Be sure to include the proper sign (+ or –) in your answer.

Question

One proposed source of clean energy is the combustion of hydrogen in the presence of oxygen. This reaction releases a significant amount of energy, and the only product of the reaction is water. The balanced chemical equation is shown here:

2H₂ (g) + O₂ (g) => 2H₂O (g) + energy

Since energy is always conserved, we can determine the total energy released or absorbed by a chemical reaction by subtracting the total energy released in forming the bonds from the total energy required to break all of the bonds.

In doing the calculation this way, if the reaction requires a net expenditure of energy, the energy term is positive and it tells us how much energy must be put into the reaction system for the reaction to occur. If there is a net release of energy, the energy term is negative and it tells us how much energy flows out of the reaction system. What is the total energy change for the reaction of molecular hydrogen with molecular oxygen to form water (based on the balanced chemical equation above)? Be sure to include the proper sign (+ or –) in your answer.

Answer 1

You need to look up bond energies for H-H bond, O=O bond, and O-H bond. Then, since H2O has 2 -OH bonds and there are 2 moles of water, you multiply by 4 to get energy released by forming 2 moles H2O. It will have a negative sign. Find bond energy for 2 moles H-H and 1 mole O=O and add them together to get energy needed to break bonds in the reactants. This will have a positive sign. Take the difference to get your answer.