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Chemistry A-level Question

I’ve attached the question, textbook page about double bonds and mark scheme below. I got tetrahedral, tetrahedral and regional bipyramidal. I don’t understand how the first one isn’t tetrahedral as I thought double bonds counted as 2 bonding pairs so the total would be 4 bonding pairs without a line pair. I don’t understand how they said it was trigonal planar.
(edited 1 year ago)
Original post by Forgotten one
I’ve attached the question, textbook page about double bonds and mark scheme below. I got tetrahedral, tetrahedral and regional bipyramidal. I don’t understand how the first one isn’t tetrahedral as I thought double bonds counted as 2 bonding pairs so the total would be 4 bonding pairs without a line pair. I don’t understand how they said it was trigonal planar.


I can't really see what your textbook is saying, but you would count the double bond as 2 bonding pairs of electrons, when determining how many bonding pairs and how many lone pairs (if any). However, when determining shape, the multiple bond counts as one, as you're really considering the shape based on regions of electron density repelling each other. There's often a bit in the text book about awkward shapes and/or multiple bonds in molecular shapes.

I'd look at this web site (for example): https://www.chemguide.co.uk/atoms/bonding/shapesdouble.html#top and also consider that there is no (well, restricted) rotation about a double bond, electron regions repel each other (basis of question), so will be planar.

Hope that helps.
(edited 1 year ago)
Original post by Forgotten one
I’ve attached the question, textbook page about double bonds and mark scheme below. I got tetrahedral, tetrahedral and regional bipyramidal. I don’t understand how the first one isn’t tetrahedral as I thought double bonds counted as 2 bonding pairs so the total would be 4 bonding pairs without a line pair. I don’t understand how they said it was trigonal planar.


When you get a double bond, you treat it like a bog-standard bonding pair. The CGP book you have there doesn’t appear to have made it terribly clear.

Carbon A can’t be tetrahedral, as that would require there to be 4 different bonds in different orientations around the carbon atom. Although the C=O bond is two bonding pairs, the two bonding pairs are both orientated the same way, so the 4 bonding pairs surrounding the carbon are not facing opposite directions (and hence it’s not tetrahedral).

It’s actually better to write about it in terms of bonding regions and lone pairs when they want a written explanation.
Double bonds can be treated like single bonds, but the double bond contains 4 electrons so like a lone pair it will take up more space and compress bond angles for other attached groups.


C=O and C=C bonds are planar. The carbon atoms are sp2 hybridised. Hybrid orbital are not taught at A-level, dont know why since its very easy and straightforward idea, but basically sp2 orbitals are trigonal planar. Not sure how you would know this unless your teacher had told you!


Atom B is just a carbon with 4 single bonds so its tehtrahedral.


Atom C - just think of it as like water, H-O-H except one of the hydrogens has been replaced by the rest of the molecule!


try here

https://science-revision.co.uk/A-level_shapes%20of_octahedral_molecules_with_lonepairs.html
Original post by TypicalNerd
When you get a double bond, you treat it like a bog-standard bonding pair. The CGP book you have there doesn’t appear to have made it terribly clear.

Carbon A can’t be tetrahedral, as that would require there to be 4 different bonds in different orientations around the carbon atom. Although the C=O bond is two bonding pairs, the two bonding pairs are both orientated the same way, so the 4 bonding pairs surrounding the carbon are not facing opposite directions (and hence it’s not tetrahedral).

It’s actually better to write about it in terms of bonding regions and lone pairs when they want a written explanation.

Thank you for replying. So double bonds are treated as normal bonding pairs because they are orientated the same so double bonds are always going to be just treated like a single bond because they are never going to be orientated differently? Just wanted to clarify, chemistry isn’t my strongest subject 😅.
Original post by scimus63
Double bonds can be treated like single bonds, but the double bond contains 4 electrons so like a lone pair it will take up more space and compress bond angles for other attached groups.


C=O and C=C bonds are planar. The carbon atoms are sp2 hybridised. Hybrid orbital are not taught at A-level, dont know why since its very easy and straightforward idea, but basically sp2 orbitals are trigonal planar. Not sure how you would know this unless your teacher had told you!


Atom B is just a carbon with 4 single bonds so its tehtrahedral.


Atom C - just think of it as like water, H-O-H except one of the hydrogens has been replaced by the rest of the molecule!


try here

https://science-revision.co.uk/A-level_shapes%20of_octahedral_molecules_with_lonepairs.html

Thank you for replying. Just to confirm for Atom A, if I put trigonal planar as the answer, and that C=O bonds are planar as the explanation, would I get all the marks or do I have to mention that the carbon atoms are sp2 hybridised and that sp2 orbitals are trigonal planar? Also are all carbon atoms sp2 hybridised? Sorry for all the questions, the teaching at my school is quite bad. I went from enjoying chemistry to dreading every lesson. I don’t really know what I need or don’t need to know and how to answer questions. My only resource is this textbook, I went from getting an 8 in GCSE to working at a D in AS then I got a B+ in the mock and then the teacher decided that it was too aspirational so she lowered it to a C. Honestly so stresssed out and confused.
Original post by Forgotten one
Thank you for replying. So double bonds are treated as normal bonding pairs because they are orientated the same so double bonds are always going to be just treated like a single bond because they are never going to be orientated differently? Just wanted to clarify, chemistry isn’t my strongest subject 😅.

That’s correct.
Original post by Forgotten one
Thank you for replying. Just to confirm for Atom A, if I put trigonal planar as the answer, and that C=O bonds are planar as the explanation, would I get all the marks or do I have to mention that the carbon atoms are sp2 hybridised and that sp2 orbitals are trigonal planar? Also are all carbon atoms sp2 hybridised? Sorry for all the questions, the teaching at my school is quite bad. I went from enjoying chemistry to dreading every lesson. I don’t really know what I need or don’t need to know and how to answer questions. My only resource is this textbook, I went from getting an 8 in GCSE to working at a D in AS then I got a B+ in the mock and then the teacher decided that it was too aspirational so she lowered it to a C. Honestly so stresssed out and confused.


Which exam board are you doing? Get the specification, past papers, mark schemes and examiners’ reports from exam board web site. They will tell you exactly what you need to know and how to answer the question (i.e. as presented in mark scheme. Note the intro where it gives info about arrows and drawings etc.).

I did AQA and this book https://www.cgpbooks.co.uk/secondary-books/as-and-a-level/science/chemistry/catb73-a-level-chemistry-for-aqa-year-1-2 followed the specification exactly (note: might be cheaper elsewhere). Hopefully there is something similar for your exam board.

I also found https://www.chemguide.co.uk/ useful and https://www.physicsandmathstutor.com/chemistry-revision/a-level-aqa/ has notes and questions/answers for practice.

I am sure there are other recommendations on TSR.

Good luck!

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