Sulfate is easier reduced than hydrogen.
Not exactly. That's where the concentration comes into play.
Let's take a less reactive metal like copper. It doesn't dissolve in diluted sulfuric acid, because H+
is not and oxidizer strong enough. However, in concentrated solutions, and in the low pH (which is typical in concentrated acid solution), SO42-
becomes a relatively strong oxidizing agent:
and it can oxidize copper producing SO2
. Note SO42-
is not an oxidizer without H+
, it is quite stable and inert.
When it comes to lithium, it is so reactive it will be oxidized even by water alone, no need for any acid (technically, as water dissociates into H+
, we can assume there is always an acid present in water). When you add sulfuric acid, it doesn't have to behave like a strong oxidizer, there is plenty of H+
and water that will easily react with the lithium itself, producing hydrogen.
I suppose in the case of concentrated sulfuric acid some SO2
can be produced by side reactions, but I would not expect it to be the main product.