In an exothermic reaction the energy of the product is less than the energy of the reactants.
Matter is a slightly archaic word for something with mass, as in the conservation of matter (which must be paired with the conservation of energy to still hold true. Mass can be converted back and forth between energy, so therefore so can matter. Of course relativistic mass is conserved as it's a function of the energy of an object in that reference frame.
The effective nuclear charge is the net positive charge experienced by valence electrons
Low pressure and high temperature
CH4 + O2 -> H2O + CO2
Then balance:
CH4 + 2 O2 -> 2 H2O + CO2
