Acetylene welding is a process in which like metals are joined by heating them to their melting temperature with an oxy-acetylene flame produced by a torch.

Used in welding metals, the reaction of acetylene with oxygen has = -1256.2

= -1256.2 .

Acetylene.

If your question is "Why is Acetylene and Oxygen used for welding/cutting NOT Oxygen and air"? This is why. Acetylene and air mixed in correct proportions produces a flame temperature of 4500F to 4800F degrees. Acetylene mixed with Oxygen in the correct proportions produces a flame well above 5500F degrees. This extra heat melts the metal faster and increases the speed you can cut or weld. Other gasses instead of Acetylene are used but do not produce as high a temperature. Oxygen is almost always used with these other gasses.

Simply burn the Acetylene (Ethyne) gas in open air. (As in case of gas welding).