Edexcel Chemistry Unit 1 15th May

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Original post by Ricky1995

"Explain why stereoisomerism can occue in alkenes, and why hex-2-ene has stereoisomers but hex-1-ene does not." (2)

If a double bond is located within a chain, the remaining parts of the chain can be either cis- (on the same side of the double bond) or trans- (on opposite sides of the double bond) to one another, this is only if there are pi bonds which is in alkenes.

So, in hex-2-ene, the H3C- and the -CH2-CH2-CH3 can be cis- or trans- to one another, making 2 stereoisomers. In hex-1-ene, carbon 1 has 2 H- attached to it, and since those are identical, there do not exist stereoisomers of that compound.

(edited 11 years ago)

Reply 181

ummm

Original post by Ricky1995

"Explain why stereoisomerism can occue in alkenes, and why hex-2-ene has stereoisomers but hex-1-ene does not." (2)

Because in a hexene molecule there is restricted rotation about the double carbon-carbon bond. Hex-1-ene can not have stereoisomers because the double-double bond is attached to the first carbon atom. This means that one side of the double bond will always just have two hydrogen atoms attached. For stereoisomerism to occur you need two different atoms/groups attached to each side of the double carbon-carbon bond.

Reply 182

Maxwell94

Original post by Ricky1995

"Explain why stereoisomerism can occue in alkenes, and why hex-2-ene has stereoisomers but hex-1-ene does not." (2)

It occurs in alkenes as no rotation can occur due to the double bond and hex-2-ene has stereoisomers because it has different groups on the C at each end of the c=c double bond

Reply 183

ummm

Original post by UserInvalid

Usually it is unit 1 chemistry that is the hardest, luckily I got away with it in my first go.

Any way I can help you guys out?

Any general tips? I really want an A in this!

Reply 184

YellowAndBlack

Original post by Ricky1995

"Explain why stereoisomerism can occue in alkenes, and why hex-2-ene has stereoisomers but hex-1-ene does not." (2)

Hex-2-ene has steroisomers as it has the ability to form different groups which can be arranged on different sides of the molecule.
Hex-2-ene can form E or Z isomerism.

Draw them out if you like ... (it makes it easier) and see you can swap groups in hex-2-ene but can't in hex-1-ene

Reply 185

YellowAndBlack

Original post by ummm

Any general tips? I really want an A in this!

I'm sure you'll be fine! I hope it's not like the biology paper today :/

Reply 186

ummm

Original post by YellowAndBlack

I'm sure you'll be fine! I hope it's not like the biology paper today :/

I'm so nervous now! I've done all the past papers and everything, it's just they always manage to slip in some weird question that throws me off. :frown:

Oh god, don't talk to me about biology. Makes me so angry. I knew everything inside out for it, and then they ask questions like that....

PS: Good luck with yours as well! :smile:

Reply 187

YellowAndBlack

Original post by ummm

I'm so nervous now! I've done all the past papers and everything, it's just they always manage to slip in some weird question that throws me off. :frown:

Oh god, don't talk to me about biology. Makes me so angry. I knew everything inside out for it, and then they ask questions like that....

PS: Good luck with yours as well! :smile:

Hahah tell me about it ! I literally knew the whole bio syllabus off by heart (cheeky pun with the heart question) but the way they asked the questions was ridiculous - Hope Chemistry isn't like this!

Good luck with yours also :smile:

Reply 188

ummm

Original post by YellowAndBlack

Same! I can recite to you word for word (well, pretty much) the entire unit in the CGP book... I reeeeeaaaallly wanted an A in that unit as well. :frown:

Oh well, just got to hope for the best on results day now! Hopefully it'll 'beat' my expectations (cheeky pun there too)...

Hopefully chemistry is actually to do with the content in the chemistry course! :wink:

I hate the whole not being able to discuss the exam until midnight thing. It's so annoying, haha!

Reply 189

fifty_six

any notes for quick revision

Reply 190

UserInvalid

Original post by ummm

Any general tips? I really want an A in this!

From doing my exam in january and doing past papers. I can remember some of the common things that are asked.

- You need to know how to write ionic and molecular equations
- definations of isotops, Relative atomic mass, Avagadro constant
- Working out Moles, using equations, also in their gaseous states, molar volume
- Collecting gas in experiments, syringe, test tube e.t.c
- writing empiricla and molecular formula
- Moles = conc x volume / 1000 (know the unites of conc and vol and know when to divide by 1000, they can trick you)
- Parts per million (one of the common questions)
- Atom economy and percentage yield (know well what is the difference between them..

- Know the defination of enthalpy changes, (neutralisation, combustion, formation e.t.c ...)
- Know the difference between exo- and endo (inside out)
- Know how to draw energy profiles..
- Calculating enthalpy changes from energy profile diagrams. (heat packs and cold packs)
- Q = mcT (know well difference in calculation for combustion ,neutralisation e.t.c ) And also know when it is exo and end (temperature changes), include signs and write in joules per mole.
- errors in experiment for heat capacity, common errors
- advantages of calometry over cup.
- Know all of the definations for standard enthalpy changes e.t.c
- Making Hess's law and working out.
- Bond enthalpies and mean bond enthalpies

- Mass spectrometer, labelling and knowing how it works
- Molecular peak/ion, calculating Ar from it. Know about isotopes and how to calculate for isotopes.
- writing in subshells, s1 p1 e.t.c (writing electronic configuration)
- Using arrows in orbitals and know why they are in opposite direction
- Trend in ionisation energy, melting temperatures.

- Ionic bonding (dot and cross diagram)
- Trends in Ionic radius
- Lattice energies, what effects it. Difference and similarities in born haber and experiment values
- Polarisation
- metallic bonding , why conducts heat and electricity, and why not.

- Hazard and risks (common ones)
- Organic, knowing functional groups and writing down molecular, structural formulars
- Know difference in aliphatic, arenes and alicyclic
- Common are alkenes and alkanes
- Naming organic compounds, with ethyl groups
- Making isomers

- Burining alkanes, General properties
- Fractional distiilations
- Homo and Hetrolytic fissions.
- Reactions of alkanes with chlorines (initiations, propagation and termination). Know how to write it in equations

- Know about cis/trans isomers. E/z Isomers
- Know why sterioisomers are formed.
- know what is electrophile
- know addition reactions of the alkenes (with halogens)
- Addition of bromine water, possible products, and also pottasium manganete, colour changes of solution
- using curly arrows in mechanisms
- Know about how polymers are formed and what are monomers

Check if you know them, and look over them.

I am off now, btw I got 114/120 in my unit 1.

(edited 11 years ago)

Reply 191

Assassin94

hey guys, i noticed a wee pattern in the past papers. in all three of the recent january exams, you have to draw out the apparatus for the experiments. and in the last 3 june/may exams there is no diagrams to draw. so if this is the case for tomorrow, happy days :biggrin:

Reply 192

TenaciousMonkey

Original post by fifty_six

Show, in full, the mechanism for this reaction in which ethene is converted to
chloroethane.

http://www.chemguide.co.uk/mechanisms/eladd/symhbr.html

The mechanism is given here, near the bottom of the page :smile:

Reply 193

Ricky1995

I really don't understand parts per million questions and how to go about working them out. Could someone give an example in context?

Reply 194

YellowAndBlack

Original post by Ricky1995

I really don't understand parts per million questions and how to go about working them out. Could someone give an example in context?

Jan 2010 look at that paper ...

ppm= number of parts of the chemical / number of parts it is contained in * 10^6

Fill in the figures and bang ...

Reply 195

Ricky1995

Thanks, also...

17.1 g of aluminium sulfate, Al2(SO4)3, was dissolved in water.
Calculate the number of sulfate ions, SO42–, present in the solution formed.
[Assume the molar mass of Al2(SO4)3 is 342 g mol–1 and the Avogadro Constant is 6 × 1023mol–1]

Reply 196

arnab

So who is ready for this exam??

Also can someone explain to me these two things please :

1. What do you need to know about Macromolecular and Simple molecular structure?

2. What do you need to know about pie bonds and sigma bonds?

Thanks

Reply 197

phaseshift

Original post by arnab

So who is ready for this exam??

2. What do you need to know about pie bonds and sigma bonds?

What exactly are they made of? When I say made of, I mean what they are actually a result of (orbitals)..

Which is stronger (there was a question asking you to compare a pi and sigma bond one year) and for what reasons? Essentially, can you compare their properties, for example; what it means if you have a double bond etc.

Where do each occur?

And not really. :wink:

(edited 11 years ago)

Reply 198

ummm

Original post by arnab

Hey,

A macromolecular structure is one in which all of the atoms are joined by strong covalent bonds in a giant lattice. These bonds are very strong, and so a lot of energy is needed to break them. Elements/compounds (such as Silicon Dioxide and Diamond) actually don't melt - they just sublime from a solid to a gas. Giant macromolecular structures are unable to conduct electricity as all of the bonding electrons are held in localised bonds, so they are unable to carry an electric charge. Plus they are insoluble!

Simple molecular structures on the other hand (Eg. Cl2 and H2) consist of groups of covalently bonded atoms within each molecule. These molecules are attracted to each other by London Forces. The greater the number of atoms in a molecule, the stronger the London forces. London forces are weak though, so don't require much energy to overcome, giving these elements/structures low boiling and melting points. The bonding is much weaker than covalent, ionic or metallic bonds. Some are soluble, depending on how polarised the molecule is.

Basically, pi bonds and sigma bonds occur in covalently bonded molecules. A sigma bond is formed when a pair of electrons in s orbitals overlap, and a pi bond is formed when two electrons in p orbitals overlap. Pi bonds have two parts to them - one below and one above the molecular axis. They are less tightly bound to the two nuclei, so compounds containing pi bonds are more reactive. Single covalent bond = just a sigma bond. Double covalent bond = pi bond and a sigma bond.

:smile:

Reply 199

arnab

Original post by ummm

THANK YOU SOO MUCH!!

Just one other question, what do u need to know about a molar gas volume = 24dm^3mol^-1?

just came across that in june10 paper and i dont even understand what that means :frown: