Question

Propanoic acid (C₂H₅COOH) is described as a weak acid. Define the term weak acid.

Write a balanced chemical equation to show the dissociation of propanoic acid (C₂H₅COOH). Physical states must be included.

 

A solution of propanoic acid and sodium propanoate (C₂H₅COONa) behaves as a buffer. Describe the term buffer solution. No chemical equations are needed.

Write a balanced chemical equation to show the dissociation of sodium propanoate (C₂H₅COONa). Physical states must be included.

 

Explain how a solution containing propanoic acid and sodium propanoate can function as a buffer solution. Your answer must include balanced chemical equations, physical states included.

Answer 1

(a)

Weak acids are those acids which does not dissociate completely in their constituent ions, or in other words, those acids which do not donate their all $H^+$ ions in the solution are known as weak acids.

(b)

$C_2H_5COOH(aq) \to C_2H_5COO^-(aq)+H^+(aq)$

(c)

A buffer solution is a solution whose $pH$ remains almost unchanged when a small amount of strong acid or strong base is added to it.

An acid-buffer solution consist of a weak acid and its conjugate base whereas a basic-buffer solution consist of a weak base and its conjugate acid.

In this case, propanoic acid act as a weak acid whereas sodium propanoate act as a conjugate base making it a acidic-buffer solution.

(d)

$C_2H_5COONa (aq) \to C_2H_5COO^-(aq) +Na^+(aq)$

(e)

In this case, propanoic acid act as a weak acid whereas sodium propanoate act as a conjugate base making it a acidic-buffer solution.

If we add a strong acid, say $HCl$ in the solution, then most of the $H^+$ ions from $HCl$ will react with $C_2H_5COO^-$ ions forming $C_2H_5COOH$ resulting in very less increase in $H^+$ ions ,hence the $pH$ will slightly change.

$C_2H_5COO^-(aq)+H^+(aq)\to C_2H_5COOH(aq)$

Also , if a strong base like sodium hydroxide is added, the hydroxide ions from $NaOH$ react with the few $H^+$ ions present. Then more of the acetic acid reacts with water, restoring the $H^+$ ion concentration almost to its original value.

$C_2H_5COOH(aq) +H_2O(l)\to C_2H_5COO^-(aq)+H_3O^+(aq)$