# Why degree of dissociation/ionisation affected by dilution? [closed]

What happens when we dilute, say, an acid which is weak, and then add water to it? What motivates or pushes it to dissociate more?

We are just adding some molecules of water, right? So, what exactly happens to a weak acid like $$\ce{CH3COOH}$$ so it starts to dissociate more?

Is it because of the fact that water being a polar solvent having high dielectric constant starts to split the $$\ce{O-H}$$ bond in $$\ce{CH3COOH}$$, or something else?

• This is not a homework question 1st of all.And I wasn't able to grasp the concept that why Ostwald's dilution law comes so. – Abner Alfred Thompson Jan 31 '19 at 4:19
• Thanks @andselisk for improving the question while retaining the ideas of the OP. – Karsten Theis Jan 31 '19 at 16:46
• @AbnerAlfredThompson The idea of a "homework question" on this site is a question which doesn't show the person's efforts to solve the problem or express why they think some given answer is incorrect. – Tyberius Jan 31 '19 at 17:35
• Well then now it does, doesn't it?So,can you please put it off hold or I need to state something more to convince you...... – Abner Alfred Thompson Feb 1 '19 at 1:58

## 1 Answer

What motivates or pushes it to dissociate more?

Perhaps surprisingly, if you could look at an individual acid molecule, it does not (or hardly) changes its behavior upon dilution. It just dissociates with a certain rate, yielding the conjugate base and a hydrogen ion. There is no "extra motivation" to dissociate, and no "tapping on the shoulder" pushing it to react.

Why is the degree of dissociation/ionisation affected by dilution?

The reverse reaction, where a dissociated acid (the conjugated base) reacts with a hydrogen ion to make the acid again, is affected by dilution. Here, two molecules have to come together, and this takes longer when the solution is more dilute.

This description is for the simplest case, where the net reaction not only describes the stoichiometry but also the mechanism of the reaction. However, even if there are intermediates, at equilibrium the forward and reverse rates are equal, and when you dilute solution, rates of the reverse steps would be significantly lowered, leading to a higher degree of dissociation.

• Why so?Why when we are diluting it,it is taking longer to come together.I would guess that the opposite should happen as there are more water molecules so the chances that say a CH3COO- ion attracting an H+ from it's surrounding water should increase shouldn't it.Also,can this be the possible explanation that since we are adding more water to it and since water has high dielectric constant so it'll ionise CH3COOH more.Also,since water is more so it'll hydrate the broken ion and surround it,make it harder for them to recombine. – Abner Alfred Thompson Jan 31 '19 at 4:25
• Try adding your argument why Ostwald’s law is counterintuitive to you to your original question. If you do, the question might be reopened. – Karsten Theis Jan 31 '19 at 11:05
• I started a new question with two specific scenarios to give others a chance to address the ideas the OP shared in this question and in these comments. – Karsten Theis Jan 31 '19 at 16:44