Proton Transfer, Strong & Weak Acids (Cambridge O Level Chemistry)

Proton transfer

• The earlier definition of an acid and a base can be extended
• In terms of proton transfer, we can further define each substance in how they interact with protons

Acids

• Acids are proton donors as they ionise in solution producing protons, which are H⁺ ions
• These H⁺ ions make the aqueous solution acidic

Bases 

• Bases are proton acceptors as they accept the protons which are donated by the acid 

Diagram showing the role of acids and bases in the transfer of protons - here water acts as a base as it accepts a proton

 

Strong acids

• Acids can be either strong or weak, depending on how many H⁺ ions they produce when dissolved in water
• Strong acids completely dissociate (or ionise) in water, producing a high amount of H⁺ ions and producing solutions with a very low pH 
• Strong acids include HCl and H₂SO₄ 
• Example of a strong acid: hydrochloric acid

HCl (aq) → H⁺ (aq) + Cl^- (aq)

Weak acids 

• Weak acids partially dissociate (or ionise) in water, producing fewer H⁺ ions in solution that strong acids and producing solutions with pH values which are closer to the middle of the pH scale, whilst still being below 7
• Weak acids include organic acids such as ethanoic acid, CH₃COOH 
• For weak acids, there is usually an equilibrium set-up between the molecules and their ions once they have been added to water
• Example of a weak acid: propanoic acid

CH₃CH₂COOH ⇌ H⁺ + CH₃CH₂COO^-

• The equilibrium lies to the left, indicating a high concentration of intact acid molecules, with a low concentration of H⁺ ions in the solution

Effect of concentration on strong and weak acids 

• A concentrated solution of an acid is one that contains a higher number of acid molecules per dm³ of solution
• It does not necessarily mean that the acid is strong though, as it may be made from a weak acid which does not dissociate completely
• For example a dilute solution of HCl will be more acidic than a concentrated solution of ethanoic acid, since most of the HCl molecules dissociate but very few of the CH₃COOH do