Problem. During the chemical reaction 2 mol of oxygen O₂ interacting with 4 mol of hydrogen H₂ produces 4 mol of water H₂O
2H₂ + O₂ → 2H₂O
Heat released during the process is equal to 9.7×10⁵ J. What fraction of mass is transformed into heat during reaction?
Solution. Mass and energy are related by the equation
E = mc²
and thus energy released during chemical reaction is
δE = δmc²
where δm is change in the mass of interacting particles. So
δm = = = 1.08×10⁻¹¹ kg
Molar mass of hydrogen H₂ is equal to 2 g, while molar mass of oxygen is 32 g so original mass of reactants is
m = 7.2×10⁻² kg
and fraction of mass transformed into heat during reaction is
= = 1.5×10⁻¹⁰ kg