Acetylene (c2h2) undergoes combustion in excess oxygen to generate gaseous carbon dioxide and water. given δh°f[co2(g)] = –393.5 kj/mol, δh°f[h2o(g)] = –241.8 kj/mol, and δh°f[c2h2(g)] = 226.6 kj/mol, how much energy is released (kj) when 10.5 moles of acetylene is burned?

Answer is: 13181,7 kJ of energy is released when 10.5 moles of acetylene is burned.
Balanced chemical reaction: C₂H₂ + 5/2O₂ → 2CO₂ + H₂O.
ΔHrxn = sum of ΔHf (products of reaction) - sum of ΔHf (reactants).
Or ΔHrxn = ∑ΔHf (products of reaction) - ∑ΔHf (reactants).
ΔHrxn - enthalpy change of chemical reaction.
ΔHf - enthalpy of formation of reactants or products.
ΔHrxn = (2·(-393,5) + (-241,8)) - 226,6 · kJ/mol.
ΔHrxn = -1255,4 kJ/mol.
Make proportion: 1 mol (C₂H₂) : -1255,4 kJ = 10,5 mol(C₂H₂) : Q.
Q = 13181,7 kJ.

Heat is involved in chemical change in two ways; chemical changes either result in heat being added to or removed from an object, and chemical changes require a certain amount of heat before they can take place.