DedicatedWizard
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I am having trouble understanding one of the January 2011 questions.It's Question 5b. The mark scheme is confusing, any help would be appreciated
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DedicatedWizard
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Bump
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HoangLe
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What is your exam board?
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GrappleX
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Post question or Link on Here and i'll give it a stab


Posted from TSR Mobile
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DedicatedWizard
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(Original post by GrappleX)
Post question or Link on Here and i'll give it a stab


Posted from TSR Mobile
http://www.ocr.org.uk/Images/65360-q...-resources.pdf

It's Question 5b :/
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DedicatedWizard
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(Original post by HoangLe)
What is your exam board?
OCR
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DedicatedWizard
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...anyone? D:
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HoangLe
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Image

Firstly they give us a table so lets look at that:

Image

The first bullet point specifies we need to talk about the halogens in the halogenoalkanes (The first column) so lets just glance through that.

You should notice in the first 3 the halogens in the halogenoalkanes are all different and this affects the results as you can see at 50 degrees that it takes longer the Cl Halogenoalkane to precipitate longer than the Br and the Br takes longer to precipitate than the I halogenoalkane.


So here you can say the chloroalkane reacts the slowest and the iodoalkane reacts the faster (as shown in the table).

Then you need to justify why and this is where your chemistry knowledge comes in:

You need to know the context of the bond strengths of halogenoalkanes.
Simply: The C - Cl bond is stronger than the C - Br bond and this bond is stronger than the C - I bond.

In summary C - Cl > C - Br > C - I

If you want to know why look here:
http://www.chemguide.co.uk/organicpr...agno3.html#top

So you can now explain that the Chloroalkane takes longer to precipitate because of their bond strengths and here you go into detail for an example and say something along the lines of :

The C - I bond is weaker than the C - Br bond
The C - Br bond is weaker than the C - Cl Bond

-----------------------------------------------------------------------------------------------------------------

Second bullet point

This is knowledge based as well:
http://www.chemguide.co.uk/organicpr...agno3.html#top

http://www.chemguide.co.uk/mechanism...water.html#top

Basically you need to know that a tertiary alcohol hydrolyses faster than a secondary and the secondary hydrolyses faster than primary.

The links above explain it but in simple terms its all in the mechanism. The links I posted should hopefully explain it.

-----------------------------------------------------------------------------------------------------------------

Third bullet point

Here you state that the halogenalkanes rate of reaction increases as the temperature increases and give evidence e.g :

The chlroalkane took 243 seconds to precipitate at 50 degrees but 121 seconds at 60 degrees

Then you use your chemistry knowledge that high temperatures means more kinetic energy and so more successful collisions (Just like the Boltzmann Distribution diagram)
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DedicatedWizard
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(Original post by HoangLe)
Image

Firstly they give us a table so lets look at that:

Image

The first bullet point species we need to talk about the halogens in the halogenoalkanes (The first column) so lets just glance through that.

You should notice in the first 3 the halogens in the halogenoalkanes are all different and this affects the results as you can see at 50 degrees that it takes longer the Cl Halogenoalkane to precipitate longer than the Br and the Br takes longer to precipitate than the I halogenoalkane.


So here you can say the chloroalkane reacts the slowest and the iodoalkane reacts the faster (as shown in the table).

Then you need to justify why and this is where your chemistry knowledge comes in:

You need to know the context of the bond strengths of halogenoalkanes.
Simply: The C - Cl bond is stronger than the C - Br bond and this bond is stronger than the C - I bond.

In summary C - Cl > C - Br > C - I

If you want to know why look here:
http://www.chemguide.co.uk/organicpr...agno3.html#top

So you can now explain that the Chloroalkane takes longer to precipitate because of their bond strengths and here you go into detail for an example and say something along the lines of :

The C - I bond is weaker than the C - Br bond
The C - Br bond is weaker than the C - Cl Bond

-----------------------------------------------------------------------------------------------------------------

Second bullet point

This is knowledge based as well:
http://www.chemguide.co.uk/organicpr...agno3.html#top

http://www.chemguide.co.uk/mechanism...water.html#top

Basically you need to know that a tertiary alcohol hydrolyses faster than a secondary and the secondary hydrolyses faster than primary.

The links above explain it but in simple terms its all in the mechanism. The links I posted should hopefully explain it.

-----------------------------------------------------------------------------------------------------------------

Third bullet point

Here you state that the halogenalkanes rate of reaction increases as the temperature increases and give evidence e.g :

The chlroalkane took 243 seconds to precipitate at 50 degrees but 121 seconds at 60 degrees

Then you use your chemistry knowledge that high temperatures means more kinetic energy and so more successful collisions (Just like the Boltzmann Distribution diagram)
THANK YOU SO MUCH! I actually understand it a lot more clearly now I wish I could rep, I ran out
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ftsoft
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yoooo
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ftsoft
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ss
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