indictae the reactant that is a bronsted lowry acid.
HCN(aq) +H2O (l)---> H3O+(aq)=CN-(aq)
i think it is HCN the weak acid substance which acts as a proton (H+) donor and CN the weak base?
You are right. The HCN donates the proton (to H2O) so that makes HCN the Bronsted acid and CN^- is its conjugate base. H2O in the equation is the Bronsted base and H3O^+ is its conjugate acid.