2S(s) + O2 (g) → SO2 (g)
CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l)
2CH2CH2 (g) + H2 (g) → CH3CH3(g)
2FeCl3 (aq) + H2 (g) → 2FeCl2(aq) + 2HCl (aq)
Corrosion is an example of ___ (redox/reduction/physical) reaction.