Spontaneous reaction is those which proceed at very fast rate. A chemical reaction which is accompanied by evolution of heat is known as exothermic reaction. Spontaneous reactions are also known as exothermic reactions. In the exothermic reactions internal energy change has negative sign at constant temperature i.e. ∆U has negative sign.
For example:
Carbon (graphite), when initiated by ignition undergoes combustion to give carbon dioxide. The reaction is exothermic reaction.
C (s) + O 2 (g) ————-> CO 2 (g) …………….. ∆H = – 393.5 kJ