Explain why the rate of bromination of methane decreases if HBr is added to the reaction mixture.

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Identify the reaction mechanism for the bromination of methane, which is a free radical chain reaction involving initiation, propagation, and termination steps.

Understand that the initiation step involves the homolytic cleavage of Br2 to form two bromine radicals, which is crucial for the propagation of the chain reaction.

Recognize that the addition of HBr introduces additional bromine atoms in a non-radical form, which can react with the bromine radicals, effectively reducing their concentration.

Consider how the presence of HBr can lead to a termination step where bromine radicals react with HBr to form non-radical products, thus decreasing the number of radicals available for the propagation steps.

Conclude that the decrease in the concentration of bromine radicals due to the presence of HBr slows down the overall rate of the bromination reaction, as fewer radicals are available to sustain the chain reaction.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Bromination Mechanism

Bromination of methane typically occurs via a free radical mechanism, where bromine radicals (Br·) abstract hydrogen atoms from methane (CH4) to form bromomethane (CH3Br). This process involves initiation, propagation, and termination steps, with the rate of reaction heavily dependent on the concentration of bromine radicals in the mixture.

Effect of HBr on Radical Concentration

The addition of HBr introduces bromide ions (Br-) into the reaction mixture, which can react with bromine radicals to form HBr and regenerate bromine. This reaction effectively reduces the concentration of free bromine radicals available for the bromination of methane, thereby decreasing the overall rate of the bromination process.

Le Chatelier's Principle

Le Chatelier's Principle states that if a dynamic equilibrium is disturbed by changing the conditions, the system shifts to counteract the change. In this case, the addition of HBr shifts the equilibrium of the bromination reaction, favoring the formation of HBr over the production of bromine radicals, thus slowing down the rate of bromination.