Proton Transfer, Strong & Weak Acids

• Physics
• Secondary Education
• Age 12,13

05 April 2023

Proton Transfer, Strong & Weak Acids

Content objectives

• I can describe the characteristic properties of bases in terms of their reactions with (a) acids; (b) ammonium salts.
• I can describe how to compare hydrogen ion concentration, neutrality, relative acidity, and relative alkalinity in terms of colour and pH using universal indicator paper.
• I can define acids as proton donors and bases as proton acceptors.
• I can define a strong acid as an acid that is completely dissociated in an aqueous solution and a weak acid as an acid that is partially dissociated in an aqueous solution.

Typical reactions of bases

Bases + acids

• neutralisation reaction occurs
• Acid + Base → Salt + Water

Typical reactions of bases

Alkalis and ammonium salts

• Ammonium salts undergo decomposition when warmed with an alkali.
• Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali.
• A salt, water and ammonia ar e produced.

NH4Cl + NaOH →NaCl + H2O + NH3

This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH4+).

Example 

Test for Ammonium Ion (NH4+)

Procedure

• Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper.
• The damp litmus paper will turn from red to blue if ammonia is present.

Proton transfer

• interaction with protons (H⁺)
• A reaction or reaction mechanism step in which a proton (H⁺) is removed from one species (the acid) and accepted by another species (the base).

Acids 

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

Bases 

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

Proton Transfer

Strong Acids

• completely dissociate (or ionise) in water, producing solutions of a very low pH 
• examples: HCl and H2SO4 

                   HCl (aq) → H+ (aq) + Cl- (aq)

Weak Acids

• partially dissociate (or ionise) in water and produce pH values that are closer to the middle of the pH scale, whilst still being below 7
• organic acids such as ethanoic acid, CH3COOH 
• For weak acids, there is usually an equilibrium setup between the molecules and their ions once they have been added to water.

                  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 acid 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 that does not dissociate completely.

Effect of concentration on strong and weak acids 

• 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.