Carbon react with dilute hydrochloric acid?
Carbon don't react with acids, except concentrated Nitric acid. Concentrated nitric acid reacts with carbon to produce Water, Carbon dioxide, and Nitrogen dioxide.
What gas is involved in the reaction between sodium trioxocarbonate IV and dilute hydrochloric acid?
It does for example with dilute nitric acid. It depends on how strong oxidant is the acid. Metallic copper must be oxidized to react with acid which can be performed by the acid itself (nitric acid, sulfuric acid) or atmospheric oxygen. Thus copper will not react with non-oxidizing acid (like hydrochloric acid) but you can notice that if you let copper with hydrochloric acid on open air, it will oxidize after time, forming green products.