Bond energy what makes a reaction endothermic or exothermic worksheet answers

The gain or loss of heat energy is what makes a reaction either endothermic or exothermic. A reaction is said to be endothermic if it absorb heat from the environment while a reaction is described as exothermic if it gives out heat to the environment. During chemical reaction, the bond energy of the reactants needs to be broken and new bond has to be formed in the products. The formation and breaking of bonds requires energy,

The mass has escaped into a gas which you can not see so the law stays true.