The chemical reactions represented by this sequence are as follows:

C (carbon) reacts with O (oxygen) to form CO (carbon monoxide)CO reacts with O to form CO2 (carbon dioxide)S (sulfur) reacts with O to form SO2 (sulfur dioxide)SO2 reacts with O to form SO3 (sulfur trioxide)K (potassium) reacts with O to form K2O (potassium oxide)