Chemistry: Post your doubts here!

[nehaoscar]

Why not B?
And doesn't D completely mix with water? - esters?

 

[nehaoscar]

How D?

 

[Mathemagical]

Thanks! So it's always trial and error? There's no way to work it out directly right?

Yes, only by trial and error

 

[Mathemagical]

View attachment 54313



Wouldn't A form H2O2 ?

And if not what will it form? like what would the equation be?

2H₂ + O₂ --> 2H₂O

It would form water.

 

[princess Anu]

but why? why wud that be not correct?

No, it would not be correct. The signs + and - are very important in Hess Law reactions, and it is important to keep them in mind while attempting questions of this sort.

 

[Mathemagical]

View attachment 54314
Why not B?
And doesn't D completely mix with water? - esters?

Esters are generally insoluble in water. Visit this link for more information: http://tinyurl.com/m7jksj

It cannot be ethanoic acid because it reacts with PCl5 to form acyl chlorides.

 

[Mathemagical]

but why? why wud that be not correct?

For enthalpy change of formation, the arrow must point from the individual constituents to the final product. If the arrow is in the opposite direction, the enthalpy change will have the same magnitude but opposite direction.

 

[Autumn98]

View attachment 54304
How to do this one?
I form the table but then I don't know what to do (which values for P and Q?)

Here....I hope it helps... if you dont get it....let me know!

 

[Mathemagical]

help with this please

This is a tricky one. The question asks how many product molecules have the ketone functional group, not how many ketone functional groups there are. Therefore, the 2 parts on either side will form one molecule each with 2 ketone groups. Add that to the 4 in the middle and you have 6.

 

[Ahmed Aqdam]

View attachment 54309

What bondings do each contain?

Iodine: Covalent and van der waals forces
Silicon dioxide: Giant covalent
Sodium chloride: Ionic
Zinc: Metallic

 

[Mathemagical]

View attachment 54309

What bondings do each contain?

I2 is a molecule, and covalent bonds are found in each I-I molecule. In addition to that, in between the I-I molecules, there are weak van der Waals forces.

 

[Autumn98]

The answer is A.

From the diagram and the arrow showing the enthalpy change of formation, we can conclude that the bottommost layer is CO2(g).

Thus, the second from bottom level has to be Option C, which shows the elements in their standard states. Working our way upwards, we can see that the change from Option B to Option A is an enthalpy change of vaporisation, which is positive. Therefore, A is the top level.

oh.. I get your point but why not B?

 

[Autumn98]

Help please??

 

[princess Anu]

somebody solve me this pleasee

 

[princess Anu]

& this too

 

[princess Anu]

For enthalpy change of formation, the arrow must point from the individual constituents to the final product. If the arrow is in the opposite direction, the enthalpy change will have the same magnitude but opposite direction.

yea right, but I'm talking about ENTHALPY CHANGE OF COMBUSTION of H2

 

[Mathemagical]

oh.. I get your point but why not B?

The change from B to A involves atomisation, which takes in energy and therefore is endothermic. Thus, A has a higher energy level than B.

 

[Mathemagical]

yea right, but I'm talking about ENTHALPY CHANGE OF COMBUSTION of H2

That's exactly the same. Recall the definition of combustion and you should realise that the arrow should be pointing toward the product of combustion.

 

[nehaoscar]

Iodine: Covalent and van der waals forces
Silicon dioxide: Giant covalent
Sodium chloride: Ionic
Zinc: Metallic

But don't all molecules have van der waals forces?

 

[princess Anu]

That's exactly the same. Recall the definition of combustion and you should realise that the arrow should be pointing toward the product of combustion.

Oh.. so it shud have been +2* enthalpy change of combustion? am I right?