Chapter U6, Problem SII3RE

Interpretation Introduction

(a)

Interpretation:

The equilibrium constant reaction must be written for the dissociation of HF(aq) to H ⁺ (aq) and F-(aq).

Concept introduction:

Acid dissociation constant (K_a) is small for weak acid. It is the ratio of concentration of products to reactants.

For a weak acid HA, K_a can be written as

${\text{K}}_{\text{a}}\text{=}\frac{\text{[H + ][A-]}}{\text{[HA]}}$, where [H ⁺ ], [A^-] and [HA] are concentrations of proton, counter anion and the acid respectively.

Answer to Problem SII3RE

Equilibrium constant reaction is given below.

${\text{K}}_{\text{a}}\text{=}\frac{{\text{[H}}^{\text{+}}{\text{][F}}^{\text{-}}\text{]}}{\text{[HF]}}.$ where [H ⁺ ], [F^-] and [HF] are concentrations of proton, fluoride anion and the acid respectively.

Explanation of Solution

Weak acid HF undergoes dissociation in aqueous solution to produce H ⁺ , F^-. Thus the equilibrium constant reaction is given as the ratio of products to reactant.

Interpretation Introduction

(b)

Interpretation:

At equilibrium [H ⁺ ] is greater than, les than or equal to [F^-] must be explained.

Concept introduction:

HF dissociates in aqueous solution to produce equal number of H + and F - ions.

Answer to Problem SII3RE

Concentration of H + will be equal to concentration of F^-.

[H ⁺ ]=[F^-]

Explanation of Solution

$\text{HF(aq)}\rightleftharpoons {\text{H}}^{\text{+}}{\text{(aq) + F}}^{\text{-}}\text{(aq)}\text{.}$

Thus from the dissociation equilibrium of HF it is clear that concentration of H + will be equal to concentration of F^-.

Interpretation Introduction

(c)

Interpretation:

At equilibrium [H ⁺ ] is greater than, les than or equal to [HF] must be explained.

Concept introduction:

HF is weak acid and so it dissociates in aqueous solution partially to produce equal number of H + and F - ions.

Answer to Problem SII3RE

Concentration of H + will be less than concentration of HF.

[H ⁺ ]<[HF]

Explanation of Solution

$\text{HF (aq)}$ ⇌ ${\text{H}}^{\text{+}}\text{(aq) +}$${\text{F}}^{\text{-}}$$\text{(aq)}$.

Thus from the dissociation equilibrium of HF it is clear that the equilibrium is more towards reactant side. Thus HF dissociates partially to produce very less number of H + ions.

Thus concentration of H + will be less than the concentration of HF.

Interpretation Introduction

(d)

Interpretation:

The direction the shift in the equilibrium needs to be explained if more H + is added.

Concept introduction:

As per Le Chatelier’s principle if any product species is added then the equilibrium is shifted to reactant side.

Answer to Problem SII3RE

If more H + is added the equilibrium would shift to reactant side that is HF side.

Explanation of Solution

$\text{HF(aq)}$ ⇌ ${\text{H}}^{\text{+}}{\text{(aq) + F}}^{\text{-}}\text{(aq)}$.

Thus from the dissociation equilibrium of HF it is clear that the equilibrium is more towards reactant side. Thus HF dissociates partially to produce very less number of H + ions. Now if more H + is added concentration of H + in aqueous solution will increase. To keep the K_a constant more HF will be produced. Thus the equilibrium would shift to left side or HF side.

Thus concentration of H + will be less than the concentration of HF.

Interpretation Introduction

(e)

Interpretation:

Whether [HF] increase, decrease or stay the same as the system proceeded back to equilibrium must be explained.

Concept introduction:

As per La Chatelier’s principle if any product species is added then the equilibrium is shifted to reactant side.

Answer to Problem SII3RE

[HF] would increase as the system proceeded back to equilibrium.

Explanation of Solution

$\text{HF(aq)}$ ⇌ ${\text{H}}^{\text{+}}{\text{(aq) + F}}^{\text{-}}\text{(aq)}$.

Thus from the dissociation equilibrium of HF it is clear that the equilibrium is more towards reactant side. Thus HF dissociates partially to produce very less number of H + ions. Now if more H + is added concentration of H + in aqueous solution will increase. So an increase in equilibrium constant (K_a) is expected. It is not possible as equilibrium constant only changes when temperature is changed. To keep the K_a constant more HF will be produced. Thus the equilibrium would shift to left side or HF side such that the ratio of concentration of products ( H + and F - ions) to concentration of HF remains the same.