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Chemistry: Post your doubts here!

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Why is the bond angle in a typical tetrahedral molecule 109.5 degree but not 90? Cuz for a trigonal molecule, a single bond angle is calculated to be 360/3 = 120 but how is 109.5 calculated logically if I wouldn't like to learn it.
 
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upload_2016-1-12_20-8-47-png.58809

How is the mechanism nucleophilic substitution?
The CH3-CH2 attaches to the NH removing HBr, and the CH2 has a positive delta... so wouldn't it be electrophilic...
Please explain why it's nucleophilic :/
 
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upload_2016-1-12_20-8-47-png.58809

How is the mechanism nucleophilic substitution?
The CH3-CH2 attaches to the NH removing HBr, and the CH2 has a positive delta... so wouldn't it be electrophilic...
Please explain why it's nucleophilic :/
I guess we need to consider the acid attacking bromoethane. In that case, the lone pair on the Nitrogen atom would make the acid a nucleophile ... thus nucleophilic substitution.
Just an idea though, I'm not sure.
 
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upload_2016-1-30_14-49-57.png
upload_2016-1-30_14-50-35.png
Why can't it be HZ + OH- ==> H2O + Z-
And pls explain the second part of the ms ... addition of base
Adding OH- ... where did the H+ come from ... there's no H+ in the equation :/
 
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View attachment 59025
View attachment 59026
Why can't it be HZ + OH- ==> H2O + Z-
And pls explain the second part of the ms ... addition of base
Adding OH- ... where did the H+ come from ... there's no H+ in the equation :/

The weak acid in a buffer solution is in an equilibrium which is : HZ <==> H+ + Z-
I guess since it's a solution, they are adding H2O.

Then, the added OH- ions combine with H+ to form water. This reduces the concentration of H+. The position of equilibrium shifts to the right and more of the acid ionises to form H+ and Z- .
 
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View attachment 59025
View attachment 59026
Why can't it be HZ + OH- ==> H2O + Z-
And pls explain the second part of the ms ... addition of base
Adding OH- ... where did the H+ come from ... there's no H+ in the equation :/
What you must understand is that there is actually no such thing as H+ existing by itself. H+ is a proton, and it would be too unstable to stay like that. As soon as it finds an H2O molecule, it binds to it to form the H3O+ ion. So really, whenever we talk about H+ in a solution in chemistry, we mean the H3O+. I suggest you think of H+ and H3O+ as interchangeable terms for the same thing.

A weak acid dissociates in a solution by reacting with water. This is what actually happens:
HZ + H2O <-> Z- + H3O+

But sometimes we just ignore the H2O and just say:
HZ <-> H+ + Z-
You can think of it as subtracting H2O from both sides of the equation, but it isn't what happens really.

The weak acid being in a reversible reaction above when placed in water is very useful for us. It can act as a BUFFER.

When acid is added (we say H+ is added), equilibrium shifts to left side based on Le Chatelier's Principle, so change in pH is resisted.
When alkali is added, OH- reacts with "H+" to make H2O molecules; H+ ion concentration is reduced when base is added to buffer.
Le Chatelier Principle means that equilibrium will shift to right to restore this "H+" and so change in pH is resisted again.

Hope that's clear.
 
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What you must understand is that there is actually no such thing as H+ existing by itself. H+ is a proton, and it would be too unstable to stay like that. As soon as it finds an H2O molecule, it binds to it to form the H3O+ ion. So really, whenever we talk about H+ in a solution in chemistry, we mean the H3O+. I suggest you think of H+ and H3O+ as interchangeable terms for the same thing.

A weak acid dissociates in a solution by reacting with water. This is what actually happens:
HZ + H2O <-> Z- + H3O+

But sometimes we just ignore the H2O and just say:
HZ <-> H+ + Z-
You can think of it as subtracting H2O from both sides of the equation, but it isn't what happens really.

The weak acid being in a reversible reaction above when placed in water is very useful for us. It can act as a BUFFER.

When acid is added (we say H+ is added), equilibrium shifts to left side based on Le Chatelier's Principle, so change in pH is resisted.
When alkali is added, OH- reacts with "H+" to make H2O molecules; H+ ion concentration is reduced when base is added to buffer.
Le Chatelier Principle means that equilibrium will shift to right to restore this "H+" and so change in pH is resisted again.

Hope that's clear.

Thanks for the detailed explanation.
So when base is added, the OH- ions, in reality, react with the H3O+ ions?
 
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What you must understand is that there is actually no such thing as H+ existing by itself. H+ is a proton, and it would be too unstable to stay like that. As soon as it finds an H2O molecule, it binds to it to form the H3O+ ion. So really, whenever we talk about H+ in a solution in chemistry, we mean the H3O+. I suggest you think of H+ and H3O+ as interchangeable terms for the same thing.

A weak acid dissociates in a solution by reacting with water. This is what actually happens:
HZ + H2O <-> Z- + H3O+

But sometimes we just ignore the H2O and just say:
HZ <-> H+ + Z-
You can think of it as subtracting H2O from both sides of the equation, but it isn't what happens really.

The weak acid being in a reversible reaction above when placed in water is very useful for us. It can act as a BUFFER.

When acid is added (we say H+ is added), equilibrium shifts to left side based on Le Chatelier's Principle, so change in pH is resisted.
When alkali is added, OH- reacts with "H+" to make H2O molecules; H+ ion concentration is reduced when base is added to buffer.
Le Chatelier Principle means that equilibrium will shift to right to restore this "H+" and so change in pH is resisted again.

Hope that's clear.
Yes :D thankyou soooo much!
 
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upload_2016-1-31_12-40-19.png
So the answer is J
But why not G?? Rotating the left side of G by 180 degrees will give F right?? :/
 
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What does the reaction of ammonium chloride and Iron(II) sulfate produce? (can it produce ammonium iron(II) sulfate???)
 
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Thanks for the detailed explanation.
So when base is added, the OH- ions, in reality, react with the H3O+ ions?
Do you mean instead of H+? Yes!

What actually happens:

H3O+ + OH- <-> 2H2O

So you see H3O+ donates the proton to OH-, so that TWO water molecules are formed.

We normally simplify this by "subtracting" H2O from both sides:

H+ + OH- <-> H2O
 
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View attachment 59070
So the answer is J
But why not G?? Rotating the left side of G by 180 degrees will give F right?? :/
Rotating the left side of G by 180° actually will not give you F. For this you must try to imagine this compound in 3D.

For G we have:
  1. CO2H that COMES OUT of page
  2. NH2 that GOES INTO the page.
  3. H that is along same plane as page
Now, if you were to rotate this 180°, you'd have a CO2H group that GOES INTO the page, and NH2 that COMES OUT of page. Try think about it. You'll see.

Is this what F is? No. F has CO2H that still comes out of page, so it is not identical to G
 
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Rotating the left side of G by 180° actually will not give you F. For this you must try to imagine this compound in 3D.

For G we have:
  1. CO2H that COMES OUT of page
  2. NH2 that GOES INTO the page.
  3. H that is along same plane as page
Now, if you were to rotate this 180°, you'd have a CO2H group that GOES INTO the page, and NH2 that COMES OUT of page. Try think about it. You'll see.

Is this what F is? No. F has CO2H that still comes out of page, so it is not identical to G
Ohh! I see ... but so when you rotate it 180 degrees wouldn't it be
1. COOH is along the same plane as page
2. NH2 goes into page
3. H comes out of page
like so:
upload_2016-1-31_18-11-40.png
Instead of "Now, if you were to rotate this 180°, you'd have a CO2H group that GOES INTO the page, and NH2 that COMES OUT of page."
 
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Ohh! I see ... but so when you rotate it 180 degrees wouldn't it be
1. COOH is along the same plane as page
2. NH2 goes into page
3. H comes out of page
like so:
View attachment 59077
Instead of "Now, if you were to rotate this 180°, you'd have a CO2H group that GOES INTO the page, and NH2 that COMES OUT of page."
I have tried thinking about it, but I still think the CO2H group would go into page after 180 rotation.
I don't see how it could be out of page initially, and then become same plane as paper. That would require non-180° rotation.
I may be misunderstanding something though. Let's see what others have to say. :)
 
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I have tried thinking about it, but I still think the CO2H group would go into page after 180 rotation.
I don't see how it could be out of page initially, and then become same plane as paper. That would require non-180° rotation.
I may be misunderstanding something though. Let's see what others have to say. :)
Haha alright :p
Just to confirm ... the stick means same plane as paper, the dotted line means into the page and the bold line means out of the page right? :p
 
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