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Exercise: Halogenation of methane

Exercise

Which order of reactivity in the radical halogenation of methane is correct?

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Bromine > chlorine > fluorine > iodine

Fluorine > chlorine > bromine > iodine

Bromine > chlorine > fluorine > iodine

Chlorine > fluorine > bromine > iodine

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Exercise

Which statements are true?

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Fluorine is the most reactive halogen. Without any special precautions, a mixture of methane and fluorine at standard pressure explodes.

The halogenation of methane is a gas-phase reaction with a homolytic bond breakage. Therefore, the activation energy of the halogenation is equivalent to the association energy of the respective halogen. The kinetics of methane halogenation can easily be described, as the association energies of all halogens are known.

The reaction entropy of methane halogenation is approximately zero, because in the reaction two molecules of gaseous starting products are converted into two molecules of gaseous products.

Chlorine is the most reactive halogen. Without any special precautions, a mixture of methane and chlorine at standard pressure explodes.

The halogenation of methane is a gas-phase reaction with a homolytic bond breakage. Therefore, the activation energy of the halogenation is equivalent to the dissociation energy of the respective halogen. The kinetics of methane halogenation can easily be described, as the dissociation energies of all halogens are known.

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Exercise

Fluorination: The activation energy of chain initiation in the transformation of fluorine to fluorine radicals has a relatively high value of +155 kJ/mol.

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In addition, chain initiation must occur particularly frequently, as each fluorine radical initiates merely a couple thousand fluorination reactions.

However, chain initiation must take place only a few times, since each fluorine radical initiates thousands of fluorination reactions.

In addition, chain initiation must occur quite often, as each fluorine radical initiates but only a few fluorination reactions.

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Exercise

Fluorination: Which statements are correct?

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The chain propagation reactions of radical methane fluorination display high activation energies. The reactions therefore occur easily at room temperature.

A chain reaction's activation energy should not be too high. Otherwise, the particularly reactive radicals that have formed in the beginning of the reaction will recombine rather than allowing chain propagation to occur.

Chain propagation of methane fluorination is particularly exothermic.

Chain propagation reactions of radical methane fluorination show low activation energies. Therefore, they occur easily at room temperature.

Chain propagation of methane fluorination is extremely endothermic.

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Exercise

Chlorination: Which statements are true?

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Methane chlorination is more easily controllable than the fluorination is, because chlorination is less exothermic.

Higher activation energy in chain initiation is one reason for the lower reaction rate of chlorination in comparison to that of fluorination.

During chlorination, only a few chlorine radicals must be generated by the chain initiation in order to maintain chain propagation.

In comparison to fluorination, chlorination reaction rates are low, because a lower activation energy is required during chain initiation.

In comparison to fluorine, chlorine's reactivity in methane halogenation is much higher. In contrast to fluorine, therefore, special measures must be taken to prevent an explosion.

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Exercise

Bromination: The first propagation reaction of the radical bromination of methane is much more endothermic. Its activation energy is higher than that of chlorination.

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Therefore, the bromination occurs even at relatively low temperatures.

In the halogenation of methane, bromine is much less reactive than chlorine, although the bromination is an exothermic reaction whose reaction enthalpy amounts to -30 kJ/mol.

Therefore, even at 300 °C the chain propagation proceeds just slowly. Bromine is much less reactive in methane halogenation than chlorine is.

In the halogenation of methane, bromine is much more reactive than chlorine. This is because the bromination is an exothermic reaction whose reaction enthalpy amounts to -30 kJ/mol.

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Exercise

Iodination: Which statements are correct?

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Radical iodination of methane does not occur, because the activation energy of the chain initiation (the formation of iodine radicals from iodine) is higher than in fluorination.

The activation energy of the iodination's chain initiation (the formation of iodine radicals from iodine) is lower than that of fluorination. Therefore, this reaction step obviously does not prevent iodination.

The first propagation reaction of iodination is extremely exothermic. Therefore, the reaction occurs even at particularly low temperatures.

The first propagation reaction of iodination is very endothermic. Therefore, methane iodination does not take place.

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