So where does the energy come from? Since energy is always conserved, we can det

Question

So where does the energy come from?

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.

Explanation / Answer

The reaction is H2(g)+ 0.5O2(g) ---->H2O(g)

enthalpy change= energy required to break the bonds-energy required to form the bonds

bond enthalpy : H-H= 436 KJ/mole, O=O : 498 Kj/molre and there are 2 O-H bonds in water. bond energy of each = 463

hence enthalpy change= 436+0.5*499-2*463 = -240.5 Kj/mole

since the enthalpy change is -ve, this much heat energy is liberated.

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