OCR Chemistry A Exam Thread (Breadth - May 27 2016 and Depth - June 10 2016) Watch

Sfgbb
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http://www.ocr.org.uk/Images/171752-...-materials.pdf

You'll have to scroll to the bottom though
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JorahMormont
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Our teacher (the head of chemistry at our college) told us that we are allowed to answer in bullet points for any of the questions, even in tables if we like! My chemistry tutor who is an OCR examiner and also head of chemistry at his college told me the same thing.
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ashbawa
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(Original post by Sfgbb)
Which reaction mechanism is most likely to come up?
I would say to be pretty confident on all of them as in the breadth paper we only had to state conditions/reagents and draw the intermediate/product so go thtrough all mechanisms: electrophilic addition, nucleophilic substitution and free radical substitution. Good luck guys x
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Saaranjan
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  1. State the bond angle and shape around each carbon atom of the C=C double bond in prop-1-ene.

How do you do this?
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adeninesbff
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(Original post by Saaranjan)
  1. State the bond angle and shape around each carbon atom of the C=C double bond in prop-1-ene.

How do you do this?
All alkenes are triganol planar, so 120 degrees
This is because they have three areas of electron density.

(P.S. WHEN I SAY ALL ALKENES, I just mean the shape around the carbon atom double bond!)
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adeninesbff
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QUESTIONS

1) In Mass spectrometry, how do you know which m/z peak to choose? Because sometimes I get confused between the possible two that seem to be very close together (e.g. if there was a slightly taller at m/z 57 vs m/z 59, which one would I pick?)

2) What is the ionic equation of Cl2 + NAOH?

3) Why do you have to RINSE the beaker after you have removed a dissolved solid from it to put in a volumetric flask? Why does distilled water not effect the solution?

4) Also, how do you know when to draw a skeletal formula or a display formula? (Some exam questions I would lose marks on because I would try to draw a display, but they would require skeletal).

5) (SORRY) How are temporary dipoles (London dispersion forces) formed?
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asinghj
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(Original post by adeninesbff)
QUESTIONS

1) In Mass spectrometry, how do you know which m/z peak to choose? Because sometimes I get confused between the possible two that seem to be very close together (e.g. if there was a slightly taller at m/z 57 vs m/z 59, which one would I pick?)

2) What is the ionic equation of Cl2 + NAOH?

3) Why do you have to RINSE the beaker after you have removed a dissolved solid from it to put in a volumetric flask? Why does distilled water not effect the solution?

4) Also, how do you know when to draw a skeletal formula or a display formula? (Some exam questions I would lose marks on because I would try to draw a display, but they would require skeletal).

5) (SORRY) How are temporary dipoles (London dispersion forces) formed?
1) If they are 2 numbers apart (57 vs 59) then it's the furthest to the right (59)

2) Cl2(g) + 2OH-(aq) ---> Cl-(aq) + ClO-(aq) + H2O(l)
With this just write the whole equation, then write the equation with ions (NaOH becomes Na+ + OH-). Remember there are no ions in gases or liquids, just in aqueous solutions. Then you cancel out the spectator ions that are the same on both sides (for this there are 2Na+ on LHS and 2Na+ on RHS, so you would cancel them)

3) You rinse with distilled water to make sure that none of the solid is left in the beaker. Distilled water is 'pure' water so has a pH of 7 as there are no salts or minerals dissolved in it which would have an impact on the original pH of solution and therefore the titration wouldn't be accurate

4) Read the question and they should say which formula they want, or if they have used any skeletal formulae in the question then I'd draw a skeletal formula

5) Electrons are always moving so in a gas (e.e Cl2) the shared electron will move for an instant and this will mean that one chlorine is slightly positive (delta+) and the other is slightly negative (delta-). As there are more chlorine molicules, the one that is adjacent (next) to the first chlorine will be affected (electrons on second one will be attracted to delta+ on the first one). And this happens with every chlorine molecule and so one chlorine molecule induces dipoles on other chlorine molecule.
Of course this only happens for a split second

Sorry for such a long essay but there you go, everything I can remember at 7AM
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asinghj
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(Original post by Sfgbb)
Which reaction mechanism is most likely to come up?
Any could, as there was none in the first paper
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asinghj
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(Original post by Saaranjan)
  1. State the bond angle and shape around each carbon atom of the C=C double bond in prop-1-ene.

How do you do this?
Think about the structure of prop-1-ene

H2C=CH-CH3

Now look at the question, 'around each carbon atom of C=C bond', which looks like

As you can see there are 3 regions of electron density and no lone pairs, (like BF3) so it must be 120degrees and trigonal planar
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adeninesbff
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(Original post by asinghj)
1) If they are 2 numbers apart (57 vs 59) then it's the furthest to the right (59)

2) Cl2(g) + 2OH-(aq) ---> 2ClO-(aq) + H2O(l)
With this just write the whole equation, then write the equation with ions (NaOH becomes Na+ + OH-). Remember there are no ions in gases or liquids, just in aqueous solutions. Then you cancel out the spectator ions that are the same on both sides (for this there are 2Na+ on LHS and 2Na+ on RHS, so you would cancel them)

3) You rinse with distilled water to make sure that none of the solid is left in the beaker. Distilled water is 'pure' water so has a pH of 7 as there are no salts or minerals dissolved in it which would have an impact on the original pH of solution and therefore the titration wouldn't be accurate

4) Read the question and they should say which formula they want, or if they have used any skeletal formulae in the question then I'd draw a skeletal formula

5) Electrons are always moving so in a gas (e.e Cl2) the shared electron will move for an instant and this will mean that one chlorine is slightly positive (delta+) and the other is slightly negative (delta-). As there are more chlorine molicules, the one that is adjacent (next) to the first chlorine will be affected (electrons on second one will be attracted to delta+ on the first one). And this happens with every chlorine molecule and so one chlorine molecule induces dipoles on other chlorine molecule.
Of course this only happens for a split second

Sorry for such a long essay but there you go, everything I can remember at 7AM
Thanks! Welp, my brain is mush now trying to remember it all
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username1530463
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(Original post by adeninesbff)
Thanks! Welp, my brain is mush now trying to remember it all
Read the textbook and do the exam-style questions from it. And if you haven't already, do the AS Chemistry Depth Specimen Paper.
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Bahcaci
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(Original post by 4nonymous)
He did it wrong. It should be:
ClO• + O3 --> Cl• + 2O2
No, that is wrong. For the overall equation you have: O3 + O --> 2O2

You cant have that equation you said, check your book(s), trust me.
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PlsJustPls
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no, the O in the second propagation step comes from the breakdown of oxygen by UV radiation - O2(+UV) --> 2O(radicals)
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Jitesh
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(Original post by asinghj)
1) If they are 2 numbers apart (57 vs 59) then it's the furthest to the right (59)
You could get a halogen in the sample which may have an isotope like Br79 and Br81 which are approx 50:50 in adundance

Really depends how big the furthest peak is, could be an (M + 2) peak etc
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Jitesh
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(Original post by Bahcaci)
No, that is wrong. For the overall equation you have: O3 + O --> 2O2

You cant have that equation you said, check your book(s), trust me.
You're right, Cl radical is an example of a homogenous catalyst which form an intermediate so we cancel them out for the overall equation, like you posted
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asinghj
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(Original post by PlsJustPls)
no, the O in the second propagation step comes from the breakdown of oxygen by UV radiation - O2(+UV) --> 2O(radicals)
Oh right, so shouldn't that be an initiation step?
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NigelK98
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Are electrophillic addition, nuclephillic substitution and radical substituition the only reaction mechanisms we need to know?
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asinghj
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(Original post by Jitesh)
You could get a halogen in the sample which may have an isotope like Br79 and Br81 which are approx 50:50 in adundance

Really depends how big the furthest peak is, could be an (M + 2) peak etc
Ok fair enough but for AS we only need to know about the M+ peak due to carbon 13 hence what I said. Also don't think we need to be able to identify a halo alkane from mass spectrometry
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asinghj
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(Original post by NigelK98)
Are electrophillic addition, nuclephillic substitution and radical substituition the only reaction mechanisms we need to know?
Yeah
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NigelK98
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(Original post by asinghj)
Yeah
Kl thanks.
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