Concept explainers
(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 (Ka) is small for weak acid. It is the ratio of concentration of products to reactants.
For a weak acid HA, Ka can be written as
Answer to Problem SII3RE
Equilibrium constant reaction is given below.
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.
(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
Thus from the dissociation equilibrium of HF it is clear that concentration of H + will be equal to concentration of F-.
(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
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.
(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
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 Ka 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.
(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
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 (Ka) is expected. It is not possible as equilibrium constant only changes when temperature is changed. To keep the Ka 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.
Chapter U6 Solutions
Living By Chemistry: First Edition Textbook
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