OCR Chemistry A F322 Chains, Energy and Resources Wed 23 May 2012

Original post by Coolina133

noo.. because they have less van der waal forces between the molecules due to less molecular contact and decreased surface area.. so they have a lower BP

That's what I meant but I was just too lazy to type it out, less and weaker VdW forces I think either are fine to use, if the molecules are further apart then they will have less of an attraction.

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Reply 583

Coolina133

7

Original post by rany345

whats the difference in steroisomers and structural isomers , can you give examples aswell :smile:

structural isomers have the same molecular formular but different structural
eg, prop-2-ene and prop-1-ene
or like butene and cyclobutane

stero: have same structural formaula but a different arrangement of atoms in space so like cis/trans isomerism
where in an alkene, each carbon atomhas 2 different groups on them but in cis and trans there must be 2 hydrogens but on different carbons
there e/z which is just each carbon having 2 different groups on each
e and trans are opposite sides
ciz and z are same side

Reply 584

Maths_Lover

2

Original post by otrivine

yes i know that :wink:

thanks but the question only wanted define:boltzman

See my edit. :wink:

That is an odd question... Boltzmann was a physicist.

EDIT - just read the original question: In decribing a Boltzmann Distribution graph, you want to say what the axes are and what the curve tells you - there is no need to give a particular example. A Boltzmann Distribution shows the no. of molecules with a particular energy plotted against energy. It shows the distribution of energies within a population of molecules and the area under the curve represents the total population of molecules. :smile:

(edited 11 years ago)

Reply 585

Original post by kevloui

ahha no problem :wink:

Another: a calculator and periodic table is needed.
In an experiment 15.22g of NH₄SCN is reacted with slightly excess Ba(OH)₂. The reaction absorbs energy, cooling the 50.0g of water from 29.9C to 10.9C

i)Calculate the energy absorbed in kJ during this reaction.
Specific heat capacity of water = 4.2g^-1k^-1 (2)

ii) Calculate the amount in moles of NH₄SCN used by the chemist. (1)

iii) Calculate the enthalpy change of reaction. Include the sign in your answer. Give answer to 2 sig fig.

Awful at this, is it bad that I STILL don't know what a sf is? I feel so infantile.

1)
4.2*50*19= -3990kj

2)
15.22/76.1= 0.2

3)
0.2/3990= +19.95

I think :smile:

Reply 586

uttamo

10

Original post by thescientist17

Please could someone explain this question to me?

If you're using formation values (not combustion) you do (enthalpy of products)-(enthalpy of reactants).
So,
[6(-394)+6(-286)]-[(-1250)+6(0)]=-4080-(-1250)=-2830 kj/mol

Enthalpy change of formation of O2 is 0 because to form oxygen you need to start from oxygen, which is the same thing, so not enthalpy change.

Combustion enthalpy changes are always negative so that's one way to check your answer is reasonable.

Reply 587

Original post by maths_lover

see my edit. :wink:

that is an odd question... Boltzmann was a physicist.

lolmygosh

Reply 588

otrivine

14

Original post by biomagic

Awful at this, is it bad that I STILL don't know what a sf is? I feel so infantile.

1)
4.2*50*19= -3990kj

2)
15.22/76.1= 0.2

3)
0.2/3990= +19.95

I think :smile:

ask me question

Reply 589

D4rth

Original post by kevloui

for combustion, its the sum of the enthalpy changes of the reactants minus the sum of the enthalpy changes of the products.

No it's the other way round for combustion it's -(reactants)+(products). In other words products minus reactants, if you do it your way you get a positive value.

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Reply 590

thescientist17

Original post by uttamo

If you're using formation values (not combustion) you do (enthalpy of products)-(enthalpy of reactants).
So,
[6(-394)+6(-286)]-[(-1250)+6(0)]=-4080-(-1250)=-2830 kj/mol

Enthalpy change of formation of O2 is 0 because to form oxygen you need to start from oxygen, which is the same thing, so not enthalpy change.

Combustion enthalpy changes are always negative so that's one way to check your answer is reasonable.

Oh okay thank you I forgot about O2 having no enthalpy change as it's an element! Makes sense now :smile:

Reply 591

Maths_Lover

2

Original post by biomagic

lolmygosh

I read your original question: "Describe a Boltzmann Distribution" and it made more sense. I thought that being asked to "Define Boltzmann" would be very odd indeed. :lol:

Reply 592

thescientist17

Original post by D4rth

Ah, I've just done this, what part do you need help with, construct your Hess cycle - arrows pointing up and then add in the values from the table

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Cycle!?

Reply 593

kevloui

Original post by biomagic

Awful at this, is it bad that I STILL don't know what a sf is? I feel so infantile.

1)
4.2*50*19= -3990kj

2)
15.22/76.1= 0.2

3)
0.2/3990= +19.95

I think :smile:

2/6
close you have to divide the first answer you got by 1000 to get the answer in kJ. The answer you have there is in J.
Your mole calculation is correct.
For the last one its q/n, so the enthalpy change is divided by the moles, you got them mixed up, but the sign in front of your number is correct being endothermic, it is positive :smile:

Reply 594

otrivine

14

Original post by kevloui

2/6
close you have to divide the first answer you got by 1000 to get the answer in kJ. The answer you have there is in J.
Your mole calculation is correct.
For the last one its q/n, so the enthalpy change is divided by the moles, you got them mixed up, but the sign in front of your number is correct being endothermic, it is positive :smile:

ask me question :wink:

Reply 595

clad in armour

16

Original post by narli

has bond enthalpy and average bond enthalpy got the same definition??

well the 'average' part comes from the fact that bond enthalpies differ for a particular bond depending on its environment i.e. the other atoms and bonds around it so an average bond enthalpy is taken to reflect those differences

e.g. c-H bond is in almost all organic molecules but depending on whats around it the bond enthalpy is different

Reply 596

Original post by otrivine

ask me question

Uses of scC02?
How is scC02 created
Give me 5 main things a green chemist would LOVE in terms of being sustainable

Reply 597

kevloui

where did that guy with the f322 revision notes and the markscheme go!! :'(

edit: dont worry, i found what i was looking for :biggrin:

(edited 11 years ago)

Reply 598

D4rth

Original post by thescientist17

Cycle!?

Ignore me - your teacher has taught it in a different way, the method is exactly the same but my teacher insists on us drawing a Hess cycle as it's more correct or something? Not sure exactly why but do it the way you know.

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Reply 599