The chemical reactions represented by this sequence are as follows:

C $carbon$ reacts with O $oxygen$ to form CO $carbonmonoxide$CO reacts with O to form CO2 $carbondioxide$S $sulfur$ reacts with O to form SO2 $sulfurdioxide$SO2 reacts with O to form SO3 $sulfurtrioxide$K $potassium$ reacts with O to form K2O $potassiumoxide$