Consider the following generic chemical equation: A + B → C + D Reactant A contains 85.1 J of chemical energy. Reactant B contains 87.9 J of chemical energy. Product C contains 38.7 J of chemical energy. If the reaction absorbs 104.3 J of chemical energy as it proceeds, how much chemical energy must product D contain

Product D contains 30 J

Explanation:

We know energy does not destroy or create, it only transforms. So, at the right side of the equation we have the energy of the reactant A and the energy of the reactant B, as a sum, and, as a result we have to get the same energy plus the energy absorbed by the reaction.

So, we have,

Ea+Eb = Eabsorbed+Ec+Ed

Ed = Ea+Eb-Eabsorbed-Ec

Ed = 85.1 J + 87.9 J - 104.3 J - 38.7 J

Ed = 30 J

= 238.6J

Explanation:

According to the law of conservation of energy, energy can neither be created nor be destroyed. It can only be transformed from one form to another.

Endothermic reactions are those in which heat is absorbed by the system and thus the energy of products is higher than the energy of reactants.

For the given reaction:

A + B ⇄ C + D

Energy of A = 85.1 J

Energy of B = 87.9 J

Product C contains 38.7 J

Energy balance:  

∑ enthalpy of the reactants + energy added = ∑ enthalpy of the products + energy released.

∑ enthalpy of the reactants = 85.1 J + 87.9 J = 173 J

energy added = 104.3 J

∑ enthalpy of the products = 38.7 J + D

energy released = 0

Equation:

173J + 104.3J = 38.7 + D + 0  

⇒ D = 173J + 104.3J - 38.7J

= 238.6J

which is the chemical energy of the product D