Chemistry HELP - Rates of reaction, redox reactions? 10 points?

An structural isomer of bromobutane (C4H9Br) can be hydrolysed using aqueous sodium hydroxide to produce butanol. This can be represented by the following equation:

C4H9Br(l) + OH-(aq) > C4H9OH(l) + Br-(aq)

This reaction was investigated experimentally and the following results
were obtained:

Experiment A -
Initial [bromobutane], mol dm-3 = 0.01
Initial [OH-], mol dm-3 = 0.01
Initial rate, mol dm-3s-1 = 4.3 x 10 -4^

Experiment B
Initial [bromobutane], mol dm-3 = 0.01
Initial [OH-], mol dm-3 = 0.02
Initial rate, mol dm-3s-1 = 8.6 x 10-4^

Experiment C -
Initial [bromobutane], mol dm-3 = 0.02
Initial [OH-], mol dm-3 = 0.02
Initial rate, mol dm-3s-1 = 1.7 x 10-3^

(b) (i) Deduce the overall order of the reaction and write the rate equation for this reaction

(ii) What is meant by the rate-limiting step of a reaction mechanism?

(iii) What can you say about the rate-limiting step for the above reaction?

(iv) Suggest which isomer of bromobutane is likely to react via this rate-limiting step. Explain your answer fully.

To deduce the overall order of the reaction and write the rate equation, we need to examine the relationship between the initial concentrations of reactants (bromobutane and hydroxide ion) and the initial rate of the reaction.

(i) Overall order of the reaction:
Comparing Experiment A (where [OH-] is constant) with Experiment B (where [OH-] doubles), we can see that doubling [OH-] results in doubling the initial rate. This indicates that the reaction rate is directly proportional to the concentration of hydroxide ion. Therefore, the order of the reaction with respect to OH- is 1.

Similarly, comparing Experiment A (where [bromobutane] is constant) with Experiment C (where [bromobutane] doubles), we can see that doubling [bromobutane] results in quadrupling the initial rate. This indicates that the reaction rate is proportional to the square of the concentration of bromobutane. Therefore, the order of the reaction with respect to bromobutane is 2.

Adding the orders together, the overall order of the reaction is 1 + 2 = 3.

Rate equation: rate = k [C4H9Br]^2 [OH-]

(ii) The rate-limiting step of a reaction mechanism is the slowest step in the overall reaction. It determines the overall rate at which the reaction proceeds. All other steps in the reaction proceed at a much faster rate compared to the rate-limiting step.

(iii) From the rate equation (rate = k [C4H9Br]^2 [OH-]), we can see that the concentration of bromobutane (C4H9Br) is squared, indicating that the rate depends significantly on the concentration of bromobutane. This suggests that the rate-limiting step likely involves the bromobutane molecule and its interaction with the hydroxide ion.

(iv) The rate-limiting step is likely to involve the isomer of bromobutane that has a more difficult or slower reaction with hydroxide ion. Since the rate equation depends on [C4H9Br]^2, the isomer with more branching (greater steric hindrance) is expected to have a slower reaction rate. This is because the bulkier, more sterically hindered substituents make it more difficult for the hydroxide ion to approach and react with the bromobutane molecule. Therefore, the isomer with more branched substituents is likely to react via this rate-limiting step.