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Using the values of bond enthalpy find the enthalpy change of the reaction

I dont uderstand this question i was given in my cambrige IGCSE Chemistry course book.

Please try and help

The water-splitting reaction is shown in the equation below:

2H2o -> 2H2 + O2

i) Calculate the energy needed to split the water molecules into H and O atoms:

2H2O -> 4H + 2O

ii) Calculate the energy change when the H and O atoms join to form H2 and O2 molecules:

4H + 2O -> 2H2 + O2

iii) is the overall reaction:

2H2O -> 2H2 + O2

exothermic or endothermic? use your answers to parts i and ii to explain your choice


kingchemist Wed, 03/07/2012 - 08:37

You need a table of bond enthalpies (energies) such as

This quotes H-O as 366, O=O as 498 and H-H as 436 kJ/mole. (Use your own data) You then need to consider which bonds need to be broken and which need to be made. Bond breaking is endothermic and bond making is exothermic.

One water molecule, H2O or H-O-H has TWO H-O bonds. So 1 mole H2O has 2 moles of H-O bonds which require 2 x 366 kJ of energy.  Two moles of water require 2 x 2 x 366 kJ. This is the energy needed to make 4moles of H atoms and 2 mole of O atoms

When you join H atoms, energy is released. For each mole of H-H bonds made from 2 mole of H atoms, you get 436 kJ. As you have 4 moles of H atoms you will make 2mole of H2 and release 2 x 436 kJ..

When you join O atoms, energy is released. For each mole of O=O bonds made from 2 mole of O atoms, you get 498 kJ. As you have 2 moles of O atoms you will make 1 mole of O2 and release 498 kJ. So for the previous two steps the energy released = 2 x 436 + 498 kJ

Compare the energy needed for the bond breaking with that for bond making. If you make more that you need to break the bonds, the reaction is exothermic. The difference is the amount of energy change.