H2 + o2

The reaction you are referring to is the combination of hydrogen gas (H2) with oxygen gas (O2) to form water (H2O). This reaction can be represented by the following equation:

2H2 + O2 -> 2H2O

This reaction is known as a synthesis reaction because it combines two or more substances to form a single product. It is also an exothermic reaction, meaning it releases heat energy.