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The mechanism for the substitution reaction between methane and chlorine in ultraviolet light involves free radicals - Leaving Cert Chemistry - Question 10 - 2019
Question 10
The mechanism for the substitution reaction between methane and chlorine in ultraviolet light involves free radicals. (i) Explain the underlined term. (ii) How are... show full transcript
Worked Solution & Example Answer:The mechanism for the substitution reaction between methane and chlorine in ultraviolet light involves free radicals - Leaving Cert Chemistry - Question 10 - 2019
Step 1
Explain the underlined term.
Answer
Free radicals are atoms or groups of atoms that have unpaired electrons, making them highly reactive species. In the context of chemical reactions, free radicals are essential because they can initiate and propagate a reaction by reacting with other molecules.
Step 2
How are chlorine free radicals formed at the initiation stage of this reaction?
Answer
Chlorine free radicals are formed when ultraviolet light breaks the bond in a chlorine molecule (Cl₂), leading to the formation of two chlorine free radicals (Cl•) through a process known as homolytic fission.
Step 3
Step 4
Write balanced equations, using dots to label the free radicals, for the two reactions that repeat in the chain reaction to give chloromethane and hydrogen chloride.
Answer
The propagation steps can be represented as follows:
- Cl• + CH₄ → HCl + CH₃•
- CH₃• + Cl₂ → CH₃Cl + Cl•
These equations show the consumption of free radicals in the chain reaction.
Step 5
Step 6
Explain why only a trace quantity of this product is formed.
Answer
Only a trace quantity of ethane is formed because the reaction between methyl radicals (CH₃•) and chlorine occurs at a lower probability compared to other competing reactions. The concentration of chlorine is usually much greater, leading to a preferential formation of chloromethane rather than ethane.