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This question is about oxygen - AQA - GCSE Chemistry Combined Science - Question 3 - 2019 - Paper 1
Question 3
This question is about oxygen. Hydrogen reacts with oxygen. 2 H₂ (g) + O₂ (g) → 2 H₂O (g) Figure 2 shows the relative energies of the reactants and products at a c... show full transcript
Worked Solution & Example Answer:This question is about oxygen - AQA - GCSE Chemistry Combined Science - Question 3 - 2019 - Paper 1
Step 1
Step 2
Determine the overall energy change for the reaction between hydrogen and oxygen shown in Question 03.1
Answer
From Figure 2, the energy of the reactants (2H₂ + O₂) is 800 kJ while the energy of the products (2H₂O) is 300 kJ. The overall energy change can be calculated as follows:
the energy change = energy of products - energy of reactants
Thus, the energy change = 300 kJ - 800 kJ = -500 kJ.
Step 3
Draw the electrons in the outer energy levels in Figure 3
Answer
In Figure 3, draw the outer energy levels of the two oxygen atoms in O₂. Each oxygen atom has 6 electrons in its outer shell. Show these as two overlapping circles, each with 6 dots representing the outer electrons and 2 shared pairs indicated by pairs of dots or a line to show the bonding between them.
Step 4
Calculate the overall energy change for the reaction.
Answer
To calculate the overall energy change for the decomposition of hydrogen peroxide, we need to consider the bond energies:
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Bonds Broken:
- 2(O-H) at 463 kJ each = 2 × 463 = 926 kJ
- 1(O-O) at 138 kJ = 138 kJ
- Total energy for bonds broken = 926 + 138 = 1064 kJ
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Bonds Made:
- 4(O-H) at 463 kJ each = 4 × 463 = 1852 kJ
- 1(O=O) at 496 kJ = 496 kJ
- Total energy for bonds made = 1852 + 496 = 2348 kJ
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Energy Change:
Energy change = Bonds broken - Bonds made Energy change = 1064 - 2348 = -1284 kJ The overall energy change for the reaction is -284 kJ.