Hydrogen reacts with oxygen to form steam - Edexcel - GCSE Chemistry - Question 10 - 2018 - Paper 1
Question 10
Hydrogen reacts with oxygen to form steam.
2H₂(g) + O₂(g) → 2H₂O(g)
Bond energies are shown in Figure 14.
Calculate the energy change for the reaction of 2 mol of... show full transcript
Worked Solution & Example Answer:Hydrogen reacts with oxygen to form steam - Edexcel - GCSE Chemistry - Question 10 - 2018 - Paper 1
Step 1
Calculate energy needed to break bonds
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Answer
To find the energy required to break the bonds in the reactants, we use the bond energies for 2 moles of hydrogen and 1 mole of oxygen:
Energy to break H-H bonds:
2×435 kJ mol−1=870 kJ
Energy to break O=O bonds:
1×500 kJ mol−1=500 kJ
Therefore, the total energy needed to break bonds is:
870+500=1370 kJ
Step 2
Calculate energy released when bonds are formed
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Answer
Next, we calculate the energy released when forming the bonds in the products (2 moles of H₂O). For each water molecule, 2 O-H bonds are formed:
Energy released for 2 O-H bonds per molecule:
2×460 kJ mol−1=920 kJ
Since we have 2 moles of H₂O:
2×920=1840 kJ
Step 3
Calculate overall energy change
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Answer
Finally, the overall energy change for the reaction is calculated by taking the difference between the energy needed to break bonds and the energy released when new bonds are formed:
Energy change=Energy needed−Energy released=1370 kJ−1840 kJ=−470 kJ
This indicates that the reaction is exothermic, and the energy change can be reported as either -470 kJ or 470 kJ released.
