Most Brønsted acids can act as oxidizing agents, because[dubious – discuss] the acidic proton can be reduced to hydrogen gas.
Some acids contain other structures that act as stronger oxidizing agents than hydrogen ions.
For example, copper is a rather unreactive metal, and has no reaction with concentrated hydrochloric acid.
Again, copper has no reaction with dilute sulfuric acid, but in concentrated sulfuric acid, the highly acidic environment and high concentration of sulfate ions allow the sulfate ions to act as an oxidizing agent.
Since sulfur is in its maximum oxidation state in the sulfate ion, it cannot act as a reducing agent.