Why triple bond in N2 is so unreactive, but in C2H2 so reactive

Electronegativity differences. Because nitrogen is more electronegative than carbon, the pi molecular orbitals (the ones making the triple bond) of N2 are lower in energy than that of C2H2.

Explanation:

Easily put... They are different compounds with different preparations and stored energy. Acetylene is formed in the partial combustion of methane so it is unstable to begin with. Triple bonded nitrogen you can think of it as a lowest energy state. Energy goes into forming acetylene, energy is released when nitrogen triple bonds..

Hope it helps..!!