Chemistry: Post your doubts here!

[smartangel]

= Bond Energy Of Reactants - Bond Energy Of Products

umm so if formation of bonds is exothermic.shouldnt the minus signs multiply to become positive?

 

[Saad (سعد)]

= Bond Energy Of Reactants - Bond Energy Of Products

Please explain the times when the MS gives Products - Reactants instead, because the Examiner uses both.

 

[Paradoxical]

Please explain the times when the MS gives Products - Reactants instead, because the Examiner uses both.

You guys seem to be using a different method to the one I usually use. I don't think I do it the way CIE says you're supposed to. I learnt how to do all my chemistry calculations from Jim Clark's book. He gives a little introduction to it on his website, maybe this'll help clear things up for you? http://www.chemguide.co.uk/physical/energetics/bondenthalpies.html
If you want, I could scan the parts of the book I have too, just let me know.

 

[raamish]

Actually, sulfur forms a double bond. In double bond, you add 2 electrons (as opposed to adding 1 in a single bond, I forgot to mention that sorry). So sulfur has 6, each oxygen has 2 making a total of 10 electrons. Now, in each double bond, 4 electrons will be used leaving 2 behind. These 2 form a lone pair of sulfur.

Likewise, if you have a triple bond, then you add 3 electrons instead of 2. But very few questions come when you determine the shape of a molecule with a triple bond.

Also, when you have an ion like NH4+. You treat the + as a loss of electron. So for example, Nitrogen has 5, each hydrogen has 4 which means 9, but an electron is lost so there's 8. The rest of the shape is determined the same way.

Shouldn't So2 form double bonds with both the oxygen molecules so that both the oxygen and S get stable. Why does it form one double bond with one oxygen and a single bond with the other oxygen??

 

[raamish]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s09_qp_2.pdf

In 1d(ii) its asking the moles of the atoms so why arent the moles calculated like this: mass/Avogadro's constant ???

 

[raamish]

umm so if formation of bonds is exothermic.shouldnt the minus signs multiply to become positive?

When they ask u to calculate the regular enthalpy change without using bond energies use this formula: AH of products- AH of reactants. And when they ask u to take enthalpy change out by using the bond energies then u use this formula= AH of Bond breaking( reactants) - AH of bond making (products)

 

[Saad (سعد)]

When they ask u to calculate the regular enthalpy change without using bond energies use this formula: AH of products- AH of reactants. And when they ask u to take enthalpy change out by using the bond energies then u use this formula= AH of Bond breaking( reactants) - AH of bond making (products)

Thank you.

Now, someone, please kindly, riddle me this.


Question 4, (f), (iii):

Paper: Click Me
Mark Scheme: No, Me!!!

... Why on planet Earth do those carbon atoms (in the mark scheme answer) have five bonds?

 

[Mobeen]

= Bond Energy Of Reactants - Bond Energy Of Products

its something like energy absorbed in bond breaking - energy released in bond forming . . dont know exact ..

 

[miss irfan]

can someone please tell what the examiner means by "use a range of atleast 40 degrees"

 

[princesskt]

tell me about q 1 d part plzzzzzzz
http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w09_qp_52.pdf

 

[IsaacNewton]

October November 2009, Paper 5, Question 2 (b) (i)
We have to calculate the moles for the Iodine reacted. It should have been simple, except that the Mark Scheme says that using the Mr for Iodine, i.e 254, instead of 127 is incorrect.
Why?
For diatomic molecules, we use the Mr, which is twice that of the Ar. But why do we do the other way round here?

 

[shan5674]

Can someone please please explain to me instantaneous dipole - induced dipole and permanent dipole-dipole forces please explain how do we identify the compounds which has either of the two forces of attraction like how do we know when a compound has permanent or induced dipole? I'm soo lost when it comes to this and also how do we know if a compound is polar or not from its shape? I dont understand any of this please help

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_21.pdf
Q1c(iv) why is it 20cm^3? :S

 

[sumaiyarox:)]

can anyone plzzz explain hw am i supposd to get the rfm of the salts in Q1 C II??
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s06_qp_2.pdf

 

[Sharjeel Larik]

Chemistry Applications Booklet SAQs answers?

 

[Ayaaa]

October November 2009, Paper 5, Question 2 (b) (i)
We have to calculate the moles for the Iodine reacted. It should have been simple, except that the Mark Scheme says that using the Mr for Iodine, i.e 254, instead of 127 is incorrect.
Why?
For diatomic molecules, we use the Mr, which is twice that of the Ar. But why do we do the other way round here?

the said in the question its iodine atoms, i did the same mistake...

 

[littlecloud11]

Can someone please please explain to me instantaneous dipole - induced dipole and permanent dipole-dipole forces please explain how do we identify the compounds which has either of the two forces of attraction like how do we know when a compound has permanent or induced dipole? I'm soo lost when it comes to this and also how do we know if a compound is polar or not from its shape? I dont understand any of this please help

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_21.pdf
Q1c(iv) why is it 20cm^3? :S

well, basically, all simple covalent molecules have instantaneous dipole- induced dipole forces, irrespective of their nature or polarity. every molecule contains a cloud of electron in constant random motion. Due to this motion, at any instant there maybe an unequal distribution of electron cloud, resulting in areas with higher and lower electron density and giving rise to partial negative and partial positive charges on the molecule, respectively. this in turns induces a dipole in the neighboring molecule so that they both attract. this is called ID-ID forces.

permanent dipole-permanant dipole force is present betwen all polar molecules. It arises when opposite partial charges of neighboring molecules attract each other. like in case of H2O which has permanent dipoles. hydrogen has a partial + charge and oxygen has a partial - charge. so the H tom is attracted to the neighboring O atom, giving rise to PD-PD forces. along with PD-PD forces, ID-ID forces are always present because these are all simple covalent molecules.

A molecule will be polar if the bonds in it are polar and the dipoles do not cancel each other out because of the syemmetrical shape. the bonds are polar if there is a large difference in electronegetavity between the atoms involved in the bond, such as in case of O-H bond. O is far more electronegetive than H and so the bond is polar. electronegetivity increases across the period and decreases down the group. so if you have a compound between Na and Cl. the difference in electrongitivity is so large that the compound is ionic. But when N2 and H2 form ammonia, the difference in electronegetivity is large enough to give the molecule permanent dipoles but not and ionic an ionic structure. s for the shape you just have to decide if the atoms are arranged in a way that cancels out the dipoles or not. line in CCl4 it does as the partially + carbon tom is surrounded by partially negative Cl atoms on all sides , but in NH3 it doesn't as there is a lone pair which is negative .

 

[littlecloud11]

can anyone plzzz explain hw am i supposd to get the rfm of the salts in Q1 C II??
http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_s06_qp_2.pdf

well, you already know the formula of the salts from the previous part. FeSO4 and (NH4)2SO4
just calculate the MR
FeSO4= 55.8+ 32.1+ 16*4 = 151.9
and (NH4)2SO4= 14*2+ 4*2+ 32.1+16*4 = 132.1

 

[leosco1995]

Shouldn't So2 form double bonds with both the oxygen molecules so that both the oxygen and S get stable. Why does it form one double bond with one oxygen and a single bond with the other oxygen??

It forms a double bond with both..

 

[leosco1995]

well, you already know the formula of the salts from the previous part. FeSO4 and (NH4)2SO4
just calculate the MR
FeSO4= 55.8+ 32.1+ 16*4 = 151.9
and (NH4)2SO4= 14*2+ 4*2+ 32.1+16*4 = 132.1

To add on, how does one identify as the grey/green residue in part (iii)?

 

[littlecloud11]

To add on, how does one identify as the grey/green residue in part (iii)?

Fe^+2 forms a grey/green or dirty-green ppt with NaOH. It's one the the cation tests. when Fe reacts with NaOH it forms iron(II)hyroxide