As mentioned in the earlier post, the other important point in Ionic Equilibrium I is the ability to identify the strength of the acid and the base. Hence, in this entry, we will be discussing about acid and base strength.
Definition of strong and weak acids:
As mentioned in the earlier post, you were introduced to the Bronsted-Lowry theory of acid and base. In addition to that, in the A level syllabus, we are required to be able to differeniate between a strong acid and a weak acid (likewise for base).
A strong acid is one who fully dissociates, while a weak acid undergoes partial dissociation. But how do we really tell whether an acid fully dissociates or not?
A simple gauge will be the measurement of the pH of the solution. A weak acid's concentration is far larger than that of the concentration of H+ ions that it produces. (You may read here for the explanation to why concentration is not a good gauge)
Understanding Dissociation Constants:
You have just read about what an acid, a base and how to define it's respective strength. It appears that would be sufficient. Now, why do we need to learn ionic equilibrium then, a chapter which is a bane to numerous students?
If you recall the definition of a weak acid, it undergoes partial dissociation. Hence, the equation below signififies the dissociation of a weak acid.
Definition of strong and weak acids:
As mentioned in the earlier post, you were introduced to the Bronsted-Lowry theory of acid and base. In addition to that, in the A level syllabus, we are required to be able to differeniate between a strong acid and a weak acid (likewise for base).
A strong acid is one who fully dissociates, while a weak acid undergoes partial dissociation. But how do we really tell whether an acid fully dissociates or not?
A simple gauge will be the measurement of the pH of the solution. A weak acid's concentration is far larger than that of the concentration of H+ ions that it produces. (You may read here for the explanation to why concentration is not a good gauge)
Understanding Dissociation Constants:
You have just read about what an acid, a base and how to define it's respective strength. It appears that would be sufficient. Now, why do we need to learn ionic equilibrium then, a chapter which is a bane to numerous students?
If you recall the definition of a weak acid, it undergoes partial dissociation. Hence, the equation below signififies the dissociation of a weak acid.
HA + H2O <-> H3O+ + A-
where, < - > refers to the reversible arrow, thus showing that HA is a weak acid.
Using your knowledge of chemical equilibrium, at a particular temperature, we will have the acid (and base) dissociation constant. This is an equilibrium constant which allows us to see the ratio of ionised products to it initial undissociated form, thus showing the extend of dissociation. It is meaningless for a strong acid and strong base to have a Ka and Kb value respectively.
Like all equilibrium constants, the Ka and Kb values are only dependent on temperature.
With an appreciation of how acid (or base) dissociation constants are obtained and its intention, in the next ionic equilibrium 1 entry, we will talk about the application of dissociation constants.
Using your knowledge of chemical equilibrium, at a particular temperature, we will have the acid (and base) dissociation constant. This is an equilibrium constant which allows us to see the ratio of ionised products to it initial undissociated form, thus showing the extend of dissociation. It is meaningless for a strong acid and strong base to have a Ka and Kb value respectively.
Like all equilibrium constants, the Ka and Kb values are only dependent on temperature.
With an appreciation of how acid (or base) dissociation constants are obtained and its intention, in the next ionic equilibrium 1 entry, we will talk about the application of dissociation constants.
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Article written by Kwok YL 2007. (edited in Apr 2009)
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