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It seems intuitive that concentrated solutions would react faster, but, when I stopped and thought about it, I got confused. Wouldn't concentrated acids and bases be in contact with less water than their more diluted counterparts, therefore being less dissociated and having less ions (H+ or OH-) available for reactions? How would completely concetrated acids (since bases would be solid) even react as acids?

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  • $\begingroup$ Acids and bases do not have to dissociate to react with each other. The proton transfer can occur directly between them, though this is rare in practice, because before you run out of water, you typically begin to lose acid to the gas phase. $\endgroup$ – TAR86 Jan 11 at 22:19
  • $\begingroup$ What if the acid is not gaseous on its own, but liquid like H2SO4? $\endgroup$ – Shannon Jan 12 at 13:13
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Strong acid dissociate, because they are stronger acids then $\ce{H3O+}$, and react with its conjugated base $\ce{H2O}$:

$$\ce{HA + H2O -> A- + H3O+}$$

Additionally, the $\ce{H3O+}$ activity coefficient steeply grows in concentrated acids, and so does the acid reactivity.

In concentrated $\ce{H2SO4}$ and oleum, the role of $\ce{H3O+}$ is overtaken by $\ce{H3SO4+}$:

$$\ce{2 H2SO4 <=> H3SO4+ + HSO4-}$$

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