This question is about hydrogen peroxide, H₂O₂ - AQA - A-Level Chemistry - Question 3 - 2022 - Paper 1
Question 3
This question is about hydrogen peroxide, H₂O₂.
The half-equation for the oxidation of hydrogen peroxide is
H₂O₂ → O₂ + 2H⁺ + 2e⁻
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Worked Solution & Example Answer:This question is about hydrogen peroxide, H₂O₂ - AQA - A-Level Chemistry - Question 3 - 2022 - Paper 1
Step 1
Give an ionic equation for the reaction between potassium manganate(VII) and acidified hydrogen peroxide.
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Answer
The ionic equation for the reaction can be written as:
Calculate the concentration of H₂O₂ in the diluted solution, considering volume is 25.0 cm³:
ext{Concentration of H}_2 ext{O}_2 = rac{ ext{Moles of H}_2 ext{O}_2}{ ext{Volume}}
Finally, adjust for the dilution to find the concentration in the original solution, which is:
ext{Concentration in original solution} = rac{0.177}{0.0500}
Therefore, the final result is:
extConcentration=1.43extmoldm−3
Step 3
State why an indicator is not added in this titration.
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Answer
An indicator is not added in this titration because potassium manganate(VII) is self-indicating. The solution changes color from purple to colorless as the reaction progresses and reaches the endpoint.
Step 4
Give the oxidation state of oxygen in hydrogen peroxide.
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Answer
In hydrogen peroxide (H₂O₂), the oxidation state of oxygen is -1.
Step 5
Give an equation for the decomposition of hydrogen peroxide into water and oxygen.
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Answer
The equation for the decomposition of hydrogen peroxide is:
ext2H2extO2→2extH2extO+extO2
Step 6
Calculate the amount, in moles, of hydrogen peroxide that would be needed to produce 185 cm³ of oxygen gas at 100 kPa and 298 K.
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Using the ideal gas law, the amount of oxygen can be calculated as follows:
Convert volume from cm³ to m³:
V=185imes10−6extm3
Using R = 8.31 J K⁻¹ mol⁻¹ and solving for moles of gas:
n = rac{PV}{RT} = rac{100 imes 10^3 imes 185 imes 10^{-6}}{8.31 imes 298}
This results in:
nO2=0.00748
From the balanced equation for decomposition, 2 moles of H₂O₂ produce 1 mole of O₂:
Therefore, the moles of H₂O₂ needed is:
nH2O2=2imesnO2=2imes0.00748=0.01496extmol
Step 7
Define the term mean bond enthalpy.
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Answer
Mean bond enthalpy is defined as the average energy required to break one mole of a specific type of bond in a molecule in the gas phase, averaged over a range of molecules.
Step 8
Use the equation and the data in Table 3 to calculate a value for the O–O bond enthalpy in hydrogen peroxide.
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To calculate the O–O bond enthalpy in H₂O₂, consider the bonds broken and formed in the reaction:
From the equation, two O–O bonds are broken:
Energy required to break 2 O–O bonds can be calculated using mean bond enthalpies from Table 3:
ext{Mean bond enthalpy for O-O} = rac{ΔH + 2 imes ext{Mean bond enthalpy of O–H}}{2}
