This question is about oxygen. Hydrogen reacts with oxygen. $$2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$$ Figure 2 shows the relative energies of the reactants and products at a certain temperature. Label the activation energy on Figure 2. [1 mark] Determine the overall energy change for the reaction between hydrogen and oxygen shown in Question 03.1 Use Figure 2. [2 marks] Oxygen is in Group 6 of the periodic table. Figure 3 shows the outer energy levels in one molecule of oxygen (O$_2$). Draw the electrons in the outer energy levels in Figure 3. [2 marks] The equation shows the decomposition of hydrogen peroxide. $$2 H_2O_2 \rightarrow 2 H_2O + O=O$$ Table 1 shows the bond energies. Calculate the overall energy change for the reaction. [3 marks]

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This question is about oxygen - AQA - GCSE Chemistry: Combined Science - Question 3 - 2019 - Paper 1

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This question is about oxygen. Hydrogen reacts with oxygen. $$2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$$ Figure 2 shows the relative energies of the reactants and pr... show full transcript

Worked Solution & Example Answer:This question is about oxygen - AQA - GCSE Chemistry: Combined Science - Question 3 - 2019 - Paper 1

Step 1

Label the activation energy on Figure 2.

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Answer

To label the activation energy on Figure 2, draw a line from the energy of the reactants (800 kJ) to the top of the energy curve (2160 kJ). This line represents the activation energy required for the reaction to proceed.

Step 2

Determine the overall energy change for the reaction between hydrogen and oxygen shown in Question 03.1.

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Answer

To find the overall energy change, subtract the energy of the products from the energy of the reactants. The energy of the reactants (H $_2$ + O $_2$ ) is 800 kJ, and the energy of the products (2H $_2$ O) is 300 kJ. Thus, the overall energy change is:

$Energy\ change = 800\ kJ - 300\ kJ = 500\ kJ$

Step 3

Draw the electrons in the outer energy levels in Figure 3.

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In Figure 3, each oxygen atom has six outer electrons. Represent the two oxygen atoms overlapping, with two shared pairs of electrons (dots or crosses) between them, showing the bonding in the O $_2$ molecule.

Step 4

Calculate the overall energy change for the reaction.

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Answer

To calculate the overall energy change, we look at the bond energies involved:

Bonds broken: For the decomposition of 2 H $_2$ O $_2$ :
- O-O bond: 2 x 138 kJ = 276 kJ
- O-H bonds: 4 x 463 kJ = 1852 kJ
- Total energy for bonds broken = 276 kJ + 1852 kJ = 2128 kJ
Bonds made: In products (2 H $_2$ O and O=O):
- O-H bonds: 4 x 496 kJ = 1984 kJ
- O=O bond: 1 x 496 kJ = 496 kJ
- Total energy for bonds made = 1984 kJ + 496 kJ = 2480 kJ
Energy change:
- Using the formula: $Energy\ change = Bonds\ broken - Bonds\ made = 2128\ kJ - 2480\ kJ = -352\ kJ$

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