In Biochemistry of a sulfuric acid world, I assumed that autotrophs would strip hydrogen from H2SO4 (which is their biosolvent) to fix carbon, just like plants on Earth strip hydrogen from water (which is our biosolvent).
But, the one world we know of with large quantities of naturally occurring concentrated sulfuric acid (Venus) also has large quantities of hydrochloric acid. Now, it seems that, given the choice between water and HCl as hydrogen sources, water is probably energetically preferred. But what about the choice between HCl and H2SO4?
Is it more energetically favorable for plant-analogs on a sulfuric acid world, with HCl dissolved in the sulfuric acid, to split sulfuric acid and release oxygen and sulfur trioxide, or to split HCl and release chlorine gas?
Or, a third option--perhaps they produce liquid Peroxydisulfuric acid instead?
(There should also be large quantities of HF, but fluoride is unambiguously harder to oxidize than chloride, so generating free fluorine is not an option!)