6 KClO3 (aq) + 3 H2SO4 (aq) → 4 ClO2 (g) + 2 HClO4 (l) + 3 K2SO4 (aq) + 2 H2O (l)
This is an oxidation-reduction (redox) reaction:
2 ClV - 4 e- → 2 ClVII (oxidation)
4 ClV + 4 e- → 4 ClIV (reduction)
KClO3 is both an oxidizing and a reducing agent (disproportionation (dismutation)).
This is a gas evolution reaction, ClO2 is the formed gas.