The dissociation of sulfuric acid in water has two steps. First, it loses one hydrogen, and second, it loses the last hydrogen. These two dissociations depend on the concentration of the mixture as well pointed in this work of Prof. Pavlov et. al.
Article Influence of H2SO4 concentration on the mechanism of the pro...
Experimentally, we can obtain a 50 % wt mixture using two procedures:
- Start with pure water and add sulfuric acid.
- Start with pure sulfuric acid and put water.
Following these two procedures, will we get the same dissociation proportions in the mixture?
I am physicist and this is not my research field. Can you recommend me any book or paper? Thank you
When speaking about dissociation of electrolytes, we are used to talk about static processes. I mean, if we have a 50 % wt concentration of H2SO4 in water, the proportions of different ions that will be in the mixture are clearly defined. We can check this proportions in commonly used lead acid battery books like:
Book Lead-Acid Batteries: Science and Technology
(p. 122)Book Lead-Acid Batteries
(p. 45)However, what happens in a dynamic system, like a lead acid battery? In such a system, the electrolyte concentration changes with the battery cycling. There are a lot of different processes, like, chemical reactions, diffusion, convection, … Indeed, it is a very complex system. In most of the researches it is supposed that more than 90 % of the sulphuric acid will be as a HSO4 ion, and the rest will be H2SO4 and SO4.
Let´s simplify the system. With a concentration of 0.01 M, the proportions of ions in the electrolyte have to be (~ 60 % HSO4 and ~ 40 % SO4). If we want to arrive to this concentration from a more concentrated mixture (> 60 % HSO4 and < 40 % SO4) we have to add water. If we want to arrive to this concentration from a less concentrated mixture (< 60 % HSO4 and > 40 % SO4) we have to add sulphuric acid. Will the system need the same time to arrive the thermodynamic equilibrium from both starting points? Is the system symmetric?
Once the final solution is obtained there should be no difference wether it is obtained with one method or another, the distribution of species in an equilibrium depends solely on the concentration as you stated. There is a difference though in the methods, when you add water to pure sulfuric acid overheating may occur until water boiling point ("do not water in acid" rule).
see here maybe https://chemistry.stackexchange.com/questions/893/why-first-water-and-then-acid
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