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Halogenalkanes have been widely used as aerosol propellants, refrigerants and solvents for many years - CIE - A-Level Chemistry - Question 2 - 2011 - Paper 2
Question 2
Halogenalkanes have been widely used as aerosol propellants, refrigerants and solvents for many years. Fluoroethane, CH₃CH₂F, has been used as a refrigerant. It may... show full transcript
Worked Solution & Example Answer:Halogenalkanes have been widely used as aerosol propellants, refrigerants and solvents for many years - CIE - A-Level Chemistry - Question 2 - 2011 - Paper 2
Step 1
Calculate C–F bond energy
Answer
To find the C–F bond energy, we first need to determine the total bond energy used to break the bonds in the reactants and then subtract the bond energy of the products.
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Step 1: Breaking Bonds\
The bond energies broken in the reaction:
- 4 C–H: 4 x 410 kJ mol⁻¹ = 1640 kJ mol⁻¹
- 1 C–C: 610 kJ mol⁻¹
The total energy required for breaking bonds is:
[ 1640 + 610 = 2250 ext{ kJ mol}^{-1} ]
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Step 2: Making Products\
The bond energies formed in the reaction:
- 5 C–H: 5 x 410 kJ mol⁻¹ = 2050 kJ mol⁻¹\
- 1 C–C: 350 kJ mol⁻¹
- 1 C–F: E
The energy released when forming products is:
[ 2050 + 350 + E = (2400 + E) ext{ kJ mol}^{-1} ]
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Step 3: Using the Thermodynamic Equation\
According to the enthalpy change of reaction:
[ ext{Δ}H_{reaction} = (2400 + E) - 2250 = -73 ext{ kJ mol}^{-1} ]
By rearranging, we find:
[ 2400 + E = 2177 ]
[ E = 2177 - 2400 ]
[ E = -223 \text{ kJ mol}^{-1} . ]
Step 2
State two reasons for the use of aerosol propellants
Answer
Two reasons why compounds like CH₃CH₂F and CCl₂F₂ are used as aerosol propellants and refrigerants include:
- Non-toxic: These compounds are generally non-toxic to humans, making them safer for use in consumer products.
- Volatile: They have suitable volatility, which allows them to turn from liquid to gas easily, providing effective propulsion in aerosol products.
Step 3
Explain damage to ozone layer
Answer
CCl₂F₂ contributes to ozone layer depletion because it contains chlorine atoms that, when released into the atmosphere, can break down ozone molecules. The C–Cl bond in CCl₂F₂ is weaker and can be broken by ultraviolet (UV) light, leading to the release of reactive chlorine species. These species catalyze reactions that destroy ozone, whereas CH₃CH₂F does not release chlorine and is less harmful to the ozone layer.
Step 4
What is meant by the term enhanced greenhouse effect?
Answer
The enhanced greenhouse effect refers to the increased capacity of the atmosphere to trap heat due to higher concentrations of greenhouse gases, such as carbon dioxide and methane. This phenomenon results from human activities that increase greenhouse gas emissions, leading to greater global warming.
Step 5