of a strong base

• The solution formed is highly basic due to the high concentration of the OH^– ions

Weak bases

• A weak base is a base that partially (or incompletely) dissociates in aqueous solutions

  • NH₃ (ammonia), amines and some hydroxides of transition metals

  • The position of the equilibrium is more to the left and an equilibrium is established

 Diagram to show the dissociation of a weak base

•  The solution is less basic due to the lower concentration of OH^– ions

Base & equilibrium position table

Strength of conjugate acids and bases

• The conjugate base of HCl is the chloride ion, Cl^–,

• Since the reverse reaction is virtually non-existent the chloride ion must be a very weak conjugate base

HCl (g) → H⁺ (aq)  +   Cl^– (aq)

acid                          conjugate base

• In general, strong acids produce weak conjugate bases and weak acids produce strong conjugate bases

• A strong base is also fully ionised and is a good proton acceptor

• For example, the hydroxide ion is a strong base and readily accepts protons:

   OH^– (aq) +  H⁺ (aq)  ⇌  H₂O (l)

• The conjugate acid of the hydroxide ion is water, which is a weak conjugate acid

• In general strong bases produce weak conjugate acids

How to distinguish between strong and weak acid

• 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^-

pH value of a strong acid & weak acid table

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

Electrical conductivity

• Stronger acids dissociate more completely, producing a higher concentration of H⁺ ions in solution

• This means stronger acids conduct electricity better than weaker acids at the same concentration

• Electrical conductivity can be measured using a conductivity meter

  • Like a pH meter, it uses electrodes placed in the solution

  • The meter displays the conductivity reading directly

Diagram to show how to measure the electrical conductivity of an acid

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

Diagram to show how a strong acid reacts with magnesium

Diagram to show how a weak acid reacts with magnesium

• When reacting with carbonates or hydrogencarbonates, both strong and weak acids produce carbon dioxide gas

• The rate of effervescence is faster with a strong acid due to the higher concentration of H⁺ ions:

  • When reacting with metal oxides or hydroxides, there may be fewer visible signs, but - the solid tends to dissolve more quickly in a strong acid than in a weak one

• These reactions often produce greater enthalpy changes with strong acids:

  • This can be observed as a larger temperature increase during the reaction