Received my (Edexcel) AS script back and have questions.

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Wigg
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Hey people, just got my script back for Unit 1 and Unit 2 for Chemistry. Decent grade I guess, but I have a few questions. My chemistry skills may be a bit rusty - holiday degradation of skills I guess

Anyway here are the problem Q's:

Unit 1


12) When methane reacts with Cl, a mixt. of products form. Which provides strongest evidence for free radical mechanism.

The answer is (I think) HCl. However, why couldn't the answer just as easily be chloromethane or trichloromethane?

21) B) ii) Two peaks of mass spectrum of Cl2 are at m/e 70 and 74. Identify the species giving rise to these two peaks.

I can't figure out why I lost 1 mark from the 2 mark question here. I wrote :

Cl35 - Cl35
Cl37 - Cl37

any ideas as to why this is wrong?
24 F I Give the displayed formula of two isomeric alkenes either of which could have given rise to this polymer. (polymer is polyethene)
26a How are atoms held together by the covalent bond in a molecule of hydrogen>

I said "Electrons are shared btwn the two H holding them in a strong electrostatic force"

which in retrospect was a bit of a crap answer. How else could I have said this?


Unit 2

17d Ozone is used to disinfect flood-damaged buildings, to remove residual smoke odours and to treat drinking water. What is the adv. of ozone as opposed to chlorine?

I said that chlorine was reactive and therefore hard to store. That was wrong.

18bii What do curly arrows shown in mechanism represent?

I said "Electrons being donated"

18biii Explain the shape of Y. Y looks like this:
CH3
|
H3C-C+
|
CH
3

My answer scoring 1/3:

Trigonal Planar around the carbocation. There is because there are no lone pairs of electrons, so the bonds are free to spread to their maximum extent around the carbocation. There are only three methyl groups.

20cii State 2 items of information (not environmental, based on renewability, or on enthalpy change of combustion) that could be used to compare methane and ammonia as fuels.

I said Activation Energy and Toxicity. Which is wrong (Probably the latter)? Why? What else could I have said?

The last two questions I don't get at all; but they both require detailed reading of a passage that I don't want to type out.

Thanks xx
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Mallika
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(Original post by Wigg)
Hey people, just got my script back for Unit 1 and Unit 2 for Chemistry. Decent grade I guess, but I have a few questions. My chemistry skills may be a bit rusty - holiday degradation of skills I guess

Anyway here are the problem Q's:

Unit 1


12) When methane reacts with Cl, a mixt. of products form. Which provides strongest evidence for free radical mechanism.

The answer is (I think) HCl. However, why couldn't the answer just as easily be chloromethane or trichloromethane?

21) B) ii) Two peaks of mass spectrum of Cl2 are at m/e 70 and 74. Identify the species giving rise to these two peaks.

I can't figure out why I lost 1 mark from the 2 mark question here. I wrote :

Cl35 - Cl35
Cl37 - Cl37

any ideas as to why this is wrong?
The horizontal axis of the graph would be M/Z, mass over charge. To get full marks you would need to show that both molecules have lost an electron, so they would need a positive charge.
24 F I Give the displayed formula of two isomeric alkenes either of which could have given rise to this polymer. (polymer is polyethene)
26a How are atoms held together by the covalent bond in a molecule of hydrogen>

I said "Electrons are shared btwn the two H holding them in a strong electrostatic force"

which in retrospect was a bit of a crap answer. How else could I have said this?


Unit 2

17d Ozone is used to disinfect flood-damaged buildings, to remove residual smoke odours and to treat drinking water. What is the adv. of ozone as opposed to chlorine?

I said that chlorine was reactive and therefore hard to store. That was wrong.
Ozone can break down to oxygen making it les dangerous, whereas chlorine would stay there?

18bii What do curly arrows shown in mechanism represent?

I said "Electrons being donated"
I guess they're being picky, but I think you would need to say the transfer of electrons?
18biii Explain the shape of Y. Y looks like this:
CH3
|
H3C-C+
|
CH
3

My answer scoring 1/3:

Trigonal Planar around the carbocation. There is because there are no lone pairs of electrons, so the bonds are free to spread to their maximum extent around the carbocation. There are only three methyl groups.

20cii State 2 items of information (not environmental, based on renewability, or on enthalpy change of combustion) that could be used to compare methane and ammonia as fuels.

I said Activation Energy and Toxicity. Which is wrong (Probably the latter)? Why? What else could I have said?

The last two questions I don't get at all; but they both require detailed reading of a passage that I don't want to type out.

Thanks xx
I hope I helped, I could only answer a few of the questions! But why don't you look at the markscheme?

Edit: Here is the unit 2 official mark scheme: http://www.scribd.com/doc/100278399/...t-2-Markscheme
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charco
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(Original post by Wigg)
Hey people, just got my script back for Unit 1 and Unit 2 for Chemistry. Decent grade I guess, but I have a few questions. My chemistry skills may be a bit rusty - holiday degradation of skills I guess

Anyway here are the problem Q's:

Unit 1


12) When methane reacts with Cl, a mixt. of products form. Which provides strongest evidence for free radical mechanism.

The answer is (I think) HCl. However, why couldn't the answer just as easily be chloromethane or trichloromethane?
I would say ethane and substituted ethanes, as this would only happen with methyl free radicals colliding.



21) B) ii) Two peaks of mass spectrum of Cl2 are at m/e 70 and 74. Identify the species giving rise to these two peaks.

I can't figure out why I lost 1 mark from the 2 mark question here. I wrote :

Cl35 - Cl35
Cl37 - Cl37

any ideas as to why this is wrong?
It is completely correct.. except that you have forgotten the positive charge on the species ...



24 F I Give the displayed formula of two isomeric alkenes either of which could have given rise to this polymer. (polymer is polyethene)
26a How are atoms held together by the covalent bond in a molecule of hydrogen>

I said "Electrons are shared btwn the two H holding them in a strong electrostatic force"

which in retrospect was a bit of a crap answer. How else could I have said this?
The electrostatic attraction of two positive nuclear centres for the double negative charge of the electron pair that lies between them.



Unit 2

17d Ozone is used to disinfect flood-damaged buildings, to remove residual smoke odours and to treat drinking water. What is the adv. of ozone as opposed to chlorine?

I said that chlorine was reactive and therefore hard to store. That was wrong.
Ozone does not impart flavour.


18bii What do curly arrows shown in mechanism represent?

I said "Electrons being donated"
The movement of electron PAIRS


18biii Explain the shape of Y. Y looks like this:
CH3
|
H3C-C+
|
CH
3

My answer scoring 1/3:

Trigonal Planar around the carbocation. There is because there are no lone pairs of electrons, so the bonds are free to spread to their maximum extent around the carbocation. There are only three methyl groups.
Three regions of electron density around the central carbon atom. This creates sp2 hybridisation and corresponding trigonal planar orientation of orbitals to minimise repulsion. Bond angle 120º.




20cii State 2 items of information (not environmental, based on renewability, or on enthalpy change of combustion) that could be used to compare methane and ammonia as fuels.

I said Activation Energy and Toxicity. Which is wrong (Probably the latter)? Why? What else could I have said?
energy release per kilogram of fuel (from combustion enthalpy)
available feedstocks, cost per kg of obtaining methane and ammonia from feedstocks.


The last two questions I don't get at all; but they both require detailed reading of a passage that I don't want to type out.

Thanks xx
OK
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18RA41M
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Which year was this from? june 2012?
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maya_fooks
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(Original post by Wigg)
Hey people, just got my script back for Unit 1 and Unit 2 for Chemistry. Decent grade I guess, but I have a few questions. My chemistry skills may be a bit rusty - holiday degradation of skills I guess

Anyway here are the problem Q's:

Unit 1


12) When methane reacts with Cl, a mixt. of products form. Which provides strongest evidence for free radical mechanism.

The answer is (I think) HCl. However, why couldn't the answer just as easily be chloromethane or trichloromethane?

chloromethane or tricholomethane could both be formed by substitution rather than by a free rdaical mechanism. HCl has to be formed by a free radical mechanism as its not adding onto anything, so I think that why HCl is the answer (i haven't explain that well at all, really you need to try and draw the mechanisms to get a clearer idea)

21) B) ii) Two peaks of mass spectrum of Cl2 are at m/e 70 and 74. Identify the species giving rise to these two peaks.

I can't figure out why I lost 1 mark from the 2 mark question here. I wrote :

Cl35 - Cl35
Cl37 - Cl37

any ideas as to why this is wrong?

as i think someone has already said you must include a single positive charge as they have been ionised (edexcel are very strict on this)
24 F I Give the displayed formula of two isomeric alkenes either of which could have given rise to this polymer. (polymer is polyethene)

you need to draw both the cis and trans versions of ethane, remember there is no rotation around a double bond so they are isomers of each other
26a How are atoms held together by the covalent bond in a molecule of hydrogen>

I said "Electrons are shared btwn the two H holding them in a strong electrostatic force"

which in retrospect was a bit of a crap answer. How else could I have said this?

the electrostatic force of attraction between the positive nuclei of the atoms and the negative pair of shared electrsns

Unit 2

17d Ozone is used to disinfect flood-damaged buildings, to remove residual smoke odours and to treat drinking water. What is the adv. of ozone as opposed to chlorine?

I said that chlorine was reactive and therefore hard to store. That was wrong

not sure about this maybe Cl could react to form HCl (I would check the mark scheme)

18bii What do curly arrows shown in mechanism represent?

I said "Electrons being donated"

the movement of electrons

18biii Explain the shape of Y. Y looks like this:
CH3
|
H3C-C+
|
CH
3

My answer scoring 1/3:

Trigonal Planar around the carbocation. There is because there are no lone pairs of electrons, so the bonds are free to spread to their maximum extent around the carbocation. There are only three methyl groups.

i'm sorry i cant visualise this, so i can't help
20cii State 2 items of information (not environmental, based on renewability, or on enthalpy change of combustion) that could be used to compare methane and ammonia as fuels.

I said Activation Energy and Toxicity. Which is wrong (Probably the latter)? Why? What else could I have said?

The last two questions I don't get at all; but they both require detailed reading of a passage that I don't want to type out.

contribution to pollution
enthalpy change of combustion
Thanks xx
I hope that was helpful, if you have any other questions, orquestions about what i said, just ask (good practice for fridays exam!!)
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maya_fooks
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(Original post by maya_fooks)
I hope that was helpful, if you have any other questions, orquestions about what i said, just ask (good practice for fridays exam!!)
Actually i think ethane would be the answer to prove its free radical as it can only be formed by the collisin of 2 methly free radicals, sorry!
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