Could someone check my answers? Please! :)

Hi TSR.

Is anyone able to check over my work, and tell me if I got the answers right or wrong. Thanks. :smile:

It's Chemistry Nuffield.

Q2. 2-bromo-2-methylpropane reacts with aqueous sodium hydroxide as shown in the equation below.

(CH3) 3CBr + NaOH --> (CH3) 3COH + NaBr

A series of experiments was carried out to investigate the kinetics of this reaction.

Initial concentration of
(CH3) 3CBr/mol dm-3
5.0 x 10-4
1.5 x 10-3
1.5 x 10-3

Initial concentration of
NaOH/mol dm-3
2.0 x 10-2
2.0 x 10-2
4.0 x 10-2

Initial rate of reaction
mol dm-3 s-1
1.5 x 10-4
4.5 x 10-4
4.5 x 10-4

(a)(i) Give the order of the reaction with respect to 2-bromo-2-methylpropane.
> My answer - First order

Sodium hydroxide.
> My answer - Zero order

(ii) Write the rate equation for this reaction.
> My answer - r = K [(CH3) 3CBr]

(b) Use one set of the data to calculate the rate constant for this reaction. Include the unit of the rate constant in your answer.
> My answer - 0.35s-1

(c) The slowest step of the mechanism is the following reaction
(CH3)3CBr --> (CH3)3C+ + Br-
Is your rate equation consistent with this infomation? Explain your answer.
>My answer - Yes, because there is only one reacting species present in the rate determining step, and the rate equation is first order overall

Thanks for your help in advance.

jamcatton

Hi TSR.

Is anyone able to check over my work, and tell me if I got the answers right or wrong. Thanks. :smile:

It's Chemistry Nuffield.

Q2. 2-bromo-2-methylpropane reacts with aqueous sodium hydroxide as shown in the equation below.

(CH3) 3CBr + NaOH --> (CH3) 3COH + NaBr

A series of experiments was carried out to investigate the kinetics of this reaction.

Initial concentration of
(CH3) 3CBr/mol dm-3
5.0 x 10-4
1.5 x 10-3
1.5 x 10-3

Initial concentration of
NaOH/mol dm-3
2.0 x 10-2
2.0 x 10-2
4.0 x 10-2

Initial rate of reaction
mol dm-3 s-1
1.5 x 10-4
4.5 x 10-4
4.5 x 10-4

(a)(i) Give the order of the reaction with respect to 2-bromo-2-methylpropane.
> My answer - First order

Sodium hydroxide.
> My answer - Zero order

(ii) Write the rate equation for this reaction.
> My answer - r = K [(CH3) 3CBr]

(b) Use one set of the data to calculate the rate constant for this reaction. Include the unit of the rate constant in your answer.
> My answer - 0.35s-1

(c) The slowest step of the mechanism is the following reaction
(CH3)3CBr --> (CH3)3C+ + Br-
Is your rate equation consistent with this infomation? Explain your answer.
>My answer - Yes, because there is only one reacting species present in the rate determining step, and the rate equation is first order overall

Thanks for your help in advance.

The correct name is 2-bromomethylpropane :yes:

The orders are correct but your value for the rate constant is wrong (it should be 0.3s^-1).

The mechanistic explanation is OK. I would say that the order of each component gives the molecularity of that species in the RDS. Hence there is only one particle of 2-BMP in the RDS.

Reply 2

jamcatton

OP

2

Thanks so much.
I know I'm asking a lot, but could you clarify for me why the rate constant is 0.3 and not 0.35 because I have confused myself?!

Thanks!

EDIT Not to worry now, think I've got it!

Reply 3

xSkyFire

19

Actually I always thought it was 2-bromo-2-methylpropane too, wouldn't 2-bromomethylpropane suggest a methyl group on carbon 1 and a bromo group on carbon 2? Then that would be 2-bromobutane o_o

xSkyFire

Actually I always thought it was 2-bromo-2-methylpropane too, wouldn't 2-bromomethylpropane suggest a methyl group on carbon 1 and a bromo group on carbon 2? Then that would be 2-bromobutane o_o

as you say, if you put a methyl group on carbon no. 1 it ceases to be a propane derivative, hence the '2' is redundant.

jamcatton

Thanks so much.
I know I'm asking a lot, but could you clarify for me why the rate constant is 0.3 and not 0.35 because I have confused myself?!

Thanks!

EDIT Not to worry now, think I've got it!

If the rate=k[A]

then k = rate/[A]

from expt 1: k = 1.5/5.0 = 0.3 s^-1

Reply 6

Rad66

6

Original post by charco

If the rate=k[A]

then k = rate/[A]

from expt 1: k = 1.5/5.0 = 0.3 s^-1

Hi, please can you explain how you came to the answer or 0.3 s-1? and what the Value of [A] is and how you came to the figure of 5.0?

Thank you very much.